587 White motor, compression question

[mstewart440]

This motor only shows 80psi of compression on both cylinders, so I pulled the head off expecting to find ugliness. The walls aren't scarred at all, feel smooth as glass. The top rings aren't stuck. The o-rings appear to be intact. Is not obvious to me why the compression is so low, and why both cylinders are the same. This ski sat for a long time and I wonder if the rings may have just lost their tension. Any ideas ?

 

[RodInEscondido]

Even tho it looks like they are sealing ok, replace the o-rings and retest compression with another tester. Many stories out here relating to getting burned by inaccurate compression testers. They are not that expensive; I have 3 and you may be able to get a loaner/rental from auto parts store.

As long as head is off, feel with fingernail on cylinder wall at top of top ring travel for a little ridge which would indicate cylinder wear; best to do actual measurement of cylinder diameter to see if maybe you can get away with rings and cylinder quick hone job to break glaze or if you need rebore and oversize pistons/rings. First things first tho: see above paragraph.

Good luck,

Rod

 

[Strizzo]

Good point rod, new engine should have around 150psi compression, and usually when they wear down to double digits they're not very close to each other either. If your tester said both were exactly 80psi then I would suspect the tester.

 

[mstewart440]

I thought the same thing. I have an old one (15yrs old) from my stock car racing days that I tried first. The valve doesn't work so the needle was bouncing with each stroke but my old eyes thought I was seeing 85 or so. I rigged up a homemade one out of a water pressure tester and it did about the same thing. Stopped by Harbor Freight on lunch yesterday and bought one and its the one that gave me 80psi on both. The bores are really smooth...I can't grab a ridge anywhere in either of them. I don't have a bore gauge anymore but a caliper on the top of the cylinder measures 76mm so they are still std.

One other tidbit...my wear ring is still tight enough that I can't turn the engine over by grabbing the flywheel. If it isn't cranking fast enough, could that cause an inaccurate measurement? It pumps up as I crank it but never goes past the 80psi mark (I may be rounding 1psi). Seems pretty fast, but maybe I'll pull the pump back off and try it again. Here it's cranking when I was checking for spark: [video=youtube_share;vdrz5C56svQ]http://youtu.be/vdrz5C56svQ[/video]

 

[mstewart440]

I pulled the pump off last night and put the head back on to try it again. Without the drag of the impeller I got about 89psi mag side and 82psi pto side. Not unexpected...compression won't just show up. I'll pull the jugs off tonight so I can see the pistons and rings. Assuming the pistons are ok, I think I'm just going to order a set of rings and an o-ring kit. Are the pistons typically marked somewhere to indicate std, .25, .50, etc? I don't think my crappy calipers are trustworthy enough to measure .25mm.

 

[RodInEscondido]

All of the small number of pistons I have seen are marked on the crown and an arrow pointing to exhaust port.

 

[mstewart440]

So here's where we are. Pulled the jugs last night and the cylinders and pistons are scored below the rings, so they are boxed up and headed to Full Bore. Its weird, the PTO side piston is scored on the intake side, the mag side piston is scored on the exhaust side. Man, I dread dealing with those damn circlips.