720 Backfire. need answer --- HELP

[123indyrt]

OK..., Owning 2 20 year old boats can test you.. I changed this motor out. I did split the lower case. Everything should be right. New rotory valve. timed within less than 5 deg. It starts / idles great. rev it a little, perfect. Now, whilst running out of the water, As soon as i go over half throttle, it backfires thru the carb. Keep in mind, when I pulled the motor, I just laid the carb off to the side. It was upside down. I've not been in this carb while i've owned it. The motor was running great prior w no carb type issues. Could something moved in the carb causing a high speed fuel issue that would cause a backfire?? Nothing else should be wrong. I think some debris moved in the carburetor

Help!!

RT

 

[quintonvincent]

Are you sure the neutral safety switch is working right? Flipping the carb over should not effect anything.

 

[123indyrt]

I know very well what the limiter sounds like. Again out of the water I can rev the motor up. As soon as I go past half throttle it backfires. I can put the other motor on the revlimiter with no issue. Not so with this motor

 

[RodInEscondido]

I would go with your first analysis regarding crud.
Not sure how it applies to your models, but that vintage SeaDoo had big problems with the fuel lines; if you have the gray Tempo fuel lines they need to go away else lots of crud gets in the fuel delivery system. If you have the grey Tempo fuel lines, in the process before running again, the carb(s) need to be rebuilt or at the very least cleaned and checked (like in pop off and leaks). These problems tend to cause a lean running condition leading to the engine eating itself.
Good luck

 

[Chester]

If it's backfiring, either the rotary valve timing or the ignition timing is out. Check both. You could have a sheared woodruff key.

Chester