One cylinder way too rich at idle?

I just checked compression and valve clearances in the last week. Compression with engine hot was #1 - 160psi #2 - 145 #3 - 145 #4 - 160. Valve clearance with engine cold on #3 was .004 on intake and exhaust valves and the other valves were all in spec. I haven't tried NEW plugs, but I have switched plugs with different cylinders with no improvement. Spark looks the same to me on #3 and #4 cylinders. It's a blue spark, but I don't know if I would call it fat. I do have the relay coil mod installed so it is getting good voltage to the coils. They are Emgo coils that are a few years old with NGK resistor ends.

I still think it has to be something fuel related because the bike runs like crap at low speeds with the #3 idle screw set out a turn and a half, but runs normal with the screw closed. I just can't think of what the problem could possibly be. The pilot screw ends looked fine when I pulled them all out. I'll have to double check the passageways for the pilot system when I take the carbs off again. I wish the engineers had designed an easier way to pull them off for servicing!

i put together a gm hei module set. it did ok, i took it off because i need to find an actual connector so i can plug it in without the wires falling out of the holes i could just cut the module and crimp it but what if something went wrong and i wanted to use the original one. overall it cost me about 60-75 to put it together.

If not already done, would assure all pilot jets are the same size (especially that #3 carb's pilot jet matches the pilot jets in the other carbs), and would also exchange #3 pilot jet with one of the other carb's pilot jet to see whether the problem moves to the other carb's cylinder.

Pilot jets are inexpensive, and while generally disliking the idea of throwing parts at a problem, it may be time to install a brand new pilot jet in #3 carb.

Given that the issue is fuel-related, and not compression-related and not ignition-related, some yet undiagnosed condition is allowing excessive fuel mixture into the carb bore, through the pilot port and/or through the mixture screw port.
Both of these bore ports receive fuel mixture that has supposedly been governed by the pilot jet.

There might be too much fuel or too little air arriving at the pilot jet, or a combination of those conditions, which could result in the reported fuel mixture richness symptoms, even with a perfect pilot jet and perfect everything else after the pilot jet.

Assure correct fuel volume to pilot jet, includes clear tube test verifying float bowl fuel level.
Assure correct air volume to pilot jet, includes undamaged intake jet and clear air passages.
If okay, re-address the pilot jet itself, and re-address flow of fuel mixture from pilot jet into the carb ports.

Good Fortune!

A bench sync with using a drill bit as a feeler gauge is good for the initial start up. But you need to go back and check the sync when running. Their are some good home made manometers in "File Base", maybe try that. I've got the yard stick and tubing one with ATF in it, and it is very sensitive for finding which cylinder is OFF.
Also as Patton said, check those Pilot jets closely. Make sure they are all the same, that non have been drilled out or are different types. Bleed/ Non-bleed etc.

Lots of good info here to get your problem sorted. My $.02 would be to second checking the fuel level with a clear tube. Its pretty easy to accidentally bend the float tang a bit or install it upside down which could cause your problem. 2nd, a vacuum sync might smooth things out a bunch, but if one cylinder is that much colder it is unlikely that it will fix that issue, DEFINITELY give it a vacuum sync once you get all cylinders running decently though. 3rd, Check that choke circuit, it gets clogged easily and you will essentially be running on 3 cylinders until the carb/cylinder gets warmed up enough by the other cylinders to start firing consistently.

When you get it out on the highway and really get on it for a few minutes do things improve? Does it take a really long time to warm up to where it will pull from a stop? Does one header remain warm after the others have started getting HOT on startup? Those are the symptoms of a clogged choke circuit in my experience.

I noticed on mine that the pilot screws are eroded pretty bad, just a thought.

Fixed the problem tonight. It wound up being a plugged up air jet for the pilot system. These are bad pictures, but you can see what happened.

#3 carb, plugged up.


#4 carb, which is clear.


It almost looked like a brass ball or something had it plugged up, like they would at the factory. It took about 20 psi to blow it out, but I never saw what came out so I really have no clue what caused it, or how long it has been there.

Thanks for all the suggestions, as I probably wouldn't have ever checked those air inlet jets. I'll pay more attention to them from now on!

Glad you got it solved!

ok good because i was gonna say dont let anyone tell you that its a muffler bearing, because that is bad news