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oldtuckunder

Mira Bogs Oh Well Back To The Lotions And Potions!

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MIRA Sprint yesterday, and just having had a good session on the RR I decided to take Vitesse on the Trailer with the 15" rims, 175 carbs fitted and run in Class 2 modified. Bit Academic as I still have to remove the LSD to try a 3.63 diff with the 13" wheels to qualify for Class 1 road going.

 

Was looking forward to a good day as she really was running sweetly. 

 

On the line for first parctice, have revs around 2.75K, green light, drop clutch, wheels squeel, we go, and then bog down, and then slowly get going.

 

Bugger haven't done that for a couple of years since I sorted the 150's properly.

 

Put it down to not having enough revs, as I'd found at Gurtson a couple of weeks previously that I was spinning to much if I used above 3K at launch.  Also Gurston you start downhill and MIRA is flat and its on the super grippy test tarmac.

 

So second practice use about 3.5 and we get away with a lot of wheel spin but no bad bog.

 

First timed run launch at about 3.25K

 

post-2759-0-32598100-1494183654_thumb.jpg

 

Bugger major bog, and as you can see despite being at just about full throttle (yellow) and low manifold vac (blue) the AFR's have gone to sub 10 and the revs (red) only slowly climb until I give up, change into 2nd at which point everything recovers and we complete the run although not trying.

 

Back in pits think I'll replay the trace on the Innovate LM2, but unlike playing log with Tuner Studio on PC where I can see lots of data points per second, on the limited logger display its only by second, so hard to identify what going on. However can see that we do go rich.  Bugger with the 150's I had lean bogs to solve and using a heavy oil was a major part of the solution, so maybe I'll try a light oil. Suck the contents out the dampers (yes I still travel with the rapid damper oil change kit) and find the lightest oil I can which turns out to be brake fluid.

 

OK Final timed run and only chance left of the day to put in a respectable time.

 

Whilst Idling I can see AFR's have moved from 12.5/13 to 13.5/14 just from the damper oil change. Launch at 3.25K 

and

 

post-2759-0-10352000-1494183653_thumb.jpg

 

Takes off well, but almost instantly I can feel its going to bog again, so lift slightly (2.33), feel it pick up again and smoothly open the throttle and we are away and running, not a bad run beat my PB at MIRA by just under 2 secs, but know I lost time through the long first left hander as because of the recovery I had only got into second for it not third so coming out I was way up the rev range range had to change up. But I'll live with a PB.

 

Now I have to work out what the F the carbs are doing. As you can see from the second trace the AFR's were only down at 12 when I detected the stammer, or was I detecting the start of the following lean spike (which even so isn't that lean at 13) and given that in the first trace with the real bog they had plummeted to 9 (which I know from experience is death to all power)

 

As at all other times, conditions everything is fine and the AFR's are where I want them to be, I'm back to trying to get the carbs to behave in that strange land that happens at launch where the revs are up, the piston has lifted enough for launch revs, you drop the clutch and go WOT, the revs drop some as the tyres bite, slowing the airflow, whilst the butterflies are fully open, and the piston and needle are?  Did the piston drop too quickly as the revs dropped and the air flow slowed so we went rich, or did it not drop and that's the cause of the lean spike.

 

HoHum the hills here are going to resound again to Alan with a full set of springs and about 10 oil weights doing multiple standing start tests again and rushing back to look at the traces, whilst hoping the transmission survives a seasons worth of launches in one go!

 

Alan

 

 

 

 

 

 

 

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Very interesting, Alan!  And good luck!

 

On launch control, this has got so sophisticated and successful in moderns that rallies are taking measures to frustrate it, as the MSA is trying to keep average speeds down on rally stages, in search of safety.

I was on the Pirelli Rally in Keilder recently and Stage 8 had a "Six Bale Weave" just after the start line, six full sized straw bales a few feet apart that had to be zigzagged, in a narrow forest track.

 

See:

 

 

John

 

PS Ah! This board won't let me upload the video, even though its less than the size limit of 32Mb.  J.

Edited by JohnD

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That's weird...... don't understand what is going on there.....

 

I'd say the problem is in good hands though!

 

Nick

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Now I am confused, I'll let you ponder the picture. Identical runs all within 15 mins at the same location.

Yellow = TPS

Red = RPM

Blue = MAP

Black/Purple = Front and Rear carb AFR's

 

Alan

 

post-2759-0-37285000-1494372213_thumb.jpg

 

 

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Confused I am!

 

Had a few days break from AFR's, family medical emergencies. and also wanted to crack on and out some time into the competition bonnet.

 

But weather was nice early Saturday evening (same time of day as the traces above) so thought I'd make some changes and try some further tests.

