Coolster 125 (110) wont run.
 

Coolster 3125r with a mislabeled 110cc engine. Newer top end rebuild.

ran great, then just stopped and wouldn't restart.

Timing was off by 1 tooth, so I adjusted that. Adjusted both valves to .005. Getting 125 psi compression. Good spark, but put in a new plug anyways.

Still won't run. It will so mn etimes fire and limp along for a few seconds, then backfire out the carb and quit.

Thought it might be a carb issue, so I slapped a new carb on it (even though the old one was only 6 months old).

still runs the same.

I'm out of ideas.

what's next?

It is just possible it wasn't the valve timing out, but the T mark. If the flywheel moves round on the crankshaft, it puts the ignition timing and T mark out. Check that TDC of the piston and T mark agree. Still most likely to be carb though, clean the old one out and put it back on.

I'll check it out. Thanks.

Did the straw trick, the "T" is TDC, or close enough I cant tell it's off. Pulled the cover anyways, it's on tight. Don't have the right puller to get it off, but I can see the key in the slot on the flywheel, and it looks OK. Cant see the slot in the crank, though. Bummed- I was convinced that was the problem.

had a new CDI lying around, no change.

as far as carb- what are the odds of a new carb being messed up in the same way as the old one? I even tried putting a little gas in the plug hole, with no luck.

"what are the odds of a new carb being messed up in the same way as the old one?" Quite high, can't hurt to clean out the old carb and re-fit it.

Just an update- cleaned the old carb, even had another one lying around. Still nothing. Pulled the flywheel, looks fine.

here is where I make myself look stupid- decided to check the spark again to see if it seemed to spark at roughly the right time.

it sparks, barely, for a few revolutions, then just kinda peters out. Obviously, it's an ignition problem. Parts are cheap, so I ordered an upgraded cdi, coil, and stator for it.

I feel stupid, my initial thought was spark, so I pulled the plug, held it to the head, and hit the starter. Saw a spark, and immediately crossed it off the list.