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Discussion Starter · #1 ·
I have this 1974 CB450 which I reassembled last fall. Bored cylinders, new pistons, new rings and a professional valve job. Afterward I found that the left cylinder compression is like 170 and the right is about 80. I can't figure out why. Valves were all adjusted to spec. New gaskets and rings correctly spaced. Would a missing cylinder stud gasket affect this or are those more about oil control?
 

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Do a leak down test. Many compression tester fittings will connect to an air compressor connection. You'll be able to tell where the air is going.
 

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Assuming that's what you mean, a missing base gasket would increase compression, but could risk oiling losses to valve train and cams.....

Compression is all about the sealing within the cylinder and compression dome.......
(piston size and rings to bore fit, ring end gaps, valve sealing at closure, spark-plug sealing to head, and of course head gasket sealing head to the cylinder)...

Quick possibilities: warped or improperly torqued head/damaged head gasket..... rings upside-down or wrong ring order, or sticking in piston grooves......bore taper incorrect..... bore diameter incorrect....cross-hatching too deep.....Throttle not opened fully during right side test (gotta have enough to compress)....Or any of the previous.....
 

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Would a missing cylinder stud gasket affect this or are those more about oil control?
if you mean one of the flat o-rings on the 2 right hand studs, no, they are there to seal the oil passage up those 2 studs to the top end and would have no affect on your compression
 

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Discussion Starter · #5 ·
I'm finally back to working on this bike after taking a year off following some medical stuff. Anyway we pulled the head......again. Flipped it over and filled the combustion chamber with mineral spirits with the valves all closed. Right exhaust leaked out immediately. I switched right and left and now it's the left that immediately leaked out. Bent valve. Got new valves and as I'm installing I'm starting to wonder. Did the boring and bigger pistons hit the valve. If you read the entire post the low compression was on the left. I believe the shop switched the valves. Is it possible or is that only possible with the timing being off? Would you simply reinstall and test or would you double the base gaskets under the cylinders just for extra clearance?
 

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My guess is the valve may have gotten bent during the top end reassembly if the cam timing was off a tooth on one cam and the engine was turned over briskly before realizing it. It doesn't take much to bend a valve a tiny bit, and the shop that switched the valves didn't look for it, didn't leak test afterward, just gave the valves the eyeball and assumed both were fine, then switched sides inadvertently so your problem moved.
 

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Discussion Starter · #7 ·
My guess is the valve may have gotten bent during the top end reassembly if the cam timing was off a tooth on one cam and the engine was turned over briskly before realizing it. It doesn't take much to bend a valve a tiny bit, and the shop that switched the valves didn't look for it, didn't leak test afterward, just gave the valves the eyeball and assumed both were fine, then switched sides inadvertently so your problem moved.
Thanks for commenting. This shop was the only one around and had, I'm guessing, about 40 or 50 bikes sitting around fixed, being fixed or waiting to be fixed. He went out of business last year. I'm starting to realize why.
 

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Thanks for commenting. This shop was the only one around and had, I'm guessing, about 40 or 50 bikes sitting around fixed, being fixed or waiting to be fixed. He went out of business last year. I'm starting to realize why.
Sorry to hear it.
 
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