QT50 Not Starting

Alright, so here is the story.

I bought a 1979 Yamaha QT50 off of craigslist. At the time it was running well and started easily. Since then I've put about 50 miles on it. As I've ridden it its become harder and harder to get it to start, to the point where it will no longer start.

I've done some diagnosing. I started with the spark plug, the spark was weak and intermittent so I replaced the plug, and when that didn't help I replaced the coil. I now have a strong clean blue spark.

After that failed to fix it I tore down and cleaned out the carb. I went into all of the jets with carb cleaner and disassembled it. It wasn't too bad, but I cleaned out what varnish there was.

Again, didn't fix it. I opened up the cylinder and found both the cylinder wall and piston badly scored. (I'm guessing from when the oil pump died, I've been running it on 32:1 Amsoil 2 stroke with 91 octane Ethanol free) Assuming this was the issue I again replaced both, and again, no fix, although I now have about 90psi of compression where before it was in the low 30s.

I've checked the reeds and they appear to be in good shape and look like they've been replaced. The bike also isn't starting with the help of starter fluid, so I'm ruling out a fuel issue.

The only thing left I can think of is that the timing is somehow off, but I don't know how to test that theory, or how you would correct that.

Anyone here have any ideas?

Re: QT50 Not Starting

clogged pipe. See if it starts with no pipe. CDI so no way to "adjust" the timing. And not sure but your compression seems low still. I was thinking that should be closer to 125-150 with new top end? pic of your work help.

Re: QT50 Not Starting

Doesn't start without the exhaust on either. The head gasket was damaged in shipping, and the seller is sending another, so that could be the compression issue. Either way it should run fine on 90psi.

Re: QT50 Not Starting

Alright, time for a bit of humility, I was checking the compression wrong, and testing with the throttle closed. With the throttle open its at 145psi.

Re: QT50 Not Starting

When you start it is the switch on start or run?

Re: QT50 Not Starting

Its one of the older QT50s that don't have the start position, so its in Run.

Re: QT50 Not Starting

Timing isn't adjustable but can be off if flywheel is out , remove flywheel cover , timing mark is on case at 7 o'clock. Its at.037",.94mm BTDC, check with light. If off check woodruff key /flywheel alignment. Look in wiki for manual if you don't have one.

Re: QT50 Not Starting

Well the woodruff key was in good shape. I replaced the stator coils and its running strong now.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account