Andrew's Mk IV Spitfire

[Escadrille Ecosse]

That is an excellent result. And you will really notice it.

[egret]

I suspected I was in a good place when looking around there was a Z4 race car on the ramp. A man tinkering with a BMW motorcycle and sidecar racer, and a "Micra" that was a tune frame race car. Oh and they wheeled out an isetta and MK2 gold GTi to get to the Dyno. An eclectic mix of interesting machinery!

[mpbarrett]

sounds a good result. Where did you go?

 

[egret]

I went to Revivals, in Thriplow.  They were really good, and interested in my car and what I was doing to it.  I'm still deep in thought about the next steps for refining the fuelling.  Maybe modifiying some new AAM needles to make them richer where I need at higher revs. I'm still undecided upon the spring removal.  My understanding is that you want the pistons to reach maximum height at max power, so reducing the spring rate will help with this at the cost of reduced depression through the venturi (which is what drives the fuel out of the jet).  This makes sense if airflow is your limiting factor (particularly if the carbs are a meaningful % of resistance in the system), but if they aren't a restriction, then all you are doing is reducing the depression which is driving fuel flow.  Given I have 1 1/2" carbs, not 1 1/4" and a standard cam, I suspect that airflow is not my issue, and that fuelling is.  So more depression would be beneficial.  Of course then I need to alter the needle profile to be sufficiently rich at this lower station.

I've also got an issue with fuel height in the float chambers being different, which likely isn't helping, the rear (left) carb appears to have a lower fuel level than the right.

Finally there seems to be a good bit of oscillation in the carburettor pistons, which isn't giving consistent fuelling and is not optimum, but this might just be an SU carb thing?

If anyone can understand the ramblings of a man who'se spend a lot of time reading books on SU carbs and pass comment then I'd greatly appreciate any and all opinion.

Here's a video of one of the dyno runs looking down the carbs.

https://youtu.be/Tb8OG95RZ14

[JumpingFrog]

Great result, considering the 75 hp DIN rating of a Mk.3 engine when new, on very different fuel...

1 hour ago, egret said:

Finally there seems to be a good bit of oscillation in the carburettor pistons, which isn't giving consistent fuelling and is not optimum, but this might just be an SU carb thing?

I'm pretty sure oscillation is normal for twin carbs on a 4-cylinder, if you think about how the cylinders are paired and the firing order, ...1-3-4-2-1-3-4-2..., the periods of flow through each carburettor are not evenly spaced so there will naturally be some oscillation. I've also experienced the situation where 2 and 3 run richer than 1 and 4 due to this.

[egret]

Well the su carburettor high performance manual has a few interesting sections in it.  It mentions that rover fitted very large carbs to one of their 4 cylinder engines as a sales gimmik, and restricted the piston travel to prevent full travel, and the poor performance that would have given. Maybe my carbs are oversized and putting the springs back in might help.

It has an entire chapter on the method of how to reprofile needles, so that feels like a good option in the future. But the most relevant part talks about how float bowl volume is critical to power output.  Given the visible difference in fuel height, this needs to be rectified before I fiddle with anything else.

Having said all that I'm not feeling negative, the car is driving much better than before. I just believe there are still some cheap/easy wins to be had

[PeteStupps]

Interesting stuff, and good to see the video. Is that with the dashpot springs removed? I'd agree with David Jumpingfrog about oscillation - doesn't look unexpected to me. Different damper oil and spring strength would change behaviour to some extent but not eliminate the wobble. Air flow down the carb throat is a train of pressure waves rather than smooth, so pistons will always react to that, I would think.

On fuel height, have you tried adjusting the float height in one or other carb? It's a bit trial-and-error, and difficult to get them bang on equal. I set mine so that (with dashpot & piston removed) lightly blowing on the top of either jet makes the fuel slosh enough to just be visible. Very unscientific... maybe a cocktail stick dipstick down the jet would be more rigorous. 

Carb size and piston opening is an interesting question; I know the upgrade from HS2 to HS4 is fairly common but, from what I've read, you need to take cam / head / exhaust mods a bit further before HS2's are a limiting factor. 

