backfire through carbs on 1999 787 SPX

[ozzy]

please help
I have a problem with my 1999 787 SPX. I have rebuilt the top half of the engine, rotary disc is timed correctly, mag timing is correct, good spark and rebuilt carbs with new seals etc. The problem that I have is a backfire through both carbs at idle and when the engine is revved. I have an R&D head fitted and compression through both cylinders is equal but high (174 psi). The only thing I have not touched is the bottom end. Could I have a problem with crank shaft/rotary valve seals???

 

[paul stubbs]

Hi, I have a 1997 xp in the same situation. Top end rebuild, rebuilt carbs, and have had no joy in solving this problem.
I noticed you posted this quite a while ago, have you since solved the problem?
I would appreciate any thoughts on this problem

 

[kustomkarl]

Hey Paul, welcome to the seadoo forum. It is interesting that no one mad a response to the original problem. I'll give this some thought tomorrow and get back to you with a couple ideas. It's almost that time...so get off line and have some fun...It's almost 10:51pm here in the states.
have a happy New Year...
Karl

 

[Dennis]

Backfire

Hi,
Sure sounds like a rotary valve that's not timed correctly. What was the method used to time it. Let us know.
Dawg

 

[ozzy]

Double checked rotary valve timing and all is o.k. Now looking at pop off pressure in carbs - could it be an over fueling problem???? somebody please helllllllllllppppppp!!!!!!!!!!

 

[Dennis]

Back Fire

If the carbs are over fueling you should be able to seem them dripping when you look down the carb throats.(you may need a mirror). You say you rebuilt the top end. Are the pistons installed correctly? Most pistons have a arrow stamped into the dome. It should point towards the exhaust port.
I would also check the timing on both cylinders. The ignition timing can be correct on one cylinder and not correct on the other cylinder if the crankshaft is twisted due to a previous problem.
Let us know, DAWG