We have an LT80 that I recently rebuilt with help from a mechanic. Cleaned the carb, blocked off the oil injector pump, and running 32:1 pre-mix. I've adjusted the throttle needle to multiple positions and still have no luck.

It tries to start up quickly but then will stall right out. When I did have it running it would stall only at full throttle and at full throttle it would begin to backfire.

It does have a fuel shut off in the fuel line.

I am out of ideas at this point...

 

So it runs but just doesn't like full throttle?
What was re-built? top end? If so, did you get one of the cheap kits?
Backfiring is not common.

 

This weekend it would run, but stall and sometimes backfire at full throttle. However, Monday when my son tried to start it and keep it running it would start right up then stall out.

It was a Pro-X top end rebuild.

To back up and provide a full history...we've owned this for about a year. I did not know about the oil injection so that tank was run dry and what ruined the previous piston, hence the required re-build. This is also why I decided to go with a pre-mix from here forward.

 

Bad reed?
Plugged pipe?
Both good possibility's.

The bottom end survived the "no oil" episode without new main bearings?

 

What pipe would be plugged?

What is considered the "back end"?

 

The exhaust pipe.
Back end? What's up that you ask that?

 

LT80 for some reason I can't see your reply on here, but I can see it in my email.

So the exhaust pipe could be plugged? It is pushing air and smoke out.

 

It could be. It happens.
What about the reed?