 

First swapped to light blue springs from the neutrals, complete disaster piston lifting way to fast and going lean, so gave up on that and swapped to heavier red springs and repeated all the above tests at the same location, results, well just about the same all launch to 5K times in 3.78 - 4.2 secs, and to be honest I think I can allow +/- 0.2 secs margin of error on the results down to me not being a robot and as I can see from the traces my right foot isn't doing exactly the same thing every run.

 

Now the really annoying thing is that in all 16 runs I haven't managed to replicate the bad bogs I got an MIRA!  So do I have a real problem or was it just the extra grippy surface at MIRA that provoked the issue.

 

Been looking back at last seasons launch logs, and the 150's nearly always at around a 3K launch responded very much like the 175 Brake Fluid 2500 launch above, i.e. pull rich, snap lean, and then start pulling richer. So for the moment I'm going to revert back to standard springs and a 20/50 oil, whilst a lighter oil may just be fractionally better at launch, I can see the heavier oil is working better at higher rev changes to WOT giving a short rich spike which is what I want.

 

So I have given up practice standing start logging for the moment if only because I'm worried this early in the season about either burning out the clutch or breaking something in the transmission line. Have a practice day at Shelsley later this month so should get in at least 8 runs, so will take all the kit so that if I experience a problem I can play with things on the day.

 

I have a suspicion that 150's respond better in the lower rev range, I think because the air volume is moving quicker and there is higher piston lift for the same volume, and I conjecture better atomisation, and then when we get into the 5.5-6K the 175's take over and can flow the required volume.

 

My mind is wandering to thoughts of how I could disrupt/disturb the airflow to the 175's at low-medium air flows only, if only to see what if any difference it makes. Almost like putting a lightly sprung loaded flap in front of the carb mouth that progressively opens with air flow. OK I know that sounds exactly like the piston in the carb, but that alters the mixture as it moves.

 

Watch this space!

 

Alan

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Would expect 150s to work better at lower airflows.

 

The lack of correlation between viscosity is doing my head in........ especially since it looks like your best result time-wise was achieved with low revs and part throttle.......

 

Suspect that a successful launch is always going to rely on the launch controller between the drivers ears so probably best to concentrate on maintaining steady thrust thereafter.......

 

If you fancy a challenge you can always come and play with my Megasquirt acceleration enrichment - don't think I've every got that quite right....

 

Nick

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The lack of correlation between viscosity is doing my head in........ especially since it looks like your best result time-wise was achieved with low revs and part throttle.......

 

Suspect that a successful launch is always going to rely on the launch controller between the drivers ears 

 

Me likewise I had viscosity just about sussed on the 150, but on the 175 its a bit topsy turvy, that one is strange if you look at that trace its got the biggest lean spike of the lot right after launch and if you notice at the peak that's about the exact point I eased on the throttle, was I feeling something? why did I back off? but it worked.

 

Also notice on all the traces after pulling down to an AFR of approx 11 just when the revs are starting to recover, the AFR's then start climbing back up to about the 13 mark for 5K (which I'm not unhappy with) with the 150's the AFR's once pulled down at higher revs just stayed there almost despite what I could do with the needles, which I think was confirmation that at high revs with the 150's there was just too much air flow over the jets.

 

The problem with viscosity damping is that it fades, so for example if the piston is low and you go WOT a heavier oil will slow the initial piston lift (from where it is) compared to a lighter oil, but only for so long as the piston will rise to the same point based on the depression. So one of the problems I think I'm looking at with the 175's is that the air flow/depression is slower at lower revs, now hold it on the line for 5-10 secs after increasing the revs and it doesn't matter what oil you are using and the lift has occurred, drop the clutch the revs drop the air flow slows, the pistons fall, and whilst with the 150's as the revs pick up the depression and piston lift is quicker, with the 175's the same air flow means less depression and thus the lift is slower regardless of oil viscosity. 

 

I'll have to experiment but I have a feeling with the 175's rather than bring the revs up and holding for a number of secs as I learnt to do with the 150's maybe an approach of opening the throttle and as soon as the revs hit target dropping the clutch may work better.

 

The harder thing may be learning to not go WOT so soon, instinct says drop the clutch and go WOT (its easier also to plant your foot) but it could be with the 175's that it might work, however as I type that I realise that stopping the piston lifting should be a similar effect. Even heavier oil?

 

Must stop looking at the Cosworth inlet system, and wondering if I could tandem a 150 and a 175 somehow  :yes: that way madness lies!

 

Alan

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Had a Eureka moment this morning re 150's / 175's, well the thought came to me in the bath anyway whilst soaking my aching joints after crawling around under a spitfire dash yesterday!

 

Was pondering why the 150's were better than 175's at lower revs, and a whole bunch of issues like why the Vitesse/GT6 were about the only 2Ltr cars of the era to use twin 150's rather than twin 175's. And it sort of dawned on me that its to do with having 6 cylinders!

