Motorcycle Repair: Yamaha MX100 1980 something, piston cylinder, partial seizure

Question
I know that you said you like to deal with Honda's but i
read an answer that you submitted, and i seem to be having a
similar problem. I have a 1980-1990 yamaha mx 100. The bike
ran okay but i noticed a lot of top end noise, and thinking
it was time for a rebuild on this old 2 stroke, i ordered
the over size piston, and had the cylinder bored. Once
everything was assembled, I tried to start it. It started up
okay, but when i went back a few days later, it would not
start.

Here it is:

good compression, I'll check the exact numbers soon.

Good spark,

clean carb, i never messed with the adjustments even when it
was running so they must be correct.

i honestly hate 2 strokes, i think they are loud, they are
dangerous to themselves, needing oil injected or mixed, and
they definitely keep the bugs (as well as the neighbors)
away

I was wondering if maybe there could be a crack or a leak in
the power valve, or the cylinder its self and it was only
masked my the small amount of oil i used to lube the
cylinder upon installation? How can i check this?

I've read a lot of half coherent questions on here, and i
hope this isn't one of them, but thanks in advance


Answer
Taylor.. yeah, not my favorite subject......

Pull the exhaust pipe and head to see if there was any piston/cylinder damage from broken rings or partial seizure after start up. You have to break them in easy for a little while.

Get a compression gauge and get a true measurement on the compression vs. whatever specs are supposed to be.

Do these have electronic ignition or points/condenser? If timing is adjustable you need to verify specs.

Usually when engines are hard to start, assuming that the engine is capable of running well, then the cause is often a plugged up idle jet.
Today's gas is only good for a few weeks before it separates and becomes unstable.

Remember that this is a factory designed racebike, not a trail bike, and as such, they are high-strung and fairly fragile in the bigger picture.

Once you get a favorable compression reading and the piston isn't scored or has a broken ring, then proceed to ignition timing, then to carburetor jets, then to any power enhancement accessories, checking each one for proper function.

Bill Silver
aka MrHonda