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hymodyne

Air/fuel Mixture And Su Carb Needles

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I have a MK III GT6 with a set of HS6 carbs. I have been able to get the air/fuel mixture right at idle, (bet. 13 and 14), but when I drive the car, starting from a full stop and acccelerating, and observe the readings, they are mainly in the overly rich (10.2) range, causing the gauge to flash all the mixture reading lights around the rim until I decellerate and settle in around 2500 rpms. I have BBT needles in these carbs. Using Mintylamb's chart, how do I select a needle set that will lean the mixture appropriately as the needles rise?

 

James

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first two (1,2) stations on needle are idle, next four stations (3,4,5,6) would govern pick-up in top gear from 20-50 mph also is part of the needle that meters the fuel for part throttle or cruising (ie. cruising @ 30mph will roughly be between #2 & #3 stations @ 50mph should be around #5 station, top sped or full throttle #7 to #9 stations). You have good a/f ratio @ idle then you accelerate (how? gradual? wide open?) to what rpm? slow back to 2500 rpm and have what for a/f ratio? You should see richening on acceleration on your gauge. Is this accompanied by stumbling, missing or etc? No pre-ignition, pinging? Timing advance reasonable? What dashpot springs do you have? What re you using for dashpot oil? What if any other engine modifications?

 

Brad

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You should be able to get the correct mix at idle with any needle using the mixture adjust nut on the bottom. Problem is (as you've noticed) it sets the whole range.

 

Could do with more reference points. What sort of readings are you getting at light throttle cruise (@ 2500 rpm say)?

 

Triumphtune book suggests BAM for standard engine with 6-3-1 exhaust and K & N filters or BAE if flowed head as well. Strangely enough these appear to be richer than the BBTs..... Do you know which dashpot springs are fitted?

 

Cheers

 

Nick

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first two (1,2) stations on needle are idle, next four stations (3,4,5,6) would govern pick-up in top gear from 20-50 mph also is part of the needle that meters the fuel for part throttle or cruising (ie. cruising @ 30mph will roughly be between #2 & #3 stations @ 50mph should be around #5 station, top sped or full throttle #7 to #9 stations). You have good a/f ratio @ idle then you accelerate (how? gradual? wide open?) to what rpm?to around 3300 rpm slow back to 2500 rpm and have what for a/f ratio?I then have A/F around 12.3 You should see richening on acceleration on your gauge.yes, but the car struggles to accelerate at all; it is sustained and only resolved by my gently decreasing pressure on the accelelerator. Is this accompanied by stumbling and A/F around 10, gauge ring of lights flashing- missing or etc?no No pre-ignition, pinging?no. Timing advance reasonable?yes What dashpot springs do you have?spoke to an SU carb specialist over here, (Joe Curto), who suggested that at this level of diagnostic dashpot springs are not that important, rather, the bias needle should be changed, to a righer one, like BAE, so that the needle jet can be raised from its present level to achieve idle. What re you using for dashpot oil? mix of tool oil and atf. What if any other engine modifications?mild street cam, accuspark ignition.

 

Brad

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You should be able to get the correct mix at idle with any needle using the mixture adjust nut on the bottom. Problem is (as you've noticed) it sets the whole range.

 

Could do with more reference points. What sort of readings are you getting at light throttle cruise (@ 2500 rpm say)?

 

Triumphtune book suggests BAM for standard engine with 6-3-1 exhaust and K & N filters or BAE if flowed head as well. Strangely enough these appear to be richer than the BBTs..... Do you know which dashpot springs are fitted?

 

Cheers

 

Nick

You should be able to get the correct mix at idle with any needle using the mixture adjust nut on the bottom. Problem is (as you've noticed) it sets the whole range.

 

Could do with more reference points. What sort of readings are you getting at light throttle cruise (@ 2500 rpm say)?2500 is the sweet spot. if I can get her into fifth and running strong at 2500, A/F runs about 14.1 . As I said in another post in this thread, the engine gains power and air readings go from super rich to moderately rich to nearly correct as I decellerate.