 

There's lots of info (some real some hypothetical) that CD type carbs like the Stromberg and SU actually like a pulse at the jet, i.e the airflow isn't a constant stream, its lots of pulses as each cylinder sucks (plus some exhaust pulse effect depending  on valve timing that may or may not help at different rpm's, but I'll ignore that for the moment)

 

Its also referenced in many places that for low-mid range performance its actually better to have an undersized carb rather than an oversized one.

 

So lets take the difference between a 4 pot 2ltr and a 6 pot 2ltr running on twin carbs!  At say 2K rpm  on a 4 pot each carb is seeing approx 33 pulses per sec, whereas on a 6 pot they are seeing approx 50 pulses per second, the other difference is that on the 4 pot is that each pulse is 50% bigger due to cylinder volume. 

 

So the 6 pot is doing lots of small gulps of air per second as opposed to the 4 pot that's doing way less big gulps, even though their total demand over the same time period is identical. So its hardly surprising that a carb that has the right characteristics for one maybe doesn't for the other.

 

So put a small 150 on a 6 pot and at 2k it sees lots of pulses/gulps each of which are moving quickly due to smaller carb size, whereas a 175 on the 6 pot would have slower moving pulses. The mixture behaviour at the jet/bridge is also going to be different, the 150 for the same air flow having to lift the piston further than a 175 would, thus each having different orifice cross sections. The implications of this extend further in that the needle to jet ratio changes much quicker on 150 than it does on a 175, so for a given increase in rpm that causes a 150 piston to rise 1/4" a 175 piston may only rise 1/8", so as all needles are machined in 1/8" steps, a 150 can exploit 2 different jetting/mixture positions, whilst the 175 only one.

 

So I'm beginning to change my mind about why Triumph stuck with 150's on the GT6 and Vitesse, I'd always thought they were being cheap and saving money, I'm now starting to think there was a lot more science to it and that for a road car the 150's are probably better suited to the engine. And whilst we know that the 150's are restrictive in the high 5K and above range and have a tendency to get richer and richer at protracted WOT, its almost the exact characteristics a manufacturer would aim at, crisp low - mid range performance that feels good, and if a driver really pushes it the mixture just gets richer and richer which is very safe for the engine.

 

Of course doesn't solve my problem where I need/want the advantage that 175's give at the top end, but does help me think about issues I'm finding in making a 175 behave in the low-med standing start range, as up until now I had been thinking that as at say 2.5K the air/fuel demand from a 4 pot 2ltr engine is identical to the fuel/air demand of a 6 pot 2ltr so carburation that suited one should suite the other, I now think I'm wrong.

 

Interesting conundrum, do I invest more time playing with 175's and low end issues and be thankful that they deliver what I wan't at the top end, or do I return to seeing if I can improve the top end characteristics of the 150's.

 

Bigger is always better!  ?

 

 

Alan

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Very interesting observations.

 

Triples further complicates things as the draw intervals vary........ best not go there!

 

Nick

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Very interesting observations.

 

Triples further complicates things as the draw intervals vary........ best not go there!

 

 

 

Quite so! especially the way triple are implemented 1&2, 3&4, 5&6   now an inlet manifold that paired 1&6, 5&2, 3&4 and that had nearly equal length inlet tracts would be interesting, although god knows what it would look like, yes your right lets not go there!  There has to be a solution using twins, or its really EFI, but I haven't given up yet! 

 

Alan

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It's the transients that mess you up and there are alot of transients in hillclimbing proportionately.  Multipoint FI or sidedrafts might help....... though even then.......

 

Do you think fuel surge in the float chambers has an influence?

 

Nick

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Do you think fuel surge in the float chambers has an influence?

 

 

 

My theory is not (but we all know about theories) I have a good Facet pump and a Filter King pressure regulator set up that is capable of delivering way more than the carbs need. In theory (again) as the float chamber on a 175 is bigger than a 150 and the air demand hence air/fuel demand at any given rpm should be the same, my guess is that as a percentage of volume the 175 should see a smaller demand at that launch point than the 150.

 

My best guess at the moment is that at launch at say 2.5-3K with the clutch depressed the engine isn't working very hard so there is very little piston lift with the 150's and even less with the 175's. As you launch unless you have the wheels spinning stupidly (and I've proved in the past that doing that gives slower 64ft times) that as the tyres bite and you put real load on the engine the revs are going to drop, as the revs drop so does the air flow through the carbs, and thus the pistons drop. I conjecture that response of the large carb as the flow drops and then starts rising again is significantly different to the smaller carb. On the 150's I discovered that using a heavy weight oil (85/140) that during that transient that the heavy oil slows the decent of the piston so that as the demand comes back as the revs rise the 150 is flowing well, and at that point the heavy oil slows the rise thus giving a better rich boost (accelerator pump effect).  With the 175 I think a heavy oil has the same effect but if the 175 piston hasn't fallen enough then there is too big an orifice left open so as the air demand comes back its all moving too slowly.

