Rich Cylinder Headscratcher

Rebuilt the engine on my '82 650 Maxim a year ago and got it running again, and have been messing with the tune for a while. I'm running low on ideas, so here's where I'm at - tell me what I'm forgetting:

The bike starts and runs fine, smooth idle and okay acceleration. But #4 cylinder is completely rich, and always ends up fouling the spark plug.

Turning the pilot mixture screw all the way in (bottomed out ) makes it run slightly better, but still turns the plug black and sooty.

Here's what's been done:
-Engine torn down and rebuilt to fix primary chain guide. Reassembled by the haynes manual - new gaskets everywhere.
-All intake boots and connections are solid, no leaks.
-valves checked and double checked.
-carbs cleaned and bench synced
-Float height on all carbs wet set and double checked. no "slow fuel leaks"
-vac synced w. YICS tool and color tuned a few times.
-spark plugs replaced and gapped. plug wire swapped out. spark verified.

Any ideas appreciated - It runs okay, but I'd like to get it running the way it should.

 

Have you checked and tested the enrichment plunger on that carb? It seems that it is not cutting off for some reason.

 

Verified the air and fuel jets were installed in the correct spot? Swapping either on #4 could cause problems. Slides pass the "clunk" test?

Compression on #4 (and across the board)?. I've never torn down the engine, but I imagine that you'd lap the valves and check the rings while you were in there.

 

Yep, I'd check all the enrichment plungers open at the same time, and that #4 isn't hung open. Als, if you got the air jets backwards it would run stupid rich at idle.

(big air jet is for idle, small is for main. It's backwards from the fuel jets)

 

I'd suggest the same thing. Check the compression to see if a faulty valve seal could be the issue.

I just dealt with a rich problem on ALL my plugs, but that turned out to be how all my floats were set (improperly!)

 

I had a stupid filthy rich problem.... my pilot and main fuel jets were reversed....

check your jets, your float hight, + see if your float is working properly. After that check your coil + plug

 

Thanks a lot for the advice on what to look at. My thought was if any of the jets were swapped, it would run very poorly or not at all. I'm going to get back into that carb this weekend and check everything over again - hopefully it's something obvious. Will report back!

 

Try lapping your enrichment plungers that might help.

 

These bikes can run (somewhat) fine on 3 cyl.

And from my personal experiences, check the stupid simple stuff too... that's usually where i went wrong....

Oh... when your bike is running, is the #4 pipe hot too?

 

Question: exactly what was done when the head was off? Lapped valves? New valve stem seals?

(And this was previously asked compression test results?

 

Did check that - #4 does fire, but it's slow to catch and warm up.

That got me thinking... pulled the carbs and got into #4 last night. All fuel and air jets were correctly installed, but this carb does have a lot of surface corrosion. I used some lapping compound and polished that enrichment plunger seat, then also took some wet/dry and polished the fuel jet pillar seats a bit. I'll re-sync today and see if there's any difference.

Valves were lapped + new stem seals during the rebuild. Timing should be correct as well, but that would be another obvious thing to re-verify.

A compression test didn't seem important after the rebuild, but I suppose it would be a quick way to catch something done wrong during that process... I'll add that to my list as well.