Vintage Dirt Bike Q & A

Dr. Robert Hawes...
You have been refered to me by the fine gentlemen at Bannedcr500riders.com as "THE" guy to go to for Elsinore help. But anyone can certainly offer up some ides here. PLEASE!

So here's my condition:

The 1980 Cr125 has a CDI ignition and it feels like the plug isn't firing consistantly at high revs. Sometimes it'll rip like nobody's business...other times it's 4-stroking (almost like a rev limiter is cutting in)....and falling on it's face...only to pick back up again very strong even without changing the throttle position. It will pull to max revs, but not consistantly.
I'm 99% sure it's NOT a carb issue (so it probably is) because there is no consistancy to the RPM the problem occurs at. The bike is an old 125 so it's pinned and screaming most of the time and it's crapping out at the higher rpm's. I've fiddled with jetting with absolutely no changes in performance at all...none...
The plug looks too clean (read SPOTLESS..yes, I know that's bad) but she smokes like a canary in a coal mine at 40:1. I might have a crank seal leak, but that's not confirmed as of yet.
What pops into your minds on this one??
Mahalo,
"Skiddy"

get the crank seal checked if that isn't it then here's what i would look at. make sure that ALL the grounds are good and clean this means engine to frame allso. check your exhaust system make sure the silencer is clear, check the coil wire allso. now check the float level and make sure fuel is flowing from the tank to the carb steadly. check reeds allso. my bet is a intermitent bad ground . or other elect componet

You might want to try closing the plug gap a bit, even down to .020" and see if that helps.
I have seen an excessive plug gap cause what your describing, especially with a borderline rich condition.