 

Now 85/140 oils are way outside the norms for damping, its just I discovered that it works well on 150's for launch, I have also discovered since that others have used the same trick on 4 pots with 175's (which makes me think there is still something in the pulse theory). Looking at the multi logs above it is noticeable that with very light brake fluid the best time was recorded, and with a light oil the 175 piston would have fallen quicker as the revs dropped, the opposite of what I discovered I needed with the 150's.

 

Play time with some lotions at Shelsley on Tuesday.

 

The other vague option running through my head that I'd love some feedback on is launch control. I have an OMEX rev limiter with a launch control function that I have never tried (mainly because it has been fiddly to get the OMEX not to rev limit at the wrong times) The launch control works by random spark killing, so theoretically on the line having set a desired launch rpm, you bring the revs up to just above launch target, flick the launch switch and then even if you open the throttle more, the revs don't rise as it kills sparks. Now I know with carbs you can only do this for so long without risking flooding the plugs, but however lets say I have the throttle open enough that unlimited it would be reving at 4K but I have it limited to 3K, my assumption is that the air flow through the carbs and hence the piston lift is where it would be at 3K, now if I drop the clutch and then release the limiter just as the tyres bite and the revs start to drop, suddenly every single stroke is firing, am I suddenly generating more power? would this prevent/override the fall of the piston? or is it a zero sum gain?  

 

Alan

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Found this comment from Chris Witor yesterday

 

General consensus is Strombergs give better throttle response than SU carburettors Twin carburettor sizes jumped from 1.5 " = 38.1 mm, to 1.75" = 44.45 mm. If the clock could be turned back, and I was a Triumph engineer, I would have opted for trialling 1.625" = 1 5/8" = 41.25 mm - Even if they had to be specially made

 

So today I made some!

 

Well to be honest I didn't but what I did do was make some sleeves to reduce the 175's, only time for a few quick test runs today, rest will have to wait until after Shelsley on Tuesday, but as a taster

 

post-2759-0-61936000-1495408305_thumb.jpg

 

Yes that's a 3.20 2K - 5K pull no bog and a whole 0.5 secs quicker than the quickest on 1.75's from 500-1k lower start!

 

Did a couple of max rpm runs and at 6K plus MAP not pulling down as it was with the 150's, so may be a good compromise.

 

Made a whole bunch of different length sleeves, from dead short so that radius stacks still work (above test) going up to about 2" extension from the carb. Takes about 1 min to swap sizes (including opening the bonnet) so I can play a few games.

 

Bound to be a catch, but lets wait and see...

 

Alan

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Well might be but unfortunately Shelsley (it appears always to be my nemesis) only provided more questions than answers. The day was great but I was plagued by a problem I have never had before and which has been taxing my problem solving (and google searching) since.

 

First run got a clean start no bog, but on first long left hander "crossing" coming out I developed a misfire/stutter in second that then cleared. Second run probably trying a bit harder misfire even worse, but strange in that zero problems prior whilst pulling to 6K plus in first before the corner or for the rest of the climb pulling similar revs after. 

 

Initial "Blame the new sleeves"  so pulled them out

 

Third run didn't notice it, must be the problem!

 

Forth run before lunch, back again!

 

Deep thought. OK third run I hadn't been in the queue long, but forth run I had been idling before start for maybe 5-7 mins, so maybe a bit of plug fouling? or a plug breaking down?  Dismissed Fuel starvation as good baffled tank, plus electric pump, plus Filter King pressure regulator.  Ok so lunchtime change plugs for new ones, plus decide that I would experiment by changing to the 13" wheels (back to power problems on hills!) plus also I knew it would change rev pattern and gearing changes and wanted to see the difference.

 

Try to start car after changing plugs, NaNa Nothing not a pop! bugg.r must have disturbed a distributor lead, check nothing amiss, check again can't see anything wrong, ok try starting it again, nothing! Ok maybe its got cold enough to need a bit of choke (highly unlikely) but try it and it splutters into life sounding appaling, glance at AFR displays and running way lean, and underneath bonnet sounds like its gasping for breath??? Blown manifold gasket or what?  Finally track the noise down to inlet manifold that had blown a sealing plug out of the capped off prv port.  Chances of that?  had driven to paddock space apparently all well, stopped, changed plugs and wheels, and bingo plug missing! So its not always the last thing you changed!

 

Five runs in afternoon 2 good no stuttering after bend, 3 with stuttering.  

 

Good day, but come home with a problem to solve. 

 

Didn't do it on 150's doesn't happen anywhere else with the 175's except coming out of that bend!

 

Examine AFR datalogs from all the runs, and the sequence was pulling to 6K plus first, change to second go WOT for about 2 secs now at 5K ease on throttle through bend say half throttle for a couple of secs, revs not dropping, and then go WOT again. The only difference I can see between a good clean pull and a stuttery one is that on a good one whilst going to half throttle the AFR's pull lean and then pull instantly rich as I go WOT, but on a bad one the AFR's stay richer and have a sort of rapid spiky pattern. Oh and both carbs behaving identically.

