This is a stock race bike put together last year. Ran fine intermittently, but consistent high rpm love was sporadic.

Stock exhaust with 135 main jets and D&D slip-on muffler.

Cleaned the carbs thoroughly...twice.
Spark plugs look fine.
Starts normally: full choke then gradually no choke after a few moments as it warms up. Idles fine, idle adjustment cable works also.

Everything's fine until you start twisting the throttle, then it starts breaking up and revving more erratic until it refuses to run past 5500-6000 rpm.

It acts exactly as if the timing is not automatically advancing as revs increase. The only thing I could tell affects advance is the ignition control module. Obviously the ICM is working well enough to deliver spark, but is it possible the ignition advance portion has gone bad while leaving the rest operable? Anyone experience something like this?

 

Loudo, let's talk about your air filter... stock headers with just a slip-on should be fine with OEM 135 mains, but if you have a hi-flow filter, like a K&N, BMC, UNI, etc., you may be getting too much air.

When you say you can't rev past 5500-6000 RPM, is this even while just sitting still, or only while riding, under load?

All the fueling circuits work together, with some overlap, but at this rev range, truthfully, I would be more concerned with pilot screw settings than mains, and more importantly, the jet needle settings. The truth is, these settings could be the cause of your problem, if they're way off in being either too lean, or too rich - either way will affect your ability to rev, and to do that smoothly.

Are your fuel screws set consistently, from one carb to the next?
How many turns out are these screws set to?

Are the needles adjustable, with various clip positions?
How many shims are present on the needles, if any?

And of course, as inquired earlier in my post, what air filter is in use?

 

Figured out my problem…wire dangling from the toggle switch I’d installed.

Evidently it was separated just enough to start and run fine and then either couldn’t handle the higher current draw upper revs demanded or the vibration at about 6k rpm caused the wire to move further away from the switch terminal. I would think such arcing would have damaged the switch but apparently not.

If the wire had moved further away, the bike would have been electrically dead as a doornail and I would have scoured the wiring starting with the switch. It’s a bit embarrassing to admit something so simple could have escaped notice.

Thinking that the unknown quantity of aftermarket D&D muffler paired with the stock jetting might have caused the problem, I had installed 145 main jets. I will sort out the jetting issue, but right now it revs to the moon no problem, no flat spots, very nice.

And to think this whole thing started simply because I wanted to hook up my Morgan CarbTune and check the carb balance.

Regarding your earlier questions (though most likely moot now) the bike has a K&N air filter and the fuel screws were set consistently.

Thanks very much for the reply, much appreciated. I’m sure I’ll be back with more questions.

Cheers,

Neil