Dual IDF Idle problem

[TifosiTimo]

Hello. Forgive me for being a Fiat owner, but thought there may be some kindred spirit here to help with weber IDFs. The application is a 2 liter Fiat twin cam. Having idle issue with #4 cylinder. The manifold has no plenum so each carb barrel individually feeds one cylinder.
Car starts and runs well, though #4 seems to be rich at idle- runs best with mixture screw all the way in.
Have done a leak down test and having nothing wrong to report there.
Have checked that the idle jet is the same size.
Removed mixture screw and air bypass screw and cleaned them and their passages.
Tried adjusting bypass screw.
Float level is in spec.
Anybody have any idea what could be wrong?
/Tim

[John Rogers]

I would check several things. First I found over the years it has never been a good idea to just pull the jets and blow out the passages as you did since the dirt or gas sludge can easily get pushed further into the car internals. I would recommend a complete disassembly and hot tank clean and then give adjustments a try. There are a couple of things that can cause one cylinder to be rich or appear rich and it could be a damaged idle adjustment seat either from wear or a crack. There could also be a crack or pin hole letting gas leak into that cylinder and those can only be see with the carb apart. Finally I would check the voltage at the spark plug to make sure it is the same as the others as if the the voltage is very low then the plug is not burning all the mixture and the plug will look rich.