Hello all from Michigan! New to the forum and owning a PWC world. Just purchased two 1996 XP 787's. Both started and ran great. I made the mistake of not bring a compression tester and bought them. When I got home, I decided to do a compression test for peace if mind. One of the machines has a pretty large difference in compression. The front cylinder is at 130psi while the rear is at 150psi. Previous owner said he recently put new rings in this machine. What is the next step for me to do and my best options? Thanks.
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1996 XP Compression
- Thread starter Marhos24
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IcyBlueMink
Active Member
Hello from Michigan as well! Did you make sure you had a decent compression tester as well? I used to use a cheaper one before my Mac one and would sometimes get low readings. If not I’d make sure to view the online service manual or buy one of your own.
It might run but not for long.....
It really doesn't matter what the numbers are, just the difference at this point. At 20 psi off one cylinder is on it's way out so when it lets go you will cause even more damage from the pieces going through the engine.
With low compression the rings are not sealing against the cylinder wall. When this happens hot combustion gasses get by the rings and start to burn off your oil film, (yes, the only thing keeping the aluminum piston from grinding against the iron cylinder walls is the oil film), so with this oil film burning off the piston will start to seize until enough aluminum from the piston transfers to the cylinder wall the piston gets stuck in the bore.
It really doesn't matter what the numbers are, just the difference at this point. At 20 psi off one cylinder is on it's way out so when it lets go you will cause even more damage from the pieces going through the engine.
With low compression the rings are not sealing against the cylinder wall. When this happens hot combustion gasses get by the rings and start to burn off your oil film, (yes, the only thing keeping the aluminum piston from grinding against the iron cylinder walls is the oil film), so with this oil film burning off the piston will start to seize until enough aluminum from the piston transfers to the cylinder wall the piston gets stuck in the bore.
Thank you for elaborating. I'll tear it down soon and take a look. Is a top end rebuild a feasible option? I've seen some opinions that if the top end is beginning to go, the lower end will soon follow.
Obviously on a 20+ year old engine it is good to replace the crank also.
If it was mine I would at least pull the engine apart inspect the crank and install new gaskets and seals when doing a top end. Air leaks on old 2-strokes are somewhat common and can ruin the engine.
If it was mine I would at least pull the engine apart inspect the crank and install new gaskets and seals when doing a top end. Air leaks on old 2-strokes are somewhat common and can ruin the engine.
Last question. I was just thinking how the PO rebuilt these engines over the winter and has not ridden them since. Knowing that, I assume they haven't been properly "broken in". Could that be the cause for differing compression? Would a proper break in possibly allow the rings to seat better?
Pulled the heads, I have some pictures that I'll attach. To me the cylinders look good. What I did find is a crack in the casting for the head cover plate (another attached picture). Going to replace that. The cylinder near where it was cracked has some rust around it. Funny thing is that's the cylinder with good compression.
1st and 2nd picture are the low compression cylinder
3rd pic is the cracked head cover
4th pic is the cylinder under the crack
5th pic is the underside if the head
6th pic is the head on the side of the crack, with some dents. Assuming from a previous issue?
1st and 2nd picture are the low compression cylinder
3rd pic is the cracked head cover
4th pic is the cylinder under the crack
5th pic is the underside if the head
6th pic is the head on the side of the crack, with some dents. Assuming from a previous issue?
That crack is just for the water portion of the head and not the compression so that will not cause low compression.
Those do not look like fresh cylinder bores so my guess is too loose of bores causing low compression.
By just putting in "new rings" the previous owner did absolutely nothing to help that engine or compression.
To get your compression back you will need to do it correctly with new oversized pistons and the cylinders bored and honed to new spec clearances.
Those do not look like fresh cylinder bores so my guess is too loose of bores causing low compression.
By just putting in "new rings" the previous owner did absolutely nothing to help that engine or compression.
To get your compression back you will need to do it correctly with new oversized pistons and the cylinders bored and honed to new spec clearances.
That's the plan! Dropping the cylinders off at the machine shop today. So with the largest cylinder at 82.55mm, what should they be bored to? I'm looking at WSM top end rebuild kits, and noticed they have pistons up to 83mm. I would like to keep the bore as small as possible so I don't need to mess with the RAVE valves. Tolerances in the manual have a .10-.12mm piston/cylinder wall clearance for new parts. So I should have the cylinder bored to 82.60mm, and order the 82.50 pistons?
I have the engine in the jetski still, so I assume I wouldn't be able to do that.
thank you!
GGuillot
Well-Known Member
That scoring on the side is from the RAVE valve, that's a heck of a clearance to have .52 mm difference between the cylinder and the piston. An overbore beyond 82.25mm is a risk of the RAVE hitting the piston, unless trimmed. In my case of the 97' GTX, this is exactly what happen, untrimmed RAVE's with 82.50mm overbore pistons. Eventually punched a hole on the side of one piston, locked it up, and it filled up water when towed, then it sat for a year.
Yeah, I'd pull it and break it down and have a look, especially if you're about to spend money on the top end. Would suck to do it again.
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