1993 XP Low Compression

[jncandrews]

1993 XP (657)
Reman crank
New RV shaft
Cylinders honed 30 over
New pistons/rings (30 over) installed
New rotary valve (minor scoring on RV cover, no grooves or depth to any of the scoring)
Complete gasket set

First time on water, jet ski ran fairly well for a few minutes. After about 10-15 minutes, loss of power above about a third throttle. Will start and idle fine.

Checked compression and only 70-80 in both cylinders.

Can scoring on the RV cover cause that much of a compression loss?

Is it possible the piston rings are installed incorrectly (ie, upside down)

Rechecked RV timing several times and it is where it should be.

Next thing I'm contemplating is pulling the jugs and seeing what is going in inside the cylinders.

No real great idea on what I should be doing at this point.

Thanks for any assistance.

Jeff

 

[mikidymac]

First are you sure your compression gauge is accurate.

Second what pistons did you use? I haven’t seen .30 over pistons as they typically come in .25mm increments over stock.

Third I didn’t se anywhere that you rebuilt the carbs or fuel system. This is the number one cause of piston and cylinder seizure and if you didn’t address it you will keep blowing up new top ends over and over.

 

[jncandrews]

First are you sure your compression gauge is accurate.

Second what pistons did you use? I haven’t seen .30 over pistons as they typically come in .25mm increments over stock.

Third I didn’t se anywhere that you rebuilt the carbs or fuel system. This is the number one cause of piston and cylinder seizure and if you didn’t address it you will keep blowing up new top ends over and over.

Mikidymac,

Yes, they are 25 over. I was going off memory (obviously faulty). The piston and ring set is WSM.

As for the compression, my gauge is 40 years old and that question came to mind. I bought a new one which showed just a slightly lower reading which I posted (70-75).

As for the carb/fuel system. All newer hoses, Fuel filter/separator checked and cleaned. Fuel selector likely original. Carbs rebuilt over the winter but after the initial symptom of power loss. Damage may likely have already been done by then.

I'm probably going to pull the jugs next to look at that area as it's easy to do and likely where my major problem is.

Do you know if a scored RV cover plate can cause much in compression loss?

Thanks,
Jeff

 

[mikidymac]

No, the RV shouldn't cause a loss in compression that dramatic.

Can you post pictures of your RV surfaces?
Did you measure RV clearance?

 

[jncandrews]

I have not measured RV clearance as it seemed I needed some material at the time which I didn't have. I will take a picture as soon as I get a chance. In the meantime, I attached photos of both cylinders. I don't see anything that sticks out but I realize the damage may well not be a readily visible thing as compared to some measurements. I'll try to get a measurement on the cylinder wall.

FYI, the skinny little o-rings which sit in the groove atop the cylinder had some type of brittle substance on them. I cleaned them off but have no idea what that would have been. I also cleaned out the groove again. May slap the jug cover back on and see if anything had changed.

 

[mikidymac]

It is really hard to tell with the naked eye but is sure looks like a lot of top ring is available.

It almost looks like the cylinders are way too big for that size of piston and that defiantly would explain the really low compression.

I also don't really see any crosshatch which is not normal for a fresh bore and hone.

 

[jncandrews]

Crosshatch was present when I first picked the jugs up from the shop. They probably have about 2 hours on them total at this point.

I see what you are saying about the space between the pistons and cylinders. I measured the inner diameter of the cylinders at the top edge and they read 77.96(mag) and 77.90(pto).

The pistons read 74.94(mag) and 74.93(pto). I have not been able to find what the piston diameter should be. I've seen 78 mentioned when looking up a piston kit for 25 over but I assume that is the cylinder diameter. Any ideas on what these pistons should have for an actual diameter?

 

[mikidymac]

Well, we found your answer and your problem.

Assuming your measurements are correct your pistons are really, really wrong and out of spec.

Standard bore is 78mm and maximum overbore is 79mm. I assume you didn't measure your bore with unactual bore gauge so that is why you would see a little smaller measured numbers like the 77.96.

The pistons however are way too small by a lot. An oem stock piston is 77.94mm so a 25 over piston should be 78.19 and yours are at least 3mm too small which is huge.
New piston to cylinder clearance should be 0.06-0.09mm and you have at least 3.02mm.

I wonder if the WSM pistons might be knockoff or something as I haven't seen them say "AUS" on them but then again I haven't used them in about 5 years.

 

[jncandrews]

I am using a digital caliper to measure with and not a bore gauge. Just for clarification, I am measuring the piston at the top edge. Should I pull the jugs and measure the piston further down for a truer spec? I know the service manual references measuring the piston clearance at the bottom of the piston skirt.

I think it would be good to pull the jugs regardless and see what all is going on with these pistons, rings, etc.

 

[mikidymac]

Yes, you would want to measure the piston down from the crown.

 

[jncandrews]

Well, sorry to mislead you earlier with erroneous measurements. After pulling the mag jug and measuring the piston below the ring level, it measures out at 78.20 or so. That top crown on the piston is considerably smaller than the actual piston diameter. A closer inspection of the jugs shows some residual cross hatching. The rings appear nicely chrome appearing on the edges with no obvious wear on them that caught my attention.

Is there anything especially important regarding installing the thin o-ring in the groove on top of the jug? Does it need anything on it? Or just install dry? (which is what I've done so far)

I'm reaching at this point as everything appears to be okay. I looked the jug cover over as best I could to see if there was a crack but could see anything. That would likely only affect one cylinder but what the heck.

I really appreciate all of your input on this.

 

[jncandrews]

Yes, you would want to measure the piston down from the crown.

Miki,

Some further information. I pulled both jugs and measured piston ring clearance. The clearance on the bottom ring is less than .016 and the first ring is right at or just shy of .016. That reading is the high end of what the manual says is acceptable for new rings. .039 is what is considered the high end for a used ring.

In addition, I found the bottom ring on the PTO piston to have broken. It likely happened when I installed the piston and rings originally as I am not using a compression tool and likely didn't get the ring properly centered over the groove dimple.

That would account for the low compression in that cylinder but don't know how that would translate over to the MAG cylinder. I'll order a new ring set for the PTO cylinder and see what that gets me. I don't see a need at the moment for a new set for the MAG cylinder at this point.

In studying this engine, it appears only the rings and cylinder head o-ring seal have anything to do with overall compression. The RV appears out of play once the cylinder is headed up and has passed the transport port.