Open high speed screw just a little?

[burtshaver2021]

On my 95XP the compression on the PTO cylinder is a little lower than the mag cylinder. Nothing crazy, all is well, I can’t remember exactly what the numbers were, something like 135 mag and 130 PTO. Can I crack the high speed screw on the PTO cylinder open just a 1/8 turn ? Will this stop the PTO cylinder from wearing faster? Does the PTO cylinder wear a little more on all these engines? Can I run the high speed screw 1/8 turn further open then factory settings on all my machines

 

[etemplet]

Not sure about this particular ski but a few carb skis recommend a PTO high speed adjuster setting greater than "lightly closed." With 130psi and a good compression gauge I'd expect to see some obvious damage to the piston on cylinder walls. You're down 20psi from new compression numbers. I'm an old 2-stroke guy so I think nothing of doing a top end. LOL

 

[burtshaver2021]

What do you mean by seeing obvious damage to piston and cylinder walls? My thinking has been 125 pounds is time to start thinking about rebuild. This would be just from regular wear, I would not expect to see any damage.

 

[etemplet]

What do you mean by seeing obvious damage to piston and cylinder walls? My thinking has been 125 pounds is time to start thinking about rebuild. This would be just from regular wear, I would not expect to see any damage.

I've some engines with damaged rings and scoured cylinders with engines at 130-135 psi compression. I don't want pieces parts ending up in the crank bearings. A happy top end is a happy engine.

 

[burtshaver2021]

I've some engines with damaged rings and scoured cylinders with engines at 130-135 psi compression. I don't want pieces parts ending up in the crank bearings. A happy top end is a happy engine.

I’m hearing you, but I’m not going to start doing top ends at 130-135. Definitely not saying you’re wrong, but I’m going to stick with my 125 pounds as a benchmark for top ends. A lot of guys use 120 pounds. The little bit I ride anymore it could be another 4 or 5 years till it’s at 125. Besides it starts like lighting both out and in the water. With that being said I do appreciate your input.

 

[mikidymac]

It isn't going to prevent any "wear" or change the life of that piston.
But.....
The rear pistons do run a little hotter simply because the crank does actually twist which slightly increases the timing on the rear cylinder and generates a little more heat so typically you would set the high speed screw just a little more on that rear cylinder.

Yamaha had such and issue with this on the 760 engines that the actually lowered the compression on the rear cylinder from the factory.