Rich Idle

[PyroJim]

Hello all,
Sorry if this has been done before.....

I have tuned up my sprint carbs and the car is running sweetly, the problem is if I set the mixture on idle to the correct value (as indicated by the colour tune) then the mixture is too weak at higher loads (bad misfire). If I then enrich the mixture so that the car drives well (and still idles well too) then the mixture at idle is too high and this upsets the MOT guys. So at the moment I have to lean it up for the mot and then as soon as I leave make the mixture rich. I have done this for the last 3 MOT's and think that I should find the root cause. I was thinking that it could be worn needles but do not want to start buying bits on a whim. Any thoughts?

(The idle speed is about 700 - 800 rpm. When the mixture is rich at idle to produce a driveable car the mixture is correct for normal driving as the spark plugs are the correct colour - just rich at idle.)

Thanks in advance,
Jim

[SprintMWU773V]

It would be helpful to know which air filter you are using and also which needles? If for example you are running a pancake filter like a K&N it will lean out at the higher end unless you have slightly richer needles. Though the issue you've mentioned would suggest the carbs are not correctly set up as even with the wrong needles it should still run OK without misfiring.

If it's leaning off to the point of misfire then I would be thinking about air leaks which would have the effect of making the car undrivable under load. This happened to me once, it was set up perfectly at idle but useless in actual driving conditions. Air leaks may occur where the carb is mounted to the manifold, breather pipes, worn spindles or even where the manifold is mounted to the head. Consider spraying some easy start around possible areas with the engine running and listen for the tell tale rise in revs.

Sure someone will be along in a bit to post a link to a carb setup guide too. Best to rule out the obvious though before embarking on a fiddling session.

[Jon Tilson]

Anything much over 60k miles I would be looking at your jets wearing oval.

This would give you a rich mixture at idle with less effect higher up.

Where are you? Last year I ran an easter SU tune up offer....

Jonners

[PyroJim]

Cheers for that. I will go and check what needles are in it and I am running deep (about 3") K&Ns on it.

I thought that air leaks would make the idle lean if the mixture under load was normal as with the throttle closed it would pull too much air through the leaks? (or have I got that totally wrong)

[PyroJim]

Thanks Jon - I am in Cornwall

Is it more likely to be the jets wearing than the needles or should they always be replaced as sets anyway?
One more question - I think that mine are set correctly but does the float chamber level have a big affect on mixture?

Cheers
Jim

[SprintMWU773V]

Really setting anything at idle is a compromise and never really gives a good result. You're hardly pulling in any excess air at idle so it's easy to get an OK setting at idle, under load though it all goes to pot. It's amazing just how lean a car will run at idle yet show it some loud pedal and they can literally splutter to a halt.

New needles and jets are fairly cheap and easy to fit fortunately. I would certainly replace needles as well if the jets are oval. The float level will have a huge effect on mixture, or perhaps rather available fuel. If you've needed to accelerate hard then with a low fuel level in the chamber you'll basically empty it and therefore you'll suffer from fuel starvation. I had this problem once, I had a fuel line full of crap which restricted the flow. If you're level is too high then the pot will overflow and everything will stink of fuel. You can have a similar effect with sticking needle valves even if the float level is correct. Again needle valves and seats can wear over time and there is evidence that they degrade using more modern fuels.

When they work carbs are great but they can be a bit fiddly and you're not quite sure what's wrong.

[AndyJ]

With those K&N's richer needles would be needed so that needs checking. Air leaks between the throttle and the cylinder head will show most under load and what ever the cause the spray test will enable you to rule them in or out, if the rev's do rise then you have to start looking further.
Worn jets would increase fuel flow, but under load the inlet depression would be greater and increase fuel flow so could confuse the symptoms.
I'd check the vacuum advance on the distributor too.
Andy.

[Jon Tilson]

In answer to your questions I have found jets do wear but needles not so much.
The spring bias needle design which means you dont have to centralise the jet in a bearing as in older SU's
means you do get more jet wear.

The float level has a big effect on mixture and needs to be set right and the same on both float chambers.

It is only adjustable by putting washers under the float valve seat in the float chamber top. You should see the float
level in each jet with the piston and dashpot removed. It needs to be the same in each...and often isn't, especially
if the car doesnt have its original carbs.

As the guys have said air leaks are also critical and must be eliminated. Perished breather tubes are the one of the most frequent causes of air leaks.

Cornwall is sadly a bit far.....the offer applies to home counties only.

Jonners

[sprint95m]

I am running deep (about 3") K&Ns on it.

I would try a standard air filter and box first.




Ian.

[Dolly-Nut]

I've found SUs to run like crap without an airbox.
Also when setting with a colortune you should richen it and then lean a flat at a time until the yellow just goes. It's a great device for balancing the mixture between carbs but once the revs are up it's a guessing game.

[AlanH]

Can you get it on a dyno?

The problem with any kind of performance mod is that just about everything can get chucked out of kilter including needle profiles and dizzy setup etc.

[xvivalve]

Isn't this the very reason cars equipped with twin SU's can be tested at 2000 rpm rather than at idle? Dispensation in the testers handbook...

[sprint95m]

If it passes the CO test at idle speed but fails the HC test, then the tester should test the latter at 2000 rpm,
this irrespective of the type of carb or fuel injection but only on non CAT vehicles.....



http://www.motuk.co.uk/manual_730.htm
It is quite a way down, Exhaust Emission non CAT Test.





Ian.

[trackerjack]

When I ran SU's I always tweaked the mixture to lean and once MOT was passed I but the jets down a flat or so till the car ran right.
My MOT guy would laugh and joke that I would do this every year

[PyroJim]

Sorry for the delay in getting back but have only just done the checks on the car.

There are no air leaks so it seems that the issue is probably the jets/needles. Seeing as the poppet valves are weak I will do a complete rebuild. It was getting quite bad - when the car ran well the idle CO was between 10-12%!!

When I removed the carbs I noticed that one of the carb mounts has seen better days - no air leaks but it almost looks like the rubber is deteriorating on the inside. I will ask this in a separate post as well but does anyone have a good secondhand one that they would consider selling?
Cheers
Jim