Topic: CB350 Twin engine issues

[NickRafAcosta]

Hi everyone,
Been a while since I've posted anything on the forum but glad to see it's still as strong as ever! Recently finished a top end rebuild on a 1972 Honda CB350 Twin (sent the head and valves out to get cut and re-machined, also cylinders are honed with new pistons and piston rings, standard size). When the engine was back in the frame I did a quick compression test, and bike was showing 175psi on both cylinders, excellent. Now after putting it through some heat cycles and getting the ignition timing spot on, I noticed the left side cylinder was running cold. Put a compression tester on it, now only showing 75psi, and only 100psi on the right side! Needless to say i was quite upset, especially since isn't my first rodeo since I've done tons of engine rebuilds. The weird part is, I next did a leak down test to see where I was losing compression, and it was only showing a 4% leak down, almost nothing for an old bike.
Any thoughts or advice would be greatly appreciated!
And if anyone is wondering
- yes cylinder is torqued down to proper 15ftpd in the correct order
- cylinder have been honed, and valves have been inspected, cut, valve seats cut, and then a quick liquid test to ensure no leaks
- cam chain tensioner was set properly as per manual
- Cam timing was spot on during rebuild too, I doubt it's a bent valve otherwise it would show immediately on the leakdown test
- Valve clearances were set to 2 and 4 thou as stated in the manual before the first heat cycle

Needless to say I'm stumped!

- Nick

[scottly]

Are you holding the throttle fully open during the compression test?

[HondaMan]

What Scottly said, and then:
1. test it hot for compression.
2. Add a squirt of oil and crank over a few times, then check it. If there is a lot of difference between the "oiled" compression (like 40+ PSI) then you're dealing with unseated rings. In the 350 this can sometimes correct itself with miles (like 500 of them) although it will struggle to idle well in the meantime. You'll know when/if they seated when it will idle like a putt-putt-putt cadence.

Normally, honing a used bore and adding new pistons and rings will result in a situation where it will never seal in these engines. I've seen that more times than I can remember. A step-bore restores it, and after that you can get away with a hone-rering operation because the bore holes will then stay round as the iron liners are then cured. In the first go-round for the engine it had uncured bores and they are not round anymore, so re-ringing usually doesn't work out well.

[low-side]

Just to clarify, did you re-check torque and valve clearance after a couple heat cycles?  That stuff can loosen up after a couple heat cycles.