So why does the PTO piston always fail first?

Krispy · 10-23-2009, 05:31 PM

Any opinions? It seems that any compression test I have ever done has shown that the PTO side piston is always in worse shape than the MAG piston. Anyone know why?

anthrobe · 10-23-2009, 06:06 PM

Here is some info for ya on rear piston problems

http://www.groupk.com/tec-rearseize.htm

Krispy · 10-23-2009, 07:02 PM

Quote:

Originally Posted by anthrobe

Here is some info for ya on rear piston problems

http://www.groupk.com/tec-rearseize.htm

Quote:

Originally Posted by GroupK

The front cylinder on all pwcs is the cylinder closest to the ignition rotor. The amount of torsioning that can take place between the ignition flywheel and the front crank pin is nearly non-existent. However, structurally speaking, the rear crank pin is much "farther away" from the ignition flywheel. When the pump (at speed) suddenly hooks up on some smooth water, the front cylinder and the ignition flywheel have enough rotational momentum to torsion the crankshaft over it's entire length. As this happens, the flywheel and front cylinder can actually get 2 to 3 degrees of rotation ahead of the rear cylinder. As this 2 to 3 degree torsioning rotation takes place, the front cylinder is still getting perfectly timed ignition sparks. However the rear cylinder is lagging slightly behind when it gets it's ignition spark. This means that the rear cylinder is firing 2 to 3 degrees more advanced than the front cylinder. Furthermore, this advanced timing is happening at the worst imaginable time...at high loads and high rpms. As any engine builder can tell you, running 3 degrees too much high rpm advance on a race engine is a guaranteed way to seize (or hole) a piston. While we don't have any iron clad proof that these presumed effects of crankshaft torsioning are an absolute fact, we feel that have enough hands on experiences and supporting information to consider it "a very probable truth". Until some one with more insight and experience can come up with a more probable truth...we'll consider crankshaft torsioning to be a reality.

I acutally beleive this, anyone else have some experence to share?

engr146 · 10-23-2009, 07:03 PM

WOW,Good Info!! this is my second rear cylinder failure,640 now my 770

Krispy,Until I pull head,I won't know anything,
My rear (PTO) carb was leaking gas,I only found this out after, when trying to find out what the problem was!

dank · 10-25-2009, 11:19 PM

If you were running to carb as lean as possible like a race ski then the rear cylinder would be the one to fail most often due to the above facts but most of us don't push our ski's as hard as someone in a race would. They adjust the carbs each day and during the event to account for water temps and air temps.

the WaTeRhAwK · 10-25-2009, 11:45 PM

slightly richening the mixture of the rear carb on dual carbs setups will offset the effects somewhat. it will not completely eliminate it from happening, but will lower the chances.