Low compression question

[Charles603]

Is it possible to get a low compression from a weak battery?
Long story short I need to rebuild my gp800 and a guy is selling one super cheap that revs out of water but only 2k in water. Changed plugs but didnt make a difference. I'm thinking of buying his and taking the parts I need but will test his ski with my battery if that could be it.


Thanks in advance!

 

[etemplet]

Charge the battery and do a compression check as you say. Usually the engines spin over pretty easy with no plugs installed. If the skis doesn't revv in the water certainly compression is a concern. Then again, good compression readings don't mean the cylinders and rings are healthy. I got 145psi on a cylinder that had half the bottom ring missing.

 

[Paul Bretz]

A weak battery can give false compression readings. Sounds like other issue wen comparing to how it runs out of water compared to on the water.

 

[mikidymac]

Keep in mind the Yamaha 800 has plated cylinders so you can’t just bore them oversized for new pistons.

 

[etemplet]

Keep in mind the Yamaha 800 has plated cylinders so you can’t just bore them oversized for new pistons.

Yikes !!

 

[Charles603]

Keep in mind the Yamaha 800 has plated cylinders so you can’t just bore them oversized for new pistons.

Yeah I got quoted 200/ea cylinder with replating... but I'm sure I'd end up needing a new piston too.

All the refurbished engines seem to be sleeved...

 

[mikidymac]

Yes the refurbished ones are sleeved which makes them run hotter. Yamaha intentionality plated the aluminum for better heat transfer.

Either way you will need new pistons and rings so that isn’t a factor.