GSX 787 low compression after rebuilt

[ottorem1]

Hello everyone
Seadoo GSX // 787 1996
(in Europe - sorry for poor English)


After assembling revised and honed cylinders, I only achieve a compression of 1.75 bar // 105 PSI on both cylinders. Pistons are new.
I did a crush test with solder wire
Thickness = 2.31 mm // 0.090 inch

I can't correct that with seals. The distance is too big
Where could the error lie? What could I do? Could I have received incorrect cylinders?

I couldn't do the combustion chamber test. I don't have a burette. Would that help?

Thanks

 

[WhiskeyD]

What size cylinder base gasket are you running? Whats the squish with no gasket?

 

[ottorem1]

What size cylinder base gasket are you running? Whats the squish with no gasket?

Hello Whiskey

Thank you very much for your reaction
I had to rebuild again first. That's why the answer is a little delayed


Thanks for your answer
The gasket is bzw was 0.45mm = 0.018

without gasket squish = 2.52mm =0.099 inch (this thime more than with the gasket??!!)

looking forward to your diagnosis

Thanks for your help

 

[etemplet]

First off, is this a new crankshaft or you are reusing the old crank? You never know if that engine ingested water and some point and bent the connecting rods. Are the pistons coming all the way to the top of the cylinder? (.090") is about (.022") over what I generally see using a (.015") gasket. Using WSM pistons I don't see the squish in the proper range but compression is at minimum 150psi when I test.

Are you sure you are getting the solder over to the edge of the piston and is the solder on top of the wrist pin? I haven't seen anything under (.064") squish on a 787 using WSM pistons. My first 787 was (.068") and I was concerned with compression. When I assembled the engine I had 155psi on each cylinder. WSM ring end gap is always at the upper end of spec. or a couple thousands over. I usually see (.018 - .020") end gap in fresh bored cylinders. I don't even check squish any longer. I put the (.015") "thinnest gasket" in the pack and go with it.

I have a lathe at home and sometimes I muse about machining the cylinders and getting the correct squish but I'm only doing that if my skis needs a top end. LOL

Did you check the end gap on the rings before installing the pistons?

You have a strange problem for sure.

 

[ottorem1]

First off, is this a new crankshaft or you are reusing the old crank? You never know if that engine ingested water and some point and bent the connecting rods. Are the pistons coming all the way to the top of the cylinder? (.090") is about (.022") over what I generally see using a (.015") gasket. Using WSM pistons I don't see the squish in the proper range but compression is at minimum 150psi when I test.

Are you sure you are getting the solder over to the edge of the piston and is the solder on top of the wrist pin? I haven't seen anything under (.064") squish on a 787 using WSM pistons. My first 787 was (.068") and I was concerned with compression. When I assembled the engine I had 155psi on each cylinder. WSM ring end gap is always at the upper end of spec. or a couple thousands over. I usually see (.018 - .020") end gap in fresh bored cylinders. I don't even check squish any longer. I put the (.015") "thinnest gasket" in the pack and go with it.

I have a lathe at home and sometimes I muse about machining the cylinders and getting the correct squish but I'm only doing that if my skis needs a top end. LOL

Did you check the end gap on the rings before installing the pistons?

You have a strange problem for sure.

 

[ottorem1]

Hello
Thank you very much for your detailed contribution. This has always been easy with my other vehicles. I'm struggling with a bigger problem here. I've already thought about connecting rods.

Attempted diagnosis: There is still far too much space in the combustion chamber. Connecting rod too short or piston wrong (if only they had a product number!!)So it's less of a compression problem than a distance problem. But how to test this thesis? I need a sub zero seal (haha)

Would measuring the displacement with a burette help?

I've taken the engine apart several times and I'm getting tired...

 

[FL-Rider]

Are the heads original, or have they been machined in the chamber? I have a different engine than you, (951) but the concept should be the same. I purchased a rebuilt engine and the heads had been shaved and the domes had been also been machined. This resulted in a large squish measurement. The way the heads were cut, it actually reduced the cc volume of the domes and caused detonation. The way I figured it out was to first visually compare the head to another head that had not been machined and then measured the volume of the heads when upside down using plexiglass with a small hole in it and a burette. I didn't have to measure the cylinder volume on the assembled engine, only the head. If you do the visual comparison and it looks like the dome/chambers have been machined, then that might be the issue and then you could order a burette to verify.

 

[etemplet]

Hello
Thank you very much for your detailed contribution. This has always been easy with my other vehicles. I'm struggling with a bigger problem here. I've already thought about connecting rods.

Attempted diagnosis: There is still far too much space in the combustion chamber. Connecting rod too short or piston wrong (if only they had a product number!!)So it's less of a compression problem than a distance problem. But how to test this thesis? I need a sub zero seal (haha)

Would measuring the displacement with a burette help?

I've taken the engine apart several times and I'm getting tired...

Man that's a bummer.

 

[ottorem1]

Sind die Köpfe original oder wurden sie in der Kammer bearbeitet? Ich habe einen anderen Motor als Sie (951), aber das Konzept sollte das gleiche sein. Ich kaufte einen umgebauten Motor und die Köpfe waren rasiert und die Kuppeln waren ebenfalls bearbeitet worden. Dies führte zu einer großen Quetschmessung. Die Art und Weise, wie die Köpfe abgeschnitten wurden, reduzierte das cm³-Volumen der Kuppeln und verursachte eine Detonation. Die Art und Weise, wie ich es herausfand, bestand darin, den Kopf zuerst visuell mit einem anderen Kopf zu vergleichen, der nicht bearbeitet worden war, und dann das Volumen der Köpfe zu messen, wenn sie auf dem Kopf standen, mit Plexiglas mit einem kleinen Loch darin und einer Bürette. Ich musste nicht das Zylindervolumen des zusammengebauten Motors messen, sondern nur den Kopf. Wenn Sie den visuellen Vergleich durchführen und es so aussieht, als ob die Kuppel/Kammern bearbeitet wurden, dann könnte dies das Problem sein und Sie könnten dann eine Bürette bestellen, um dies zu überprüfen.

 

[ottorem1]

Hello - Also a special thank to your answer

Good idea. I'll try that once. This would also make the measurement more accurate. However, I first have to find the right material
Thanks