 

Triumphtune book suggests BAM for standard engine with 6-3-1 exhaust and K & N filters or BAE if flowed head as well. Strangely enough these appear to be richer than the BBTs..... Do you know which dashpot springs are fitted?

 

Cheers

 

Nick

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Looks like the manifold picks up wrong air at idle.

Indication is overlean mixture when engine is pushed at 3000rpm.

Make sure Brake booster is okay and all hoses are tight etc.

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Looks like the manifold picks up wrong air at idle.can you explain this please?

Indication is overlean mixture when engine is pushed at 3000rpm. at this point when accelerating, the mixture is super rich.

Make sure Brake booster is okay and all hoses are tight etc. no power assist on brakes.

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I am German, so maybe I use the wrong words.

A pushed engine is when gas pedal is not pushed

and you roll down a hill in gears and engine is reving 3000

 

"Wrong air" means that there is a damaged gasket or flange that lets

air come into the manifold between the carbs and engine.

 

That weakens the mixture at idle what is compensated by setting the needle jet down

and when engine runs faster manifold pressure rises and does

not take that amount of air like before and in addition takes

more mixture from the carbs, that still suffers from the rich setup of needle.

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<p>Go with Joe's recommendation. I have 70 GT6.  Engine .030 over, HS6's, BAE needles, yellow springs, 30 wt  in dampners, TT modified head,TT fast road 83 cam, 6-2-1 header. I don't have a/f gauge but personally I feel  I'm a little rich. Plugs don't look to bad but exhaust smells rich. I only have a C/R of 8.9 could be reason or maybe fuel standoff from carbs. You go for a drive or work on car while its running and you'll have an aroma of unburned hydrocarbons following you for the rest of the day. I'd be interested  in your a/f ratios after your needle change, be sure and post your outcome.</p>

<p> </p>

<p>Brad</p>

<div style="left: -1000px; top: 56.29px; width: 1px; height: 1px; overflow: hidden; position: absolute;"> </div>

 

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Spoke to Joe Curto a day or two ago, and he helped me iron out some carb issues. Unlike before, I can now maintain a steady, stable idle at around 1000 rpm, with HS6 carbs with velocity stacks and an airbox, a mild street cam, ported intake and stock exhaust. put in BAE needles and switched to ATF for dashpot oil, and the car will, as said, idle around 1000, and A/F reads around 13 to 14 at that speed, but as soon as I try to accelerate, the ratio enrichens dramatically and remains so until I am releasing pressure on the accelerator. On a 20 mile drive today, I stopped and leaned the mixture at the jets by turning them up about two flats before two full turns. this made the idle mixture read leaner, reading 17 consistently, but the adjustment meant that at road speeds and when accelerating, the mixture would remain around 11.3, instead of 10.2. With the mixture in this setting position, ten flats down, the car was prone to falter at idle, and would not maintain a low idle speed unless I was turned at least two full turns, usually 13 or 14 flats down. timing at 14 BTDC

 

When I drive the car, pushing down on the accelerator is out of the question; she will shudder and falter, ratio gauge flashing all its lights and reading 10 until I release pressure on the accelerator. Parked, I can increase rpms smoothly, until about 4K, then the shuddering and overly rich mixture occurs again.

 

The exhaust leaves you and your clothes smelling like, well, exhaust, so I know it is rich.

Any suggestions would be appreciated,

 

James

 

<p>Go with Joe's recommendation. I have 70 GT6. Engine .030 over, HS6's, BAE needles, yellow springs, 30 wt in dampners, TT modified head,TT fast road 83 cam, 6-2-1 header. I don't have a/f gauge but personally I feel I'm a little rich. Plugs don't look to bad but exhaust smells rich. I only have a C/R of 8.9 could be reason or maybe fuel standoff from carbs. You go for a drive or work on car while its running and you'll have an aroma of unburned hydrocarbons following you for the rest of the day. I'd be interested in your a/f ratios after your needle change, be sure and post your outcome.</p>

<p> </p>

<p>Brad</p>

<div style="left: -1000px; top: 56.29px; width: 1px; height: 1px; overflow: hidden; position: absolute;"> </div>

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Ok. What Triumph V8 said about manifold air-leaks may be relevant. I'm not sure whether you got his point before.