What was the rolling road bloke's explanation for removing the springs? Was it running too rich / less power beforehand? 

[egret]

I'm not certain if the springs were in or not with the video.  I'll have to check the timestamp, but I think this was before we took them out.

I've got it on my list to look at the fuel floats and try and adjust these better.  The rolling road man said to ensure you set them with the smallest weight on the needle valves, and don't just turn them upside down using the entire weight of the float.

Also I think that if I had the option I'd be running HS2s, it's only that I have the HS4 setup that I'm using it.  I feel like a reliable set of HS2s would be comparable to a fuel injection setup.

The reasoning behind removing them was that we'd run out of richer needle options and were still overly lean at the top end, so getting the needle up faster would put more fuel through.  It did seem to do this, but I still suspect that a richer needle with the springs back in might be the optimal solution.

[Nick Jones]

I agree that HS2s are big enough on a 1300 unless it’s fairly seriously tuned up in airflow terms. The HS4s are a bit big.

As I understand it, the dashpot springs  are used to set the working range of the carb so that the dashpot is fully up only at maximum airflow. 

Taking the springs out makes things richer by exploiting needle taper, but the flip side is it reduces vacuum in the throat so less fuel is drawn out…. 

[RichardB]

Your thoughts do make a lot of sense, as someone who went on this journey and had a lot of fun fettling the carbs on my Spitfire. The book you have is brilliant and it all works, I ended up making the little sticks with numbers on to put in the dash pots and custom needles on the drill press. You can get it into a very good state of tune without a rolling road, and you could even fit a wideband sensor to your exhaust if you want to confirm the results you think you are achieving. Ultimately I went down the fuel injection route as I liked optimising the car so much, in that way I guess the book and tuning carbs is a bit of a gateway drug towards EFI.

As Nick says I think the logic you've been given may not bear out in reality, but at least you've now got one good setup to fall back on, and can use that as a benchmark to experiment further.

[egret]

Uneven fuel levels in the jets can be attributed to mismatched float valves.  I have solid plastic (i.e. non-adjustable) floats, with one viton tipped valve, and one metal tipped one.  Interestingly the seats also have different diameter holes in them, so looks like a bit of a cobbled together system, for once I'm not guilty!

I've got two new viton tipped needle and seat kits ordered which along with everything else will complete the trigggers broom aspect of the carbs rounding out new butterflys, needles, and jets.  Anything else goes wrong and it might be the straw that pushes me to fuel injection...

Photos sto show some of the nastyness that the ethanol has caused on the bits

[egret]

Slow progress is being made, I'm hoping for good weather this weekend to push it forward again.

I've got new float bowl viton tipped needles and seats swapped in which will hopefully go some way towards stopping the mixture leaning at higher revs.  I've also measured the carburettor piston springs I have in my collection, the measurement technique wasn't great so don't pay too much attention to the actual figures, but I believe it's acceptable for comparison purposes.

The ones in originally (which we took out during the rolling road session) were 8oz, and I have spares at 4.5oz.  I also have some 2.5oz springs, but these are for the later HS4's I got as part of a spared box and are the larger diameter springs.

I think I'll try the softer springs based on the idea that spring rate creates the vacuum (under steady state this is the only thing, the dampers can be ignored), and more vacuum will give better fuel pressure, atomisation etc. so better fuelling with all thing being equal. Yes it means I'm on an earlier station on the needle so a smaller opening for the fuel, but I suspect that the added vacuum will help as it's running lean even at higher needle stations.

 

 

 

[egret]

It drives better with the springs out.  Maybe I'll try the other carbs, with even softer springs next.

I can almost hear the cogs turning on the justification of a wide band sensor purchase!

[Escadrille Ecosse]

12 hours ago, egret said:

It drives better with the springs out.  Maybe I'll try the other carbs, with even softer springs next.

It probably will do as it will be over-rich now where you are looking for driveability.

12 hours ago, egret said:

I can almost hear the cogs turning on the justification of a wide band sensor purchase!

Never used one, because I never had one, because they were out of my league price wise back in the day.

But given how cheap they are nowadays seems perfect for a job like this.

[Nick Jones]

Wide band is a very useful bit of kit….. why work in the dark when you can turn a light on…..?

