1997 XP 787 - Backfire, Flooding, Low Compression on 1 Cylinder

[Hondo]

Need help on this one! Possibly even a therapist!

1997 XP 787 - Backfire / Flooding / Low Compression on 1 cylinder

I just replaced a non-working coil with a pretty corroded one that had cleaned up to get a spark on this ski that was running perfect last year. Got ski to crank and sounded fine on trailer (Idled great and rpm's were strong). It sat for a few days after getting it fired off, and I took it to lake to test. It fired up fairly easy on the trailer, but once on the water it wouldn't get much past 2k rpm. It seemed to be flooding out. Put it back on the trailer and it had a couple of backfires when trying to get it started. Tried cleaning fuel filter, changing plugs, nada. Nothing worked. Finally gave up at the lake with this one for the day. Ski has never had issues with backfiring and was a fresh rebuild on top end 2 yrs ago with very few hours - also carbs were rebuilt at same time & ski ran perfect last season.

After I got it back home it would crank with some coaxing but only if you turned the gas off to prevent flooding - then once it cranked, it had a pretty consistent backfire - especially at high rpm's. Also - now it wouldn't idle & just seemed to die when I let off the gas. When I pulled the plugs, they always seemed to be wet with fuel - which made me wonder if the coil wasn't igniting hot enough, or if the fuel may have gone bad over the winter.

* The gas was still left over from last season so I drained & replaced with fresh (ski is set up for 50:1 premix).
* Problem not solved - next suspected this used coil wasn't sparking hot enough - How do you check "spark strength"? I know that if you accidentally bump the spark tester, it shocks about like any other plug wire - could almost be a defibrilator! Even after geting shocked, I still question if this might have been the cause of what leads me to what I found next.............
* I did a compression check - MAJOR PROBLEM - I only have 85 lbs on the front cylinder and show about 145 on the rear. It looked like it was backfiring from the rear cylinder when I had it running with the breather off. While hooked up to the hose, I also noticed that there may be a small water drip that appears to be coming from one of the small "freeze-plug buttons" on the exhaust. Ski was winterized so that was extremely odd, but I have an extra exhaust if it can't be patched or repaired.

My question/s: Could the backfire cause engine/head gasket to blow? With a pretty fresh top end, I'm suspecting it's not piston/rings/cylinder issue (hopefully). REALLY hoping it's not internal water damage where some of the water wasn't blown out and caused cylinder or head to crack etc.

After I get the compression back up to par, I'm back to where I started - could a weak coil or stator cause backfire/flooding which might have lead to this whole problem. Or is the old fuel more than likely the culprit? Possibly a bit of both!? I'm just really paranoid now that I'm going to do a rebuild & then end up doing this all gain. I'll post anything I find once I get the head off and/or have engine torn down hopefully this weekend. I'm hoping it's simply a blown head gasket. Is there an easy way to verify if a gasket is the issue? I REALLY don't want to take this motor out again if I don't absolutely have to!

Any help on this one would be great, because this thing just seems to keep going from bad to worse!

Thanks!

 

[CReynoldsMIZ]

I dont know if the cylinder base gasket would really have an effect on your compression. More than likely it is a piston/ring problem. Take the head off and you should see fairly quickly if there is a problem, none the less, you cant run the ski with 85/145 compression and not have a problem. The water leaking at the exhaust is a very common problem, and can be repaired fairly inexpensively.

 

[Hondo]

Removed head - obvious damage (metal nicks from pounding top of head & piston). Took of cylinder - looks like a ring caught the power valve and blew off most of the top of the piston above the ring. I found a small piece of a ring in the exhaust port. Cylinder has one tiny nick where the ring caught on the way down before it blew out - probably will need at least a weld, if not a re-sleeve. Top end is bored 60 over already, so I'm not sure how much further it could be bored.

I didn't notice a way to adjust the power valves and I'm pretty sure they were not ground down at all when rebuilt - only cylinder bore. However they didn't seem to have a clearance issue upon rebuild. If it isn't ground/adjusted, is this a common failure? If so, anyone got a link or proper procedure on a way to adjust it, or how much is alright to shave off of it without sacrificing functionality at stock bore? Will make sure this is accommodated for on both cylinders before reassembly.

 

[Hondo]

Bad to worse - was about to order all parts for half a top end (the blown side) and noticed I put my pistons in backwards!!! It made it a whole season before a ring tip caught. No idea how I missed it with the giant scribed arrows that were on top of the pistons when I built this one, but the holes on my pistons are facing the intake/exhaust when they are supposed to be on the other side! WTF!!?

Should I go ahead & just do a full top end on both sides since there's no telling how much damage was done to the other ring/piston!? Also, anyone got a hookup on re-sleeving? I'm on my last bore with these cylinders so I will probably either have to do a full re-sleeve and go back stock bore, or I do have another set of worn stock bore cylinders out of a GX 787 motor that I can probably just punch up +.020. Are all of the 787s the same? I think the extra parts I have are from a 96 and my xp is a 97. If cylinders are the same, then boring the spares might be cheaper than the re-sleeve.