Honda’s factory service manual (FSM) lists as 150-170psi as within specification for cylinder compression. I did some measurements when I first got the bike and found the right side firing at 170psi and left side at 119psi when warm. I let the bike sit for a week with Seafoam Deep Creep in the cylinders, topping them up every couple of days. I got the bike up to running temperature and did another measurement.
Right: 165. Left: 119psi.
Per the FSM, I added a tablespoon of motor oil to the cylinder with low compression and re-tested. I got the same result: 119psi. This indicates that the compression issues were caused by something in the head of the engine and not in the piston rings. Possible causes include the head gasket, valve timing, clearance, or seals. I didn’t see any leaks around the head gasket, so I decided to try my luck with adjusting the valve clearance. I didn’t see any reason to check the right cylinder because it was running well already. Don’t fix it if it ain’t broke.
There’s three things you have to do in this operation: Hold the crank at top dead center (TDC) for the cylinder you’re examining (“LT” mark on the generator rotor) with one hand, and with the other hand alternate between slightly adjusting the tappet clearance adjuster screws and checking the rocker arm clearance with feeler gauges.
This is harder than it seems because the appropriate clearance is measured in thousandths of an inch, and even a couple thousands off will cause problems. The adjustment screws only need to be turned a thousandth of an inch in order to translate that change to the rocker arms. It took about an 30 minutes for me to adjust each of the two screws. That’s half an hour sitting on the ground, resisting the inertia of the cylinder falling from TDC, trying to turn a flathead screw a thousandth of an inch and accidentally going a few hundredths in the wrong direction each time. Ugh.
Once I got the valve clearances into spec, I did a quick compression test on the cylinder and read 155psi, up from 119psi! That’s with the carburetors off the engine, so the engine was cold. With the engine warm, it’s likely that that number would be a bit higher. Either way, the cylinder is now in spec