Post rebuild compression on 951

[Muckstuffer]

I’m looking a purchasing a used 2001 sportster LE with a rebuilt 951cc engine. The owner has paperwork to show that it was bored oversize with new pistons and rings. The invoice also shows a crank rebuild kit but not sure what that included. The boat is in a heated storage area for the winter. After the rebuild, it was water tested by the shop that did the work but never completely broken in. The current owner started the boat with no water for about 15 seconds without any water just to prove that the boat ran. After this, I did a compress test on the cold engine and only got values of 120 on the pump side and 110 on the other. Is this motor viable or a time bomb?

 

[Matt Braley]

That compression does seem a little low. Many times the gauge is at fault, cheap ones often give low readings. The rings do take a little break-in time but you usually only gain 5psi at most. Stock compression is 135 so if the gauge is right then I would have to question their rebuild.

 

[Muckstuffer]

That compression does seem a little low. Many times the gauge is at fault, cheap ones often give low readings. The rings do take a little break-in time but you usually only gain 5psi at most. Stock compression is 135 so if the gauge is right then I would have to question their rebuild.

What effect does the motor being cold have. Should compression go up when warm?

 

[XP 787]

Full throttle while doing compression test?

 

[Muckstuffer]

Full throttle while doing compression test?

No it wasn’t. I’m guessing that will make difference?

 

[Matt Braley]

Opening the throttle helps it pump up faster but if you take it for a good spin it will get there. A cold engine should test fine.