Working on a 300EX with elusive problem:

Stumbles/misses consistently starting at @3/4 throttle - well below rev limiter range, until it gets very warm, usually about an hour of hard riding then suddenly revs fine. Compression is good - all mechanical, fuel and ignition systems have been examined and it runs smooth and powerful up til cutting out at upper-mid RPM.
Aftermarket pipe and Jetting up 2 or down 1, changing throtttle slide clip position,
float level, airfilter-airbox cover on/off makes no improvement but of course makes run worse
at extreme settings.
Obviously there are lots of possibilities such as weak valve train components, a simple ignition / electrical problem etc… , but after extensive examination and the fact it clears up AFTER GETTING HOT, I am wondering if the CDI or other component could have a thermal related problem and is shifting the rev limit.
Useful suggestions not anecdotes are appreciated especially if anyone had similar symptoms and identified the problem.
TIA
Badapple

Currently Operable fun rides:
300EX
KX 80
200SX
PUCH Moped
71 SL 70 Motosport
Generic Minibike
Craftsman ATV (mower conversion)
Kawasaki 440 Intruder (Sled)
'50 Chevy Deluxe
'50 Ferguson TO20
Grasshopper 721

 

Hmmm i'd check all your wire connections. You say it runs fine after an hour or so of hard riding, maybe all the jostling around could somehow make the connection again or something. Just a guess.

 

Sounds like a fouling spark plug. Its rich at low end and leans out a bit on the top. It clears up when hot because the combustion finally gets hot enough to burn off the carbon. Check the plug and jet accordingly. thats the only way to get rid of it unless you have a bad ignition.

 

If I were you, I would try going up a couple more sizes on the main jet...Could be slightly lean.

 

You know you have to love all these guys that have been coming here for years asking for help and when they are given advise they don’t have the decency to give us an answer of if it worked or not or what did end up working. How about sharing a little???? I swear I read more dead post with no resolutions than anything else when searching on here. [img]i/expressions/face-icon-small-sad.gif[/img]