Weak idle until hot

[Grimly]

Any bike without a hand shifter and manual spark advance is for wimps.

Need a foot clutch, too.

[DimitriT]

Happened again this morning and she cut out twice in traffic. Ran fine a few minutes later when warmed up. I'm wondering if it's a float level problem. Could a needle valve get stuck closed and starve a carb for fuel until it warms up enough to get unstuck? It really feels like it's firing on three until it warms up full.

The odd thing is it's idling fine off choke when it's cold. It's a weak idle only after running for 5 min

[chuck hahn]

Stop fishing. Time to go back to square one and verify everything. Start with cam timing. Valve clearances. Ting advance working right and not stuck and no freeplay in the counterweight springs. Then dynamic timing. Carb sync with guages. Mixture screws set right. Are plugs sooty or ash grey... .. everything matters. To me it sounds that when its hot it leans out for some reason. Are the pilot and main jets the right size according to the manual.....

[chuck hahn]

Service manual here in case you dont have one.

BikeCliff's Website

[DimitriT]

Stop fishing. Time to go back to square one and verify everything. Start with cam timing. Valve clearances. Ting advance working right and not stuck and no freeplay in the counterweight springs. Then dynamic timing. Carb sync with guages. Mixture screws set right. Are plugs sooty or ash grey... .. everything matters. To me it sounds that when its hot it leans out for some reason. Are the pilot and main jets the right size according to the manual.....

Appreciate the advice. The leaning out is what I'm thinking as well. Maybe it's a case where some cylinders are rich on idle and some are lean.

[chuck hahn]

I would look up what stock jets are per tge service manual. IFF you dont have STOCK pipes and airbox go up one size on jets. Also i asked what the plugs look like. Sooty is rich and TOO RICH makes it run like crap because it can burn alll the gas. Same with ashy white plugs. too lean and not enough gas to produce power. You need to check it all and guessing isnt gonna cut it

[DimitriT]

Here's a pic of the plugs numbered 1 - 4 from left to right.

https://immortalsunited.com/apps/memories/s/XAE2RaTds34TrkP

After looking at these I adjusted idle richness on #4 so that it is just barely firing when cold. We'll see if that fixes it.

[chuck hahn]

1 a little rich
2 and 3 a tad lean
4 way too rich

[DimitriT]

So the weak idle seems related to fouling and temperature. I think it can be explained this way: Cylinder #4 is way rich so it can run a bit stronger when cold and, while #2 and #3 are lean and run poorly when cold. As the bike warms up #4 starts to misfire as it is too rich but the bike isn't warm enough for #2 and #3 to fully contribute to the idle. When the bike is fully warmed up #2 and #3 finally contribute but #4 misfires and fouls up with soot. So there's an intermediate temp which can be a problem. Also I was noticing a tendency for weak idle after decel in gear. I think a rich cylinder will foul more easily when stopping at the bottom of a hill because the throttle has been shut and there's less air for it to burn the extra fuel.

So I adjusted the idle richness on #4 (very small increments needed on this one) just to the point where it is firing when cold. Did the same on #1. I will run it like this for a while and see if the intermediate temp cut outs get better. When I have a little more time I will adjust #2 and #3 a little richer. #3 may have some other problem (float level) as it doesn't contribute well to the idle. That will need to wait until winter when I will pull the carbs for a full service.

I used to own a Gunson colortune which may have helped here but I sold it as it wasn't too accurate. Too bad there's no oxygen sensor bung or any way to measure anything. Probably not practical with four carbs.