Suddenly running lean

[PER]

Cheers.

My car has suddenly started to run lean and stumble/missfire as soon as boost comes on. I know its lean because of high readings on my LC-1 lambda...

I have changed the fuel pump, fuel filter and checked the fuel pressure. All seems fine.

Any suggestions?

Could a sticky float valve cause this?

It seems like the carbs don't "read" that boost is coming and that they don't add the extra amount of fuel needed to compensate for the extra amount of air...

If anybody has any suggestions, please feel free to enlighten me on this subject...

Edited August 11, 2008 by PER

[910Esprit]

Per

Have you checked fuel pressure, on the road, under boost conditions? i.e. does the fuel pressure remain circa 4.5 psi above the boost gauge reading?

I dont think there is anyhing special about the carbs during 'boost', they are still acting upon the air passing through the venturi. (except for the fact that the fuel pressure is increased to compensate for the + pressure) I.e If there was a mechanical problem with the carbs I would also expect it to manifest itself in off-boost conditions.

Having said that, it would be certainly worth removing the top cover to have a look and to blow through the main jets.

Does your spark plug colour back-up the suggestion that the car is running lean? In the past I have had very similar problem caused by faulty spark plugs. (albeit on a different car).

cheers Steve

[USAndretti42]

I'm not sure how the turbo is set up but what do the carburetor float chambers vent to? They need to be vented to an area which sees boost. If there is a leak in the circuit the float chamber pressure will be low when you are on boost which would reduce the pressure difference across the jets. This, in turn, will reduce the fuel flow leaning out the mixture.

[910Esprit]

The carbs are sealed Dellortos and are vented to the plenum. In turn the variable fuel pressure regulator is 'vented' to the plenum, so fuel pressure and air pressure are automatically kept in equilibrium. I reckon an (air) leak would would affect both circuits equally and keep all in balance? I think a test of fuel pressure is showing circa 4.5psi above indicated boost 'proves' all is well with that circuit.

cheers Steve

[PER]

The fuelpressure is ok when idling, but I haven't had a chance to read the pressure while driving yet. I will need someone to read the pressure gauge while i'm keeping my eyes on the road :-) New sparkplugs are going in tomorrow to see if that helps.. I will also have to check the float valve and fuel level in the float chambers..

Thanks for the input so far guys..

[s2_esprit]

When I had this problem it turned out to be a vacuum (it also reads boost too) line under the intake plenum that was cracked. The fuel pressure is always regulated to 4.5 psi over boost, so if vacuum line is leaking it can not regulate properly. If the system if hooked up properly this line is also tied into to the boost gauge. When I had this issue I was only reading 4 psi boost.

Cheers Jim