 

The web seems almost silent on the symptoms, apart from a million fuel surge in tank issues, which I'm just not buying with my set up.

 

But finally found a similar discussion on  TRIKING forum (which isn't people on three wheeled bicycles, but rather a modern 3 wheeler rather like an old Morgan but with a MotoGuzzi engine).  Problem turned out to be float levels and their transition in corners. Which got me searching with a few different terms which finally yielded a single thread on an American Triumph forum for a TR6 on Strombergs with a similar problem. Again the major suggestion being fuel surge in the tank, but a couple of little nuggets if not conclusive that have given me something to work on.

 

http://www.triumphexp.com/phorum/read.php?13,829499

 

Now all I have to do is find a test track where I can replicate the problem, to see if I can cure it.

 

If anyone has any bright ideas how I might provoke/engineer the problem please let me know. PS I can't afford to hire Shelsley, Oh and we are back there in a couple of weeks for a competition and I'd really like to solve this first!

 

Alan

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That comment from kas on your link was what I mentioned some time ago.  I never explored it but I recall Kas said they added a second vent hole on the other side.  That said, do you know about drysumping the bowls ala Formula Atlantics back when they had carbs.  Only really useful on high G cars yet could be a solution here?

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That comment from kas on your link was what I mentioned some time ago.  I never explored it but I recall Kas said they added a second vent hole on the other side.  That said, do you know about drysumping the bowls ala Formula Atlantics back when they had carbs.  Only really useful on high G cars yet could be a solution here?

 

Ah a second vent hole makes far more sense than turning the tops around.

 

I have a number of spare carbs here that I'm going to start pulling apart, 

 

Hmm couldn't resist a quick peek right then, and its going to be an interesting investigation.

 

OK the 150's I was running were a pair of NOS CDS ones for the last of the UK MK3 GT6's i.e. no emmisions, they have a tiny vent hole that runs from logical outward edge of the float chamber to the air filter side of the carb, so theoretically on a LH if there is surge up that side in the bowl petrol could be pushed up the vent and dribble into the filter side of the carb.

 

I have a pair of 175 CD-2's from a Lotus Elan and they have a similar set up, except that there is a small drilling in that vent tube to the outside of the carb, so if petrol was pushed up the vent tube as well as going to the air filter side, it could just dribble down the side of the carb.

 

Now the 175's I'm actually running are a pair of very late CDSEV's  they have a completely different vent system that if full emmisions are fitted the vent depending on throttle position goes to charcoal canistor, and then gets drawn by vacuum back to the inlet manifold side of the carb. Without emmisions the normal mod is to blank off the feed to the charcoal canister, and then cross drill from that port to the port on the inlet manifold side. NOt a bad mod for normal conditions, but does mean that if you have high manifold vacuum, and surge that covers the vent tube in the filter bowl that you could actually suck fuel direct from the bowl into the inlet manifold via the vent tubes.

 

Interesting, I'm going to have to pull them apart and take a good look at the vent tube system again.

 

Alan

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Was going to say fuel surge in the float bowls.....  Fuel temp might also be a factor as modern fuel boils awfully easily and if it's warm you might gets some unexpected effects just from sloshing it around.

 

Triumph day at Prescott a few years ago I had comment that mine was among the few cars that picked up cleanly accelerating out of Pardon and when we thought about it, it was mostly the injected cars (Mike Bestards Spit 6 was another) than could do this and the carb cars that were fluffing......

 

Nick

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Now the 175's I'm actually running are a pair of very late CDSEV's  they have a completely different vent system that if full emmisions are fitted the vent depending on throttle position goes to charcoal canistor, and then gets drawn by vacuum back to the inlet manifold side of the carb. Without emmisions the normal mod is to blank off the feed to the charcoal canister, and then cross drill from that port to the port on the inlet manifold side. NOt a bad mod for normal conditions, but does mean that if you have high manifold vacuum, and surge that covers the vent tube in the filter bowl that you could actually suck fuel direct from the bowl into the inlet manifold via the vent tubes.

 

Interesting, I'm going to have to pull them apart and take a good look at the vent tube system again.

 

 Ok for anyone subsequently reading this thread in search of a solution what I have written above is bolloc.s, or partially bolloc.s which can be just as bad.

 

If you really want to know how the late vent tube system works on the CDSEV carbs the following link is one of the best.

 

http://www.buckeyetriumphs.org/technical/Carbs/CarbsI/CarbsI.htm

 

My mistake is that I always confuse the return pipes from the charcoal canister (which do go to carb mixing chamber) with the pipes that go from the vent tube arrangement to the charcoal canister which are on the air filter side. If you haven't got all the emissions kit then capping off the return pipe ports and the feed ports from the vent tube assembly makes sense, with one proviso, in the great link above there is actually an error in the drawing, the vent pipe to the charcoal canister actually comes into the carb to the right of the float chamber vent tube to another drilling through from the air cleaner face (where it is blocked with a plug) A lot of playing with tubes and blowing had me confused until I found one other person who also seemed to have deduced the same thing and had an elegant solution.