 

What he's saying is that if you have a smallish manifold air leak, at idle this can have quite a big effect on on the mixture as it represents a fairly high proportion of the total air flow. This causes you to need to set the mixture quite rich to compensate for the extra air. At larger throttle openings the leak becomes more or less insignificant because it is such a small proportion of total flow but you idle setting has made everything too rich. Your cruise setting might not be too badly affected as you'll not be using much throttle at that point.

 

If you are confident there is no air leak then:

 

Check float heights and needle valves. These more normally cause rich (and richening) mix problems at idle so I doubt this is your problem.

 

Check that the pistons will lift smoothly the way up. Pistons that stick part way will cause the mixture to go mad rich. You should check this with the dampers out and then in. More resistance will be felt with the dampers in, but they should still go all the way up. I once came across the situation where someone had used the shorter Dolomite Sprint dashpots with full length damper rods. This stopped the pistons going all the way up, so the engine basically drowned in fuel when he hoofed it....

 

What dashpot springs do you have? Yellow are 8oz and red are 4.5 oz. These are the commonly seen ones, though there are others. Stronger springs tend to make a particular needle run richer across the range by causing more vacuum in the carb by holding the piston down.

 

Note that the BAEs you are using now are richer than the BBTs you had before so running (even) richer is not unexpected.

 

You might find this book useful

http://books.google.co.uk/books?id=CTz2EAp2ua8C&pg=PA60&lpg=PA60&dq=SU+spring+ratings&source=bl&ots=gt6pibuCpO&sig=hlCU2scghFbOp8d065HHah7LlbU&hl=en&sa=X&ei=dIEXU5PWOumr4AS1j4DQDw&ved=0CE0Q6AEwBg#v=onepage&q=SU%20spring%20ratings&f=false

 

It explains things much better than I can!

 

Cheers

 

Nick

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thanks Nick,

 

I discovered I have long dahspot pistons instead of short ones. One short came with the carbs, but I went with the long ones.

I've changed to two short pistons.

I also found a vacuum line from the carbs to the dizzy that had a hole in it and replaced it.

I'll see how it runs tomorrow.

 

James

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switching from BBT to BCY needles has made a tremendous difference in performance. Many thanks to Mintylamb for his chart!!

It has taken my car from something that is nice to look at, to something I can drive safely on the highway.

 

Again many thanks for this resource and the assistance of folks on this site.

 

Cheers,

James

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Hi,

 

I hope you don't mind me joining your thread so late.

 

I'm using  HS6s which I've recently rebuilt and also have some mixture problems. I didn't know you could check the air to fuel ratio while driving the car. What are you using for this?

 

Thanks,

 

Chris

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You need a wide band lambda sensor (aka O2 sensor) with controller and either a dashmount display, a laptop for datalogging or probably can be done using bluetooth and smart phones these days.  I use an Innovate LC1 permanently installed and attached to my MS ECU.

 

http://www.innovatemotorsports.com/products.php

http://www.wbo2.com/

 

Nick

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You need a wide band lambda sensor (aka O2 sensor) with controller and either a dashmount display, a laptop for datalogging or probably can be done using bluetooth and smart phones these days.  I use an Innovate LC1 permanently installed and attached to my MS ECU.

 

http://www.innovatemotorsports.com/products.php

http://www.wbo2.com/

 

Nick

Thanks for your reply. This looks like a fair bit of work and expense but probably worth it. One to add to my wanted list.

 

Chris

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switching from BBT to BCY needles has made a tremendous difference in performance. Many thanks to Mintylamb for his chart!!

It has taken my car from something that is nice to look at, to something I can drive safely on the highway.

 

Again many thanks for this resource and the assistance of folks on this site.

 

Cheers,

James

Its been a while, but what a/f yout get at different load with your BCY needle?

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