717 backfiring through carb

[fgb26]

hi,guys, i have a seadoo explorer with a 717 rotax and the last time i took it out i lost top end power with spluttering and backfiring through the carbies, it is as though it has an intermitting spark problem. I was told it might be the mpem/coil - checked and changed but no difference.Removed stator and it tested okay. Carbies were pulled apart and cleaned, new plugs and still no success could the rotary valve have moved? Or could the stator be faulty still, any ideas that i could try?:emoticonb thanx frenchy

[dsolie]

Re: 717 backfiring through carb

Backfire through the carbs usually means a timing issue. The rotary valve could be off because you stripped the rotary gear inside the cases. Has anything else happened recently? (Sucked in water, oil injection problems etc). Do you notice any brass shavings anywhere near the carbs or inside the oil lines (big lines into the case)?

[fgb26]

Re: 717 backfiring through carb

hi dan,checked for brass in oil doesn't seem to. nothing else happened to it just took it out for a ride was going great all of a sudden it bogged down and wouldnt go past half throttle without spluttering forgot to mention it starts and idles fine i also changed rectifier still know luck.Ps. Checked timing found it to be ok 147deg using a degree wheel.:emoticonb

[dsolie]

Re: 717 backfiring through carb

nice! What is your current compression on the motor?
When does it usually backfire then? It starts and idles well, so do you have the backfire at 1/2, 3/4, full throttle, or all the above?

[fgb26]

Re: 717 backfiring through carb

hi dan, half throttle it starts missing very badly and any further it starts to backfire through carbie's comp on both cyl are 148psi. Ps i noticed a bit of oil leaking from behind flywheel could i be loosing crank case pressure?:emoticonb

[dsolie]

Re: 717 backfiring through carb

Yes you could! There are 3 large lines that come from the oil tank. 1 goes to the oil pump by the carbs, the other two are for the rotary gear bath. Both those lines go into the case. One behind the flywheel and one back by the starter. If one of your lower oil lines have cracked, you have possibly stripped your rotary gear because of a lack of lubrication in the rotary bath.

or

If you have a bad crank seal, that could cause some problems. You need to pull the front cover off and report what you see.

If the oil leak is nothing major, just a small leak in the line or connection, and no other damage done, then replace the line. You should also check your rectifier. Open your grey electrical box and unplug it. See how it does then. Just remember, once you unplug the rectifier, you are no longer able to charge your battery off of the magneto.... Make it a short test :emoticont or you will run out of juice.

[hydro X]

Re: 717 backfiring through carb

Check that one of the lower bolts on the intake manifold ( RV cover ) hasn`t fallen out. Had one that had the same symptoms..... and there it was, missing bolt.

[Bill O'Neal2]

Re: 717 backfiring through carb

Put a new rectifier in it and it will run smoothe again.

[fgb26]

Re: 717 backfiring through carb

Hi bill,you gave the same answer when it first happened a few months back so i did change rectifier and opem no different i have been trying to adjust carbs and found it would peak but cannot get consistency checked fuel filters dismantaled carbs could not find any problems any thing else to check? is there an adjustment on needle and seat as it seams to peak up and down as if its running out of feul.:emoticonb

[dsolie]

Re: 717 backfiring through carb

What's your pop-off. Have you tried replacing the needle and seat?

[Bill O'Neal2]

Re: 717 backfiring through carb

Carbureator problems rarely will cause backfiring. I think you may have some other major issue with this motor. I bet if it were here sitting in my shop I could figure it out real quick.

Backfiring, is usually ignition related, or RV timing related, but can be caused by such things as a badly siezed or holed piston, twisted crankshaft, major air leak into the motor, such as a cracked intake manifold ( common on 720's) or a really bad crankshaft seal.

To be a carbureator problem causing the motor to backfire, it would be running very lean in order to cause a backfiriung motor. Run like that for very long at or near WOT, and it will sieze a piston or burn a hole in the top of it.

First take a very close look at the intake manifold for cracks. You might try starting the motor a spraying some starter fluid around the base of the carbs and intake manifold. If it revs up while spraying, you probably have a cracked intake manifold.
I once had one in here that took days to find the problem (backfiring). It had the wrong diameter oring on the RV cover and was sucking air from around the RV cover.

[fgb26]

Re: 717 backfiring through carb

hi bill, i checked intake manifold for cracks none that i could see but i do have a oil seal behind flywheel that im loosing a bit of oil from surely it couldn't be causing the problem? but what i did notice is that i held throttle half oppen for about 10 sec motor would pick up revs but motor would not shut down even pulling leads of it kepted running? end up covering carbs to stop it.

have a great christmas and new year:beer

[dsolie]

Re: 717 backfiring through carb

You have an air leak!

[kbloom]

Re: 717 backfiring through carb

not to sound like an *** Dan but you say he has an air leak. Ok! Now how does he fix it?

[fgb26]

Re: 717 backfiring through carb

hi,if its a air leak theres only one place it can come from and thats the intake side and theres no leak that i can find as i have pulled everything of this heap of **** next thing is to burn it.can the mpem cause missfire (send timeing out)?