 

http://bullfire.net/TR6/TR6-20/TR6-20.html     (nb note is right at the bottom).

 

OK thats cleared that blooper up, back to the problem.

 

Alan

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That comment from kas on your link was what I mentioned some time ago.  I never explored it but I recall Kas said they added a second vent hole on the other side.  

 

Was going to say fuel surge in the float bowls.....  Fuel temp might also be a factor as modern fuel boils awfully easily and if it's warm you might gets some unexpected effects just from sloshing it around.

 

Triumph day at Prescott a few years ago I had comment that mine was among the few cars that picked up cleanly accelerating out of Pardon and when we thought about it, it was mostly the injected cars (Mike Bestards Spit 6 was another) than could do this and the carb cars that were fluffing......

 

 

Curiouser and Curiouser said Alice.......................

 

Well have spent the last two weeks trawling the internet for any hint of a mention of Stromberg's and left hand corners, I think I have inspected half the contents of the internet, and come up with only a handful or possibly relevant postings, funnily enough most of which date back a decade or two and usually include somewhere a posting from GT6Steve :yes: on the topic, and of course the great GT alternatively supplying an interesting fact, interspersed with outright condemnation of anyone else!

 

The extra vent drilling came up a couple of times but no details as to how where, the best (again a GT6Steve post) indicated that a former ASC competitor told him about the fix back around 2000, but was at a point where I think Steve had given up and gone Webbers.

 

The other problem with the Left Hand Turn Stromberg problem, is that from reading there appears to be two not one problems. There is the long left hand turn where the explanation of the fuel surging blocking the vent, the float dropping on the fuel angle but no fuel incoming due to blocked vent. And there is the very short sharp left turn stutter, where that just doesn't make sense, and the problem I had.

 

A few theories and suggestions thrown up over the years. 

 

Scrap the Stromberg's

Seems to have been a reasonably popular choice over the years, however countered by some really experienced racers in the US who have never had a problem with them! 

 

Raise the float level 1-2 mm 

Increase Fuel pressure 0.5-1 psi

Both these seem to leave the carbs on a knife edge of flooding

 

Change the Needle valves from 1.75 to 2.25  

I have managed to find a pair! but haven't tried them yet

 

Don't swap the 150's for 175's but get the 150 counter bored so that they are actually 1.5" right the way through rather than 1.25" at the inlet.

This is similar in concept to me playing with reducing the 175's which I haven't got any further with. NB if anyone has a pair of GT6 MK3 150 strombergs for sale please let me know as I don't want to experiment with my almost new ones, and the other sets of 150's I have are the earlier ones with the lifting bridge choke that aren't suitable.

 

So as they say an ounce of real experience is worth a pound of theory, and we had the Abingdon Sprints this weekend. Great event 8 runs on two courses in one day.

 

I also as you will see from this thread had two problems to resolve, the Start Line bogs from Mira (and a couple from Shelsley) plus the stutter out of the left hander at Shelsley. So what did I do in preparation? Nothing!  Well to be honest all I did was drop the float bowls and check the float heights, they were perfect to spec 16mm.  And decided to revert to a 75w90 synthetic dash pot oil and standard springs.

 

I didn't make any other changes as I'd run out of time, and had reached a point where I didn't know what to try.

 

Abingdon is an airfield course, flat but with long fast G pulling corners (left and right) plus some nice sharp left and rights where you exit at full throttle etc, so should be ideal to data log and find out what was happening.

 

So I now have some great data logs of the problems, to work on?   NO!

 

I have some great data logs, but everything was dam near perfect.

 

No bogs off the line (I think this may be as I have now adapted the launch technique so that instead of bringing the revs up, holding, dropping the clutch and going WOT as I could with the 150's, of just bringing the revs up at the last moment, dropping the clutch and then gently squeezing the the throttle open as we pull away)

 

Not the slightest hint of any stutter/stammer during or coming out of any long or short left hand bend.

 

In fact if I look at the logs the AFR's are almost perfect everywhere, and you cant watch the CD carbs responding perfectly to every throttle/rpm input/change. In fact I'd go far as to say you have to be a genius to program EFI to respond as nicely, and as the carbs are doing it based entirely on a single input of airflow its quite remarkable. I'll try and put together an annotated log later so you can see.

 

So I couldn't a test road anywhere to replicate the left turn problem (which was one of the reasons I hand't changed anything) and Abingdon where I thought I had every chance of doing so also turned up nothing.

 

So I'm a bit stumped, if I can't replicate the problem then its pointless changing anything as I cant test if it works. Well I could test if it makes things worse but that seems a bit pointless.

