Seadoo xp 787 Loud shot gun backfire out of exhaust will not start

[xprider02]

Hey guys I just put a new engine in this 97 seadoo xp 787.

I try to start it and it backfires through the exhuast and will not start. The backfire is loud and it has done it multiple times. Haha my neighbors came over to my shop and thought I got shot or something because it was so loud.

compression is 150 in both cylinders
rv timing is right on the money (I checked it multiple times)
The flywheel key is not sheared
The mag cup is aligned properly with the flywheel.
It has great spark
The carbs are jetted right and clean
I did premix the gas and I have forgging oil in engine for lube for first start up (but this is something I do with every engine I build so that there is extra lubrication for break in and lube while oil pump catchs up)
The rv cover o-ring is good and brand new
the carb gaskets are good and not leaking air
the crankshaft is not twisted

any ideas what could be wrong? I am kinda stumped

 

[xprider02]

bad gas wouldn't cause this would it?

 

[Minnetonka4me]

possibly...did you ever try a different set of plugs?

 

[xprider02]

no I have not tried another set of plugs but I will try!!! I just was looking over exhaust and the rear hose that is after the water box is blown out. I would deffinitly have to say from the backfire. so before I can try starting it again I need to get a new hose.

Would the voltage regulator/rectifier cause this or the MPEM? I tested resistance of the ignition coil and pulser coil and both are right in spec.

 

[Minnetonka4me]

Well I can have another hose on the way for $40 shipped tomorrow!

KISS...keep it simple stupid. I had a snowmobile I beat my head on rebuilding the carbs 3 times, then accidently put a different set of plugs in. I still feel dumb for not trying it sooner.

also clip the plug wires a 1/4" and reattach the cap.

 

[xprider02]

I tried clipping the wire leads already and new plugs. I wouldn't think plugs would cause this to happen. Normally a backfire out of exhaust is caused by fuel in exhaust igniting. I did not build this motor. I checked and made sure the pistons were in stalled correctly and that the crank is not twisted and they both are good. everything checks out!!! I tested the stator and it checks out ok too! I am stumped on this one. any other ideas?

 

[jhjesse]

If the plugs are straight out of the box new, it is possible that one of them has a cracked insulator.
Happened to me once a couple years ago...still haven't seen the cat that was standing behind the ski when it backfired.

Long gone!!

 

[Minnetonka4me]

I dont have any idea what else you can try...you have checked everything I would check.

 

[xprider02]

FINALLY I Figured it out!!!!! It was the MPEM. I plugged in a new MPEM and it fired right up with no backfire!!!!!!!!

 

[Minnetonka4me]

Wow...would have never thought that would be it...gotta love seadoos!!