compression
- Thread starter nfusion
- Start date
J
Jack Klarich
Guest
While this is on the low side it is consistent, if the bike runs good and does not use oil I would ride it til it NEEDS a top end job JMO After you oil the cylinders and confirm there is NO change in compression
he gets off work later this day we will try the oil and I will report back our findings and I thank everyone for there assistance
dolt
Well-Known Member
The standard 95" BB kit includes the SE203 cams and cast flat top pistons. At sea level and with good ring seal and a fresh top end, the CCP should be about 170psi. If the readings are close from cylinder to cylinder, that means that the cylinder wear has been consistent. A difference of 125psi to 170psi would be enough for me to pull the cylinders and hone to fit some .010" pistons, or, better yet, bore to 3.932" and increase displacement to 98".
When checking compression on a CV carbureted motor, I pull the carb out of the boot to get an accurate reading. As BigAl66 points out, the slide is vacuum operated and will not move without motor running; it needs engine vacuum to operate. Holding the throttle open without the motor running simply opens the butterfly but not the slide. I would pull the carb from the boot and check compression again to confirm the 125psi reading.
The next step would be to follow with a leakdown test which will tell much more about the condition of the top end. JMHO
When checking compression on a CV carbureted motor, I pull the carb out of the boot to get an accurate reading. As BigAl66 points out, the slide is vacuum operated and will not move without motor running; it needs engine vacuum to operate. Holding the throttle open without the motor running simply opens the butterfly but not the slide. I would pull the carb from the boot and check compression again to confirm the 125psi reading.
The next step would be to follow with a leakdown test which will tell much more about the condition of the top end. JMHO
dolt
Well-Known Member
Calculated 170 vs actual 150 isn't so bad; a little north of 10% variance. I have yet to put a motor together and have the actual CCP be dead on with the calculated CCP. So, if you are looking for a reason to freshen up the top end, if she's not burning oil (as Jack suggested), still start and idles properly, has good oil pressure, still has adequate power and no blow by, she's got a few more miles to go before a fresh top end is necessary.