[egret]

I'm rebuilding my other carbs which look to be in better condition.  One thig I've noticed is that the jet bearings don't end flush with the bridge as the do on the older ones, and as I'd expect.  They also are a funny nut size, bigger than 11/16" by a hair, but smaller than 19mm so removing them for inspection hasn't happened yet.

Does anyone have any opinion on how much of an issue this could be?  It feels like a path to leak fuel if the fuel height is higher than the top of this bearing. I dont think I can test this easily without putting them on the car.

Has anyone else come across anything similar?

[Escadrille Ecosse]

Hmmm. Never seen that before. My understanding is that the jet bearing (and jet) should be flush with the bridge. Not so much a fuel leakage issue as a carburettor not working properly issue because you're not getting a consistent or correct depression at the jet.

Not sure how the situation could have arisen unless a PO has put a washer between the body and jet bearing.

As for the nut sizes - yeah seem to remember that with some of them. Bit of a pain

Edit. Not sure if the fixed and bias needle jet bearings have different heights which if they do would also explain this.

Edited June 27 by Escadrille Ecosse

[JumpingFrog]

I'm not sure about the jet bearing issue, I've never had an issue there and also think that it should be flush, or more flush than yours is at least. It could just be that the tube has not centred properly? That or someone has fitted a new jet bearing and left the old washer as well as fitting the new one?

But I also want to warn you that you might have issue with those jets, the black jets are for SUs on a 45 degree angle like on the A-series and B-series. The tubes are longer and easily kink, and then you'll have starvation issues as it'll collapse from the bend, been there and done that... I recommend you track down the proper red coloured jets.

Edited June 27 by JumpingFrog

[egret]

So the jet bearings come in different sizes, meaning I'm more committed to the new frankenSUs as I only have the parts for one complete set of working carbs. The lower bearings also happen to have a finer adjusting thread on them.

Thanks for the comment on the jets, my choke connection linkages are bit rubbish which I think gives funny rotational alignment, stretches the tubes, and caused kinking issues with the old red ones I had. The recent (part way through the rolling road) change to the black jets seems to have improved this, but I'm keeping an eye on them.

[Escadrille Ecosse]

Not sure what that means exactly for you but I would strongly suggest that the final result involves the jet flush with the bridge.

Otherwise you will find getting a good tune across the range of operation pretty much impossible as there will be no consistency in the fueling.

It will certainly be a major factor in the driveablity issues you were having earlier.

As the correct bits are available from Burlen and having gone to all the trouble and expense to this point it would be a shame not to just sort this.

Edited June 27 by Escadrille Ecosse

[egret]

Sorry, I'm not being clear! I've swapped the jet bearings from the older carbs into the newer ones, to give me a decent set of working carbs with the jet bearings flush with the bridges.

Getting a better set of choke linkages is on my list of things to do, but will also involve getting a set of the correct red jets, so over £100 to sort something that I believe is working fine for now.  I will keep an eye on it though.

[Escadrille Ecosse]

No apologies needed. You've had a bit of work on your hands there. 

Somewhat eye watering cost for the choke bits!

Hopefully the rest will do the job nicely for a good while

[egret]

Well the new carbs run really well.  I know this is silly, but I'm rather pleased that I managed to foul my plugs.  Given that I've struggled to chuck enough fuel at it in the past - I've never fouled the plugs before - this gives me hope that I'm now not restricted by fuelling.  So plenty of tinkering in the future to get it running sweetly, but I'm feeling really positive about this now.  The wideband sensor is calling, but the children are now very interested in coming with when I take it for a drive and the Mrs is now badgering me about a rollover bar and inertia reels (at least for passenger).

I did take it to a local general car meet on sunday morning with my littlest and other than a rather embaressing 2krpm idle I think caused by the plugs clearing, it behaved very well.  I think I was the scruffiest car there other than the intentional type2 rat vans, but everyone was very welcoming.  I did made the others look even better by association!

[Escadrille Ecosse]

Great news.

Looks great in that photo

 

[Nick Jones]

3 hours ago, egret said:

I made the others look even better by association!

Welcome to my world 

And good work on the carbs.