Lugging at low RPM -- lean, plugged pilot?

[buggrin]

The last time this happened a complete carb cleaning fixed. Now again it misses, and sputters when trying to roll up from 2K. The sputtering stops above 3500RPM. The missing started after a hard few miles of showing off. If I pull the choke one notch the problem goes away. This time a carb cleaning didn't make a difference. Richening up the bottom doesn't seem to help.

The 78 XS11 is completely original (except wear items & Motion Trends fairing & TKAT fork brace, brakes, etc) and has served me well for 32+ years & 70K miles. With the stock carbs/jets it averages 45+MPG and still pulls like a mule.

Along with the missing, the dreaded backfire & popping started when using the engine for a brake. I understand this indicates lean.

Because the problem started after some hard running at high RPMs, and goes away with partial choke, I'm guessing that some crap shook loose and plugged an orifice. The first cleaning apparently missed it.

Does anyone have experience with this or any suggestions as to what was missed?

After being "married" for this long I've gotten used to the old gal.

[motoman]

I'd check cam chain slack adjuster after rotating to center, if it is the original adjuster........sound like an out of place place to check?........don't ask how I know.

[TADracer]

Yes, check the tensioner and also check the carb boots where they meet the head. You could have a vacuum leak on one or more of the boots right there and the boots are subject to engine vibration. Will cause a lean mixture at low RPM, while on the pilot circuit, that goes away when the throttle is opened up a bit. It goes away when you open the choke because you are richening the mixture. This will also cause poor idling conditions that get worse as your motor heats up.

[buggrin]

I'd check cam chain slack adjuster after rotating to center, if it is the original adjuster........sound like an out of place place to check?........don't ask how I know.

OK, I won't ask. But am sure curious, Been so long since it was checked I'll need to get out the well worn shop manual.

[buggrin]

Yes, check the tensioner and also check the carb boots where they meet the head. You could have a vacuum leak on one or more of the boots right there and the boots are subject to engine vibration. Will cause a lean mixture at low RPM, while on the pilot circuit, that goes away when the throttle is opened up a bit. It goes away when you open the choke because you are richening the mixture. This will also cause poor idling conditions that get worse as your motor heats up.

Makes sense. An yes, the idling does leave something to be desired. I'll check using the method described in the notes.