Surging, bucking, stumbling -- please HELP!

XJers:

My 400 Seca that I use for daily commuting developed an issue a couple of weeks ago that is annoying and dangerous.

At the start of my commute, everything works fine. Crisp acceleration, smooth running, etc.

Toward the end of my commute, after about a 35-mile high-speed internet jaunt, I usually have a mile or two of stop and go traffic and then a couple of miles of city streets.

In these low speed sections, my bike has started stumbling and surging between 3000-4500 rpm. If I try to hold the throttle steady in this range, the bike surges and bucks.

If I give it throttle, it just kind of falls on its face and nothing will happen for a couple of seconds, then it picks up and takes off.

At high-speed/high rpm everything is fine.

I initially thought the throttle slides must be sticking, but I polished the slides and bores, and they work slick as toad snot, and when I chop the throttle I have all kinds of engine braking and that nice moan that signals they are slamming shut.

I also changed my in-line filter to make sure there weren't fuel delivery issues.

The carbs are spotless, and I blew out all the circuits and cleaned the jets this weekend, but the problem is still there.

Is the 3000-4000 rpm range where a transition between circuits happens? Why would it only happen at the end of my run -- heat-related?

Anybody have any ideas I can try? I'm not real comfortable in traffic, knowing that when I twist the throttle, nothing may happen.

Please help!
Herb

 

I know that for me the float settings are pretty critical, if you have a lower fuel level in the basin in one carb it can starve the fuel supply and the one cylinder can splutter Do the the float level test - you can just about do it with clear tube with the carbs on the bike, but better to pull them off and do on the bench, and then you can do a quick bench sync at the same time. I bench sync by snipping off a bristle from a wire brush and then slide under the butterfly and tug with tweezers, feeler guage is not sensitive enough, get number 3 set so that the butterfly is just about holding the bristle and then move through 1 2 and 4 the same way. Its not as good as the vacuum test, but you would need a YICS tool for that, but made a big difference in idling and running for me.

 

Being a 400 you only have 2 cylinders right? A sync of the carbs may the just the ticket it needs. Should be pretty easy with only 2 carbs. Also checking the fuel level in the bowls isn't a bad idea either. As a test on the heat thing you may want to wrap the bottoms of the carbs with some tin foil and see if the heat from the engine is causing the fuel in the carbs to cause a vapor lock in the carbs. The tin foil should insulate some of the heat from getting to the carb bowls. Stop and go traffic is not kind on the heat a bike put out.
3 to 4000 rpm is probably where the main jets start to kick in.

 

Right, it's a twin.

I set the float levels and synched it a couple of months back. I'll check them again, but they should be OK.

Thanks,
Herb