'74 CB450 Low Compression in Right Cylinder

[Finnigan]

I'm sort of stumped. This is my 4th 450, "all original" and I was told the engine was never out of the frame with 4k on the clock. Since I purchased it was very hard to start but ran strong when it got warm. I check and found that the right cylinder read 75psi while the left was 120 (carbs and exhaust off the bike). After adjusting the valves meticulously the values didn't change much. A quick squirt of oil in each spark plug hole and both cylinders went up to 180-190psi. Ok, time for a top end refresh. Dissasembly was a quick and easy, everything looked good mechanically and according to the service manual the cylinder taper and out of round are in spec AND the ring gap for all 6 rings is right where it should be. To check my own work I wen't back and repeated the procedure making sure to measure in the respective cylinder using an inverted piston to put the rings in the same spot each time, almost exactly the same values with 5 thou diff on one measurement. On a side note there isn't much to see inside the cylinder walls, they look pretty good.

So I'm confused, if the oil trick worked I assume there was blow-by (I think that's the correct term) past the rings meaning they were out of spec and needed replacing. Is there something else I should be checking for?

 

[irk miller]

Did you check gap just at the top of the cylinder, or all the way down to the bottom of the stroke? No scoring? Rings clocked?

 

[teazer]

Adding a small amount of oil is usually a way to detect a worn bore/rings but if they are OK have you leak tested the valves?

If the head is still off, turn it upside down and pour kerosene into the combustion chambers and watch for leaks out of the ports - obviously with the valves closed....

If the head is back on, try adding compressed air to the motor through a plug hole and listen for air seeping out of ports of into the crankcases. A leakdown tester (4 stroke type) would be even better but you can use a compression test connector and plug it into an air line if it has a suitable snap in connector. You don't need 100psi. 20 or so should be enough to hear the source of leaks. And keep fingers out of the way because air pressure can spin the motor over pretty fast.

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[Finnigan]

Did you check gap just at the top of the cylinder, or all the way down to the bottom of the stroke? No scoring? Rings clocked?

Only one location, about 1/3 of the way down the whole piston. I figured if my taper was within spec, measured in 3 different places, it would be representative. I might try more locations tonight.

 

[Finnigan]

Adding a small amount of oil is usually a way to detect a worn bore/rings but if they are OK have you leak tested the valves?

If the head is still off, turn it upside down and pour kerosene into the combustion chambers and watch for leaks out of the ports - obviously with the valves closed....

If the head is back on, try adding compressed air to the motor through a plug hole and listen for air seeping out of ports of into the crankcases. A leakdown tester (4 stroke type) would be even better but you can use a compression test connector and plug it into an air line if it has a suitable snap in connector. You don't need 100psi. 20 or so should be enough to hear the source of leaks. And keep fingers out of the way because air pressure can spin the motor over pretty fast.

This is the one I use but there are others

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Everything is off still, I pulled the torsion bars and valves out to check for pitting or if anything was obvious. I can reassemble and put rubbing alcohol to check as I don't keep Kerosene in my garage, I've used that in the past.

I was trying to figure out if oil on the pistons could have gotten pushed onto the valve seats and sealed those, making me think it was a ring problem but really was just valves not seating.

 

[irk miller]

Glaze and carbon can do it. Scoring on the cylinder wall and/or ring can do it. Carbon in the ring grooves can cause a stuck ring and do it. It's also possible that oil could seal a leaking head gasket.

 

[Finnigan]

There is definitely carbon on the valves and in the ring grooves on the pistons. I guess the plan is to clean and inspect everything as well as possible, replace seals and gaskets and maybe use some gasket sealer rather than putting them in dry.

Fingers crossed, thanks for the help.

 

[Finnigan]

In anticipation for the rebuild I ordered everything I thought I might need including a 70mm cylinder hone (FlexHone) I don't think its needed but should I do it anyway?