low rpm stumble

[Robert Haller]

My bike has just started to sputter and stumble when I accelerate from idle or low rpm. It is fine above about 4k rpm and idles fine. Any suggestions what to check (starting with the simplest causes ). I just filled it with gas, would bad gas or clogged filter yield those symptoms?

[dragxs11]

I'm having a similar problem now and I've been in the process of jet changes and carb calibration for awhile. What we both need to do is get our bikes to someone who can test the hydrocarbons at idle and cranked up. Right now I'm a little lean at idle, then the mixture gets rich when the butterfly opens. My theory is that the heavier mixture changes the weight of the air and causes a momentary stall. You can play with your pilot needles on the top to change the idle mixture and see which way helps. For me, backing them out was helpful(richer). Where they should be set varies for a couple of reasons. Elevation, and aftermarket parts primarily. Just keep track of the # of turns in adjusting. The smartest thing for both of us I think is, as I said, to have them matched up by someone with the right equipment. That way, theres no guessing.

[dragxs11]

Sorry Robert,
I just read your thread a little closer. If this all of a sudden just started it could be a lot of things. I don't think anyone can just tell you whats wrong. But I have a couple of ideas.
Could be bad gas, I guess.
Try new gas, thats easy enough.
Clogged filters? I don't think so.
Poor spark, due to plug fatigue or foul
Anything that could change mixture
Dirt sucked into carb pilot circuits causing a mixture change
Sucking air through cracked carb boots, etc.

[Ken Talbot]

Just before you filled up with gas, did you happen to run low enough to switch the petcocks to reserve? If so, this might have dislodged a wee bit of spooge (dirt, perished rubber, etc) that might have been sitting undisturbed for quite some time in the petcock body(ies) or one or more of the reserve flow fuel lines.

[skids]

One of two things could be the culpret, I think. If it is still a stock arrangement for the carbs (filter, jetting, pipes), and the pipes are not shot, I would look into the pilot circuit:

1) It seems that the holes beneath the pilot screws are OK if she holds an idle. The pilot jets and/or the three pinholes in the top of the ventouri bore (near the throttle plate's resting place) are plugged. Too lean at midrange to ignite

2) The air jet provides aereated fuel by "fizzing" the fuel that comes up the pilot jets. If it is plugged, the vacuum would suck fuel faster than it should, causing an overly lean condition causing the engine to stumble at a gradual throttle twist.

Of course, there is the issuue of the pickup coil wires on these bikes. To test, you can disable the movement of the pickup coil plate by unplugging the vacuum hose to the No. 2 carb (the one that goes to the ignition cover) and put a short hose on the carb's nipple and kink/crimp it over to prevent air from entering that carb. If there is a dead spot in the pick-up wires, you will know, because your bike will act better.

[skids]

Ugh! I meant to say "overly rich condition"

Originally posted by skids

2) The air jet provides aereated fuel by "fizzing" the fuel that comes up the pilot jets. If it is plugged, the vacuum would suck fuel faster than it should, causing an overly lean condition causing the engine to stumble at a gradual throttle twist.