 

Anyway back at Shelsley next Sunday, so can see if the problem(s) are still there, and will take new float jets etc, so if it appears in morning practice I have a chance to make some changes lunch time before the timed runs. Seriously wondering if I can replicate GT6Steve's old suggestion of making the float bowl securing bolts into wing nuts so that they can be dropped quickly!

 

Alan

 

Oh PS. On Nicks suggestion that is could be heat in the float bowls causing this modern crap fuel to boil/vaporise, I went equipped with laser thermometer so after a couple of runs where I knew we had idled for a fair time before the start, at the end of the run I flipped the bonnet and checked. Inlet manifold circa 88deg (about right) Carbs 45deg and felt cool, as a comparison Master Cylinders on bulkhead 43deg, so phenolic spacers and the huge heat shield I have that keeps carbs in cool inlet flow seem to work. So I'm going to park that suggestion. However worth noting the difference between inlet manifold temp and carb temp, if the carbs were bolted direct to manifold without spacer I think they would have been way hotter!

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Wow I just won my first Motor Event (well Class) in about 35 years (and in the same car!)

 

Shelsley came good for me! Have I broken my Jinx?

 

Come Saturday night when I went to refuel, I wasn't sure if we were going to make it :unsure: started the car and had what sounded like a sort of rumble or rather like something rotating at about cam speed knocking/tapping. Bit hard to identify where its was coming from (as exhaust a bit noisy from header collector to pipe joint (dam little 13" wheels means I clout the exhaust on virtually everything)  Anyway took it for around 15 mile refill run and when I got back noise was worse  a bit, but I still couldn't identify, seemed worse when listening from RH rear. Tightened manifold bolts and it got a bit worse! So started hoping it may just be either a manifold gasket blow, or a blow from a cracked header joint. It was too late to start stripping, and to be honest I thought my Shelsley Jinx had just struck again!. So decided sod it will drive to Shelsley in the morning (drive because I have decided to stop trying to fight the odds and dropped into pure road going  for a few events) Thought it might go bang on the way, but it drove like a dream (country lanes early morning with the hood down are just fun!) and also with the bonnet down and the main exhaust blowing a bit I couldn't hear the new noise anyway. Did notice that the popping on over run (which it always does a lot) was actually getting quite bad! By the time we got to Shelsley and lifted the bonnet the new noise was now quite bad, but I still couldn't locate if it was manifold gasket or break in the header somewhere.

 

Decided sod it just continue and see if it holds together. Took first practice gently really just practising the correct line, second practice thought go for it a bit (as it had been pulling well on the first) and starting nudging towards by new bogey time (but could see that others were doing better) Opted out of a third practice run that everyone else took (for UK getting hot 30C air, and under bonnet?) thinking I'd save the car for at least one competition timed run in the afternoon (noise was getting loud now, but engine still pulling well) First run after lunch engine nice and cool (had had a longer rest than most!) just warmed the engine up enough to get oil over 60 (oil water heat exchanger meant I could do this quite quickly) and just went for it! (expecting a load bang or total loss of power at some point) bug.er me as I crossed the line saw the clock and spotted I had beaten my bogey by just under a second!

 

And that at the end of the day was enough to win the class (did do one further run, not as quick, slightly slower than my bogey).

 

So what happened to the Start Line bogs, and fast left hand stutter problems I had been battling?

 

Don't know, just disappeared!, despite the new problem it went like a train all day. No, as in previous reports I hadn't changed anything as I didn't know what to change.

 

What was interesting was that by afternoon runs (real hot under bonnet temps), a lot of other people running on SU's were complaining about gutless engines, loss of power, apparent misfires, and changing plugs, coils, removing filters etc, to try and resolve with no effect. Whilst my unfashionable Strombergs (admittedly in the cooler air chamber I have created) behaved as though they were having the time of their life.

 

Did the higher temps actually help with my problem? Has it mysteriously gone away? Or is it waiting to bite somewhere else?

 

Anyway well pleased with the car and the weekend!

 

Alan  

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I think your careful development and preparation re. the heat shielding was paying off yesterday.  Fuel vaporisation is a huge problem with current fuels in carb cars.  We built Spitty specifically with this in mind - electric pump at the rear, fuel pipes well clear of anything hot, several gaskets between manifold and carbs - but still it gets issues in hot weather.  Chris just invested in a rather nice SS heat shield.  We only fitted it Friday so I guess I hear if it worked or not later.  Today will be a fairly severe test - it's hot here.

 

I also wonder about fuel variation.  Usually I'm a shameless (inverse) rate chaser and buy the cheapest crap I can find.  Typically this has been Morrisons or Asda, though recently a local BP station has been running them close on price and is more convenient being directly on my route to work.

 

However this weekend I needed mower fuel and went to Texaco 'cause it's much nearer than the really cheap ones and much cheaper than the really near one (gouging b'stard Esso station on A303).  So the Vitesse was carrying the mower can and got a drink too - just plain ordinary Texaco 95.  Possibly unrelated but seems to be running extra sweet right now.......  Maybe Texaco put the expensive stuff in the wrong tank by mistake......?

