I'd like to see a list of reasons you guys could come up with (not just carb related) that would cause occasional slight missing/stumbling at higher rpms (6 to 8000) on part throttle and wot. Any help appreciated.

 

1) corroded or loose spark plug cap, damaged plug wire, faulty/dirty/old spark plug
2) corroded or loose coil ground wire
3) corroded or loose ignition wiring connectors
4) dirty fuel screen
5) water in tank/float bowl

 

+1 for roadglider's list.
I don't know if a very dirty air filter would cause this condition as well or a vacuum leak?

 

Thank you. I have a starting point. I'd like to hear anything you feel may be carb related also.

 

What's your jetting?

 

Bike is jetted with DJ 128 main, DJ needle at 3rd notch, #38 pilot, fuel screw at 2-1/2 turns out. Snorkel has been removed and there's an HMF slip on with quiet core in it. Float is at 17mm like the book says. I've tried mains ranging from DJ 120 up to 132. Also messed with the needle in all positions with the different main jets. Tried the N1TC needle and the stock needle with washers. Checked the fuel screens. Tested the coil. New plug. Purged the float bowl and fuel tank. Idles and runs great when its not stumbling. Stumbling doesn't always happen.

Seems to me like it's not a carb problem with all that trial and error but I could be wrong.

Any other ideas? It's driving me crazy. There's a prize waiting for whoever nails it first. Haven't yet looked at all of roadglider's suggestions.

 

Might be a bad Throttle Position Sensor. I recall some posts about that but have no idea when that was. Do a search for TPS. You might get lucky and someone posted up an issue like yours and came up with a remedy.

 

Intermitant "stumbling" problems can be really difficult to find but they are usually related to either ignition or fuel delivery. It's hard to find because it's working most of the time. Some more things to check.
1)partially plugged gas tank vent/ kinked fuel line
2)partially plugged carb vent
3)the safety interlock switch stuff like the kick stand/clutch lever even the key switch or kill switch
I know that these types of problems can drive you crazy. You might try to get one of those neon in line spark plug caps. It plugs in line with your spark plug wire. If the problem is ignition related you can see the missing in the neon light (can be a little tricky when your riding). That way you can at least determine if the problem is indeed ignition related.Good luck

 

Have you checked your valve clearances? Perhaps if they are too tight they might cause the erratic behaviour? Just a thought.

 

Yep. Checked valves also. One exhaust valve was out so I shimmed it. All good now.

Had another thought. Timing. But how can I check it with a timing light without oil shooting out the little window?