Weber IDF problem??

I have dual 40IDFs. Current issue is: whilst idling, one cylinder thoat can be blocked off at the air horn with no effect on the idle speed of the engine. The idle mixture screw for that cylinder also has no effect. The idle speed is steady however. Also RAW, and I mean RAW fuel is coming out of the exhaust pipe - enough to wet the ground where the exhaust points to! Spark is present at that cylinder. I have replaced the following:points,plugs,dizzy cap,leads,coil and carbs have been professionally soaked, cleaned and blown out. Any ideas? Help!!!!

Have you done a compression test on that cylinder?

Yes. 137 psi. Other cylinders were 132 , 138, and 142. Engine has 8.0 to 1 compression and is ~55000km old

What happens when you pull the pair lead for that plug??

ie IF it is #1 that you think has the problem and you pull the lead to #2 does the motor die??

What happens when you rev it??? Does it come alive???

When you pull THAT plug is it wet??

Did it run better before??? or is this a new installation???

Carbs were soaked but were the jets???

I'll try that pair trick and see what happens and report back. Bit late now unfortunately...may upset the neighbours. These carbs have been on this engine for about 12 years now actually. I had a backfire up a carb (just one of those occassional Weber pops you get) which set fire to the air cleaner (that was fun!) and its all gone pear shaped since then. The carb that caught fire was on 1 & 2 cylinder bank. I am having trouble with #4 throat. They have been totally cleaned out to make sure nothing was stuck in them since that "fateful incident". It runs pretty good over about 2500rpm which suggests to me something in the idle circuit. Thanks for the feedback so far.

graeme86 wrote:It runs pretty good over about 2500rpm which suggests to me something in the idle circuit.

HAS to be idle circuit.

Just a matter if jet or passage.

Just a follow up to this...FYI I think I found the problem...the coiled up return spring on the throttle plate appears to have been jammed between the manifold and the carby. It just sits at a slight angle and is thicker than the distance from the throttle shaft to the base of the carby..so if it is not held straight up to clear the manifold, it holds the carby slightly off the manifold when tightened up...causing the massive air leak. Seems to run quite nicely now!