Compression imbalance after rebuid

[Adidasandy]

How long does it take to run in a full Yamaha 701 rebuild?

1mm over sized rebore and pistons, new gaskets, seals etc

Ran 15ltrs max through it today varying the speed. Re torqued head bolts and girdled head bolts with very slight re-torque needed.

Brought it home to check it over.

Carried out comp test with good gauge. Wide open throttle, 1 plug in and held stop button whilst cranking. Rear achieved 200psi and the from 175psi (running a AM head with 33 domes)

Is this due to the running in period to have the rings mated correctly?

 

[SXIPro]

Did you try it with both plugs out just for giggles?

 

[Adidasandy]

Did you try it with both plugs out just for giggles?

Nope. Always done it this way.

 

[Mike W]

Nope. Always done it this way.

Try it with both plugs out. You get faster cranking that way. And make sure your gauge fitting is tight and the O ring on the fitting is getting a good seal.

 

[Rushford_Ripper]

Stuck piston ring?

 

[Storbeck]

That amount of imbalance is not caused by break in (really no imbalance should be caused by break in), you either have something wrong with your measurement or something wrong with your motor.

If you have an aftermarket head with o-rings, I would pull it off and visually check for damage, then swap domes and see if the change in compression follows the domes.

 

[iQRTurbo]

Piston isn’t in backwards is it?
Ring on exhaust port= no bueno

 

[Quinc]

May need to replace the oring on the dome.

 

[Adidasandy]

Is this the engine from the Blaster that had a several air leaks?

Yes

 

[Adidasandy]

That amount of imbalance is not caused by break in (really no imbalance should be caused by break in), you either have something wrong with your measurement or something wrong with your motor.

If you have an aftermarket head with o-rings, I would pull it off and visually check for damage, then swap domes and see if the change in compression follows the domes.

New domes but will remove head today to inspect

 

[Adidasandy]

May need to replace the oring on the dome.

Brand new, will recheck

 

[Adidasandy]

Piston isn’t in backwards is it?
Ring on exhaust port= no bueno

No they in correct mate

 

[Storbeck]

New domes but will remove head today to inspect

I meant visually check the cylinder, like if it has a big scratch that's fairly obviously the problem.

 

[Adidasandy]

You did do an Air Leakdown at 8psi for 10 minutes, not 2 minutes, 10 MINUTES right?

Yeah I did. It did drop down to 6psi (lost 2 psi)after 8 mins.

 

[Adidasandy]

I meant visually check the cylinder, like if it has a big scratch that's fairly obviously the problem.

No scores or scratches. Dome O ring in cylinder 1 screwed. Must have caught it on install I guess.

 

[Rushford_Ripper]

Cracked cylinder? Girdled head?

 

[Adidasandy]

Cracked cylinder? Girdled head?

Cylinders checked and bored by SERCO and yeah girdled head.

 

[Adidasandy]

It has to hold 8 psi for 10 minutes, If it's air-tight it will easily hold 8 psi for an hour...

Find the leak...

I'm on it.

 

[Rushford_Ripper]

Ever check for warping? Just trying to rule out simple measures is all.

 

[Adidasandy]

Ever check for warping? Just trying to rule out simple measures is all.

It got done at machine shop but good call.