96 speedster strange backfire

[speedyboyl]

I bought a 1996 speedster twin rotax about 4 week ago and for the first three days i had no problem with it, ran out fine. I took it out of the water and when i put it in the next weekend the port engine ran fine for about 5 min then shut off. It would restart but with a bad shimy and backfireing through the rear carb. i have gone throught the shop manual from start to finish, moved the carbs from one engine to the other, took the carbs apart and cleaned them, even took off the rotary valve cover and resealed it. i can't seem to find out what is wrong, i have gotten it working fine inso that it no longer backfires at idle but as soon as you give it some gas it starts to backfire again. the other engine runs fine on both sets of carbs and the plugs are giving the proper spark. If anyone else has had this problem or somethign similar please let me know before i have to take it to a professional, and spend a ton of money, thanks

 

[seadoosnipe]

Rotary valve.

I read this yesterday about carbs backfiring and thought it may be applicable to your situation. Very good information and worth checking into.

Backfireing through the carbs on the Rotax engines usually means you have rotary valve or rotary valve timing issues. If you removed the valve when you took the intake off, you will need to re-time the intake timing. Also, you need to check if the rotary valve shaft gear has been damaged. When you remove the rotary valve cover to check/retime the valve, rotate the pto while holding resistance on the valve and check for "dead" spots. The valve will stop turning in sequence with the crankshaft. Also feel for excessive play on the brass gears.

 

[speedyboyl]

:cuss: I figured out the problem, a blown piston. i ordered a new moter and it should be in today, i hope to get it broken in by this weekend. thanks for the help

 

[seadoosnipe]

Piston?

Hate to hear you have a blown piston and will be replacing your engine. At least you'll be ready for the spring!.....
Question. Did you take a compression on the motor and if so, what was the reading on the bad cylinder? Just curious. It will be useful in future posts I'm sure.

 

[speedyboyl]

sorry i didn't do a compression test on the motor, i would have had to put it back together to do so. The new motor is in and i have had good enough weather the past two days to do most of the break in, so if the weather holds i might get on more weekend out of her before winter

 

[seadoosnipe]

Great!

No need to apoligize, just glad your up and running again!.......