Rm400 Bomb

[Flat Track Pete]

Hey guys I know I'm a little out of place being a flat track racer but, Anyone want to tell me why a 78 RM400C that was running a little rich would blow the top off the piston when it's wound up tight. I've done that to two of my motors so far. The only thing I can think of is that I used my 79 400N ignition because the 78 ignition went bad. Do you think it might just be spark knock ( as Suzuki is so clear on how to set/check ignition, just line up the marks, RIGHT ). I'm tired of putting $600 in this motor every other half-mile race. If anyone could lend any info. you'd make this riders day. Thanks Pete

 

[Rich Rohrich]

Pete - If you can, post a clear picture of the piston and the plug. They should give some clues as too the root cause, but it's likely that the high speed ignition retard in the new ignition isn't matched to the pipe characteristics on the 78 engine. It's also possible that the plug is running too hot and it's keeping the ignition retard from functioning. If that's the case the plug may run too hot and act sort of like a glow plug firing on it's own regardless of when the spark is timed to fire.

The look of the piston and the plug will tell the story.

 

[Flat Track Pete]

There was nothing left of the plug. The piston has a scar from the plug to the exhaust port, must of it ending up in the pipe. I run a special made Gary Nixon 79'rm400N pipe. It's supposed to be made for high revs (it's about 5' from inlet to stinger with about a 24" mid section thats 5" diameter). The rest looks rich. I run 93 octane and klotz's verson of Castol R, a bean oil mixed with sythetic. Sorry pictures aren't avalible at this time.