Low compression after rebuild.

[bakaracer]

Ok. I spent the day trying everything. I pulled the head and measured the bore. book says no bigger than 1.574. Mine is 1.579. Sorry, I don't believe .002 is giving me enough blow by that it won't even kick. I got 30psi on the gauge WET. How ever, I did order another head and cylinder. Gaskets are all good. Everything was tight.

And I looked and looked. The rings did not have any markings for top or bottom. The service manual doesn't mention it either.

check the ring end gap!!!!! measuring the bore diameter from the top is not going to give you the right measurement. all bores taper narrow at the top and wider at the bottom. follow the manual on how to check ring end gap. you then have to check the clearance of the piston to wall clearance. How much did you port the exhaust port? what direction did you port it? check the reed for cracks.

[motormike]

You say that you didn't expect .002 to be critical.......yet your stated measurements indicate a difference of .005......measurements should be taken of the cylinder at several varied locations in the cylinder.....top, bottom, left to right, front to rear.................

[tom waite]

check the ring end gap!!!!! measuring the bore diameter from the top is not going to give you the right measurement. all bores taper narrow at the top and wider at the bottom. follow the manual on how to check ring end gap. you then have to check the clearance of the piston to wall clearance. How much did you port the exhaust port? what direction did you port it? check the reed for cracks.

I will check the ring end gap next. I measured the bore with bore indicator in several different spots. It all seemed consistent. I ported the exhaust port towards the deck about 1/8". I checked the reed during rebuild and it was perfect.

You say that you didn't expect .002 to be critical.......yet your stated measurements indicate a difference of .005......measurements should be taken of the cylinder at several varied locations in the cylinder.....top, bottom, left to right, front to rear.................

Yes. .005. Sorry. I typed the wrong number. Either way, I don't think that much would give me less than 30psi.

[bakaracer]

if you ported it 1/8 in towards the deck, you can pretty much toss the bore in the garbage. Thats about 4.5mm you ported which puts that exhaust port way out of even being riden. if you ported it about 1mm then that would have be better but 4.5mm, that bore is not going to work. it may start but your lucky if the bike moves. thats where all your compression is going. on the up stroke pretty much all the air fuel mixture was lost out the exhaust port from being open to long.

[Wheelman-111]

Greetings:

Uhhh, photography painting and Maff are hardly my strong suits, but 25.4/8 is 3.175, not 4.5mm.

But 3mm "Porting" is still way too much and I agree with everything else Baka-san said.

[bakaracer]

Greetings:

Uhhh, photography painting and Maff are hardly my strong suits, but 25.4/8 is 3.175, not 4.5mm.

But 3mm "Porting" is still way too much and I agree with everything else Baka-san said.

close enough

[motormike]

I just learned that 25.4 mm= 1 inch.........I'm embarassed i didn't already know.............

[richdavison34]

As an engineering student, I can't help but love Maff jokes.

I have a favorite shirt that I wear on test days.

It reads......

If Calculus was easy, it'd be your mom.

[tom waite]

if you ported it 1/8 in towards the deck, you can pretty much toss the bore in the garbage. Thats about 4.5mm you ported which puts that exhaust port way out of even being riden. if you ported it about 1mm then that would have be better but 4.5mm, that bore is not going to work. it may start but your lucky if the bike moves. thats where all your compression is going. on the up stroke pretty much all the air fuel mixture was lost out the exhaust port from being open to long.

This was my original concern so I ordered a new cylinder before anything. It came in yesterday. Installing today.