 

Nick

 

PS.  Your strange noise sounds (indirectly obviously) like it could be a manifold blow either due to a crack, leaky gasket or leaky joint.  Fits in with the popping on the over-run too.

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THERE SHE BLOW'S! 

 

Had to have most of the day off travelling and visiting aged relative, where my body had been promised to fix a broken curtain rail, whilst everyone else sat around drinking and saying how nice it was to be able to sit outside.

 

Now its easy to change a curtain rail isn't it! And as a good boy scout I had taken about every tool I could conceive I might need (knowing no tools at destination) Plus a complete new Rail Kit longer than required (took what I was told and added a couple of feet on for good luck).

 

Old rail brackets had been nailed up by some "Master Craftsman" with masonry nails into plaster, so as I took it down I had chunks of plaster falling of the header rail! Also discover when I tried drilling header rail for new brackets that the dam thing had been formed by some other Master Craftsman out of solid plaster no inner timber or anything. So now have to go off hunting in strange town for a DIY store to buy Wooden Batten, Polyfiller etc.  3 hours after I started finally have repairs done, solid wooden header rail, and new rail and curtains hung!   So now its a journey back cross country in the rush hour, and finally get home around 7pm.  Had originally thought 5 ish and then I could get on with stripping the tunnel, starter, and headers to find the blow. 

 

Bug.er it normally takes me at least an hour to get all the electrics disconnected and the tunnel out (plus and hour and a half at the end reinstalling), just so I can get at the two nuts on the bolts holding the starter motor on. Look again is there anyway I can get at those nuts from engine bay side? Nope impossible!  But what if I cut a chunk out the Transmission Tunnel?

 

post-2759-0-62011800-1498172357_thumb.jpg

 

Easy!  10 mins later starter off!  OK I will have to make some jointing plates when putting it back, but from now on removing the starter is going to be a walk in the park!

 

So now all I have to do is break the joints between the collector box and the two header pipes and I can get the headers off. Bug.er the engine doesn't seem to be burning as much oil after this years rebuild, in the past the joints have all been a bit oily and have come apart easily, this time they are rusted solid (and they were only refitted 3 months ago) and it takes half an hour just to pull/bash/cajole/threaten the two parallel joints apart. But finally they come and the headers can come off! All done by about 8.30 (just about clawed back the lost curtain rail time)

 

post-2759-0-98987600-1498172356_thumb.jpg

 

Whoops look like they started mating and have burnt a hole through each other where they touch so they can exchange bodily fluids gasses! 

 

Now I know that all the header wrap looks tatty, but that stuff is the dogs bollocks, after two wrappings in the best I could buy in the UK which just turned to powder in less than a year, this stuff an American friend into drag racing sent me, and its survived two and a half seasons so far, sure its brittle, but it hanging in there!. My point? Loads of people say don't use header wrap on mild steel as it attracts damp and starts the steel rusting between the wrap and the header, but 1) I'm sorry but I'm a bit anal about under bonnet temps and I want that heat out and away and 2) as the rest of the headers inside the wrap are solid its the bits that weren't wrapped that have rusted/burn't through.  Why aren't they wrapped there? because with wrap between them I couldn't fit the headers. OK pondering that with wrap on lot more heat trapped and going down the pipes (just what I wanted) but maybe heat focused on the touching faces that aren't wrapped causing steel to burn? rust? away.

 

So what to do?  I have a new set of stainless headers in a box that I was saving for a rainy day, and maybe (if I didn't think it may be snake oil) get zircon coated before fitting. But by the time I have beaten them into shape and fitted them (New Headers Never Fit) then pulled them again, then got the sensor bungs welded in, and then either get them coated (£££!) or get some more header wrap from the States (£!) the season will be almost over! Certainly won't make Castle Combe on the 28th!

 

So I think I'm going to cut some plates in the morning and see if a clever friend can weld them for me!  Having seen the video that either John or Nick posted a few months back about how beating headers in all sorts of ways seemed to make virtually no difference, what I think I will do it beat these two points in when welded, so I can actually get some wrap in between them, and hopefully stop it being a high heat transference point!.

 

But at least problem found!

 

Alan 

 

PS. Was the engine running so well at Shelsley because the headers could cross breath? Darn wish I hadn't asked that question! 

 

 

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Well, that's fairly ugly....... and certainly explains the odd noises. I was imagining cracked welds in the "armpit" of one or two joints, not damn great rot holes!  I imagine it's because they've actually been in contact some of the time and there's been fretting.

 

Looks like a 6-2-1?  Is the stainless one a 6-3-1?  No idea whether "cross-talk" has been affecting performance but looks like hole enough for cross-talk to have been happening!

 

Good luck with the repair!

 

Nick

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