74 CB360 - Horrible compression after rebuild

A

AleCb

New Member
Hey guys, so i picked up this bike from a salvage yard for $250 a couple months ago. Did a compression test before the teardown and both cylinders were 125. I had a shop install new intake and exhaust valves, valve seals, hone the cylinders, and set the piston ring gap (new rings). I assembled everything (new gaskets, and oil seals) then did a compression test, both cylinders read 35psi. This was a cold engine, choke in the run position, throttle wide open, and no header pipes on. I put on a header pipe the did the compression test again. With the header on it raised it up to 40. Then i put some oil in the cylinder and it still read 40. I tried flipping the choke just in case i had it wrong but it didn't change anything.

Any ideas whats wrong with it? Could i have timed the camshaft to sprocket wrong? I dont know, im so lost haha. Ive read a couple places that if you cant hold your finger on the plug hole while kicking it, compression is normally good. When i do this, my finger gets shot off pretty violently, it seems like its fine but the tester is telling me otherwise. I even rented another tester from another store but i got the same readings. Any ideas?
 
If pressure didn't increase with oil in the cylinders, that would indicate something with the valve train. Clearances too tight? Valves not seating properly?
 
noexit said:
If pressure didn't increase with oil in the cylinders, that would indicate something with the valve train. Clearances too tight? Valves not seating properly?
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Im pretty sure I got the valve gap set correctly, I set it, checked it, then checked it again and it was spot on. I'll adjust it again to make sure it is set right, then I'll do a compression test again. Any other ideas ?
 
AlphaDogChoppers said:
Holding the throttle open doesn't do anything unless you also hold the slides open.
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I could be wrong, but doesn't holding the throttle open, open the slides?
 
I could be wrong, but doesn't holding the throttle open, open the slides?
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Not on CV carbs. Their slides are opened by vacuum. Still, holding the throttle open will allow more air in than not. I would still do it.
 
Try a different compression tester. if it's $20 harbor freight one in a red case throw it away or get you cash back. We went through 3 of those.

If it's a good one, try it on a different bike to see what that shows. Adding oil is to check the rings but enough oil in there will raise compression just because of the space it takes up.

Normally, the fact that you tried two gauges should confirm that it's the bike and not the gauge, but I'd try it on a different bike just to check it. Oil or garbage in the hose will cause low readings and so will a leaky or sticky valve in the hose.

At that stage I'd check valve timing. I'd expect something to bend if it was that far off, but something is way out if that reading is accurate. Pop the head cover off and check the cam sprocket position in relation to the crank and make sure you are checking the correct side. At top Dead Center the valves on that side should both be slightly open (overlap) or both fully closed (ignition cycle).
 
Did you set the ring gap to 120 degree spacing on the pistons before putting it together or did you line the gaps up?
 
I just checked the valve clearances, one was slightly tight, readjusted and did another compression test. Still around 35. I guess its time to pull the valve cover off.
 
was thinking if it was the gasket he would probably hear it fanny farting....
 
If you had a shop do all of that work I'd take it to them and make them sort it out before you dig in to deep and they blame you for it.
 
You said new valves were installed. Did they recut the valve seats and lap everything in or just drop in new valves? If the valves arent in the seats you'll get poor compression.
 
No matter how the compression was one, it's really low.
Did you 'kick vigorously' at least 5~6 times or until needle on gauge stopped rising?
BTW, CV carbs, the slides never close completely there is always a pretty substantial gap under them, even when closed
OP stated he put it together, it's probably going to be quicker to just take it apart gain and check for damage
 
tobydmv said:
You said new valves were installed. Did they recut the valve seats and lap everything in or just drop in new valves? If the valves arent in the seats you'll get poor compression.
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Lapping isn't really done anymore. If your machinist is lapping your valves, they're doing it wrong.
 
True, I just couldn't be bothered to argue about it.
Personally, I haven't lapped valves for 20+yrs
 
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