Need help with 760 compression

[jhall752]

I recently bought a superjet so here is what it has. 62t case 64x topend, b pipe. I just switched to duel 38’s from duel 44’s and I just put a new ADA head with 38cc domes.

I just checked compression and I have 175 in the front and 145 in the back. The pistons and sleeves look really clean.

I am stumped on how my compression is so different. Anyone have any ideas on what I can look into to get them even?

 

[kevbo]

Either you have a damaged bore/stuck ring or the ADA head isn't sealing. Its easy to tell by the pisser stream. Steady= no leak, broken=head not sealing to cylinder. BTW Yamahas can be difficult to seal without a gasket. Sometimes you have to resurface the deck and shim the domes.

 

[maxboost]

Make sure you got the same domes, I’d measure them. Seems like I recall couple people saying they were marked wrong in the past

 

[jhall752]

Make sure you got the same domes, I’d measure them. Seems like I recall couple people saying they were marked wrong in the past

How do I measure the domes?

 

[maxboost]

How do I measure the domes?

Find something you can measure cc’s like a syringe or measuring cup, set the domes upside down with the spark plugs in them and measure how much fluid it holds and compare to the other one

 

[bkspeedwerks]

If you cant measure the domes, switch them front to back and if the low compression follows the dome you know you either have a differnt cc dome or that dome is not sealing to the deck.

 

[Harbor]

Pull the head and check the oring on the dome, prolly slipped out... needs to be installed with grease in the grove and on oring to help it stay put while installing it

 

[jhall752]

Don’t the 760 64x usually run different compression originally. I remember reading that somewhere. Or was that just based on tHe stock head?

 

[bored&stroked]

Don’t the 760 64x usually run different compression originally. I remember reading that somewhere. Or was that just based on tHe stock head?

That's based on the oem head nothing else.