Misfiring/lean on boost.

[Brigsy]

Ok ive been trying to fix a mates gtt today. Standard engine/boost.

Basically the problem is lean/mis-firing on boost. Approx 4.5-5krpm in 3/4/5 gears the afr's will lean right off & it misfires.

I have changed a load of bits today, the car is running 100% better but not right by any means.

Ign components have been changed, i.e plugs/cap/leads/tdc sensor for new.

I have checked the fuel pressure, 3-4psi on idle & 12psi on boost (4psi above 8 at carb top) The fuel pump/filter are brand new, Known working fpr fitted. I have bypassed the pump relay with no joy. The fuel pressure holds 12psi when misfiring.

Reference pipe to fpr is OK have pressured up & no leaks.

I have checked the jets in the carb, all cleaned out, car runs mint off boost/low load situations.

One way valves on inlet are fine, well one was kaputt but have changed now.

Copuld the inlet gasket be fecked? causing air leak/lean on boost? A couple of the nuts are missing?

[Andrew Cooke]

little filter on the carb inlet?

[Spooky]

Ive had a few nuts missing on the inlet and its not made a significant impact but its worth making sure they are all there :)

Changed the aei? Maybe thats going kaput, could be losing voltage with increased revs?

[Brigsy]

Took the pre carb filter out & made no difference :crap:

Havent changed the aei yet as i havent got a decent spare, needs to be changed though.

I have changed the fpr today, one way valve & carb gaskets. All have made a significant difference to general running but no cigar.

[Andrew Cooke]

have you pulled the emulsion tube out?

[Brigsy]

Not yet as i ran out of time, i have given it a decent spray of carb cleaner with the air corrector out though.

Also the jets in carb are 130main & 110 air corrector 120 2nd stg, so should be rich at all times really but it struggles to hold 11.8-12.1 before it goes super lean at higher revs.

[Andrew Cooke]

all the enrichment fuel has to go through the emulsion tube, so it's worth a look in there. It might just be full of swarf from drilling the 2nd stage :(

[Spooky]

1.2mm 2nd stage? hmm...

i ran mine with an oe cam at 1.35mm lol

[Brigsy]

Cheers Andy, i have instructed my mate to give it a clean. I did find a small amount of crystallised petrol in the main jet so theres a possibility there is some crap floating about. It makes sense really as i have decent fuel pressure & i assume flow at the needle.

Its strange really as the car ran fine for at least a year, then one day it decided it was going to mis-fire.

The carb was jetted for a considerable amount of boost at one point spooky;) as soon as its running a bit better im going to downsize the jets a bit.

[Spooky]

i was thinking at the time surely i wont need to go further than 1.2/1.3mm but the car was spot on for me :)

like you say, its weird how its fine one min and changes for whatever reason the next min :cry:

[Scoff]

does it go lean then misfire, or misfire then lean ? the misfire can only be atributed to the fueling if it's seen to go lean before the misfire occurs. wideband will always show lean during a misfire so don't let that throw you off the scent :)

[Andrew Cooke]

does it go lean then misfire, or misfire then lean ? the misfire can only be atributed to the fueling if it's seen to go lean before the misfire occurs. wideband will always show lean during a misfire so don't let that throw you off the scent :)

true, and if you throw enough fuel at it to put the fire out the lambda sensor will also show lean, even though it's swimming in fuel.