cruising in 2nd gear holding throttle study and

[Downeaster]

jd750ace, that is a very informative and to-the-point post. I've copied it and will add it to my carb files.

I've been dealing with a left-cylinder issue since I got my '79 back on the road and intend to make it go away this winter. Only at idle and just enough to be annoying so I've spent the summer riding, not tweaking. At ANYTHING above a dead idle, it runs like a raped ape, but the left cylinder cuts in and out at idle, especially when cold.

 

[gggGary]

DE have you gone Pamco? I did my first pamco this summer and was blown away at the improvement in running. Easily the best improvement to a points bike you can do. Always hard to decide where the problem lies but removing one variable is never a bad thing.

 

[Downeaster]

DE have you gone Pamco? I did my first pamco this summer and was blown away at the improvement in running. Easily the best improvement to a points bike you can do. Always hard to decide where the problem lies but removing one variable is never a bad thing.

Yup on the PAMCO (see my signature line...), and yer right, HUGE improvement. Mid-range and top end is MUCH cleaner.

I'm guessing I either have a very small air leak, sync issues, a partially plugged jet or some combination of all of them.

As soon as it's too cold to ride, the engine is coming back out for a head retorque and a teardown on that left carb.

 

[jd750ace]

With the high vacuum at idle, when the butterflies are nearly closed, the smaller leaks, such as shaft seals, or the possibility of the air bleed hole being obstructed on your left cylinder, are made all the more critical to balancing the mixture. Once the plate opens, the path of least resistance is past the plate in the throat of the carb, the vaccum decreases, and the air velocity increases through the throat, and siphoning begins shortly from the pilot jet, and it starts behaving better, as the air supply has improved. It would be helpful to know if the left cylinder is rich or lean at idle with the throttle closed. It's going to stumble on that cylinder, too rich or too lean. You may be able to use your God-given exhaust gas analyzer (nose to the exhaust) and see if it smells rich or puffs black. If it does, I'd say the air bleed passage would be suspect. If not, then the mixture screw's pickup path would be suspect. I wouldn't think the pilot jet would be the issue, since it goes great with the throttle cracked open. With the PAMCO, I would think that any possibility of an electrical problem would be minimized, unless somehow the timing can be off for the one cylinder.
No expert, of course, but just my thoughts.