Idle Stumbling and Stalling

[shanemoon]

I've been having idle issues with my 1981 XS11 that I'm hopefully someone here can help with.

For some background:
1. Converted to standard gravity fed petcocks.
2. Octy has been removed.
3. Vacuum ports are plugged.
4. 4-in-1 straight through exhaust with minimal baffling.
5. Pod filters. (the correct ones)
6. Jets are sized up from stock to #145 and #45

Recently on idle, it will randomly stumble and pop...typically accompanied by smoke from either the air filter or a big puff from the exhaust. Eventually it'll stall out.
Idle sits around 1100 RPM.

When I'm riding, it runs fine. And If I manually throttle up the idle to around 1600 RPM, it'll typically run pretty good.
But it's also worth noting that I've tried to just increase the curb idle screw and I'm currently bottomed all the way out on it.

What I've done:
1. Ran Seafoam through the fuel.
2. Adjusted mixture screws. (spark plugs are a good color)

Carbs got rebuilt and cleaned somewhat recently. About 100 miles ago.


Videos and pictures attached.

https://www.youtube.com/shorts/fdouuaaRzsY

[DEEBS11]

If it's backfiring through the carbs it could be timing (cam and/or advance unit) or valve lash. Or a major air leak/lean condition.

[DEEBS11]

Oh, and of course, a good sync.

[Radioguylogs]

It seems doubtful that it could get bad enough to cause those symptoms, but have you adjusted the cam chain tension?

It's low hanging fruit.

[DiverRay]

If the cam chain adjust does not fix it, I would look at the valve lash as a second option. When you replaced the jets, what jets/brand did you use? If you do not have all Mikuni jets, that IS your problem. I've had carb problems on every XS that did NOT use all original or Mikuni jets as the aftermarket jets are NOT sized correctly. I'm thinking your pilot jets are the wrong size, or there is something going on with the floats and the level is too high.

[shanemoon]

If the cam chain adjust does not fix it, I would look at the valve lash as a second option. When you replaced the jets, what jets/brand did you use? If you do not have all Mikuni jets, that IS your problem. I've had carb problems on every XS that did NOT use all original or Mikuni jets as the aftermarket jets are NOT sized correctly. I'm thinking your pilot jets are the wrong size, or there is something going on with the floats and the level is too high.

Thinking there might be something with the floats. When the bike sits for a while with the petcocks off, I get fuel spilling out of the filters when I turn them back on. Then if I bang on the side of the carb bowl with my fist, it stops. Like the floats are getting stuck for some reason.

[DiverRay]

Check the float gasket. Most of them will intrude into the float bowel, and they can cause the floats to stick open. If you have the metal floats, check and make SURE they don't have any holes. Pull and shake each float. If you hear something inside, there is a hole in it. They can be repaired, but it's easier to replace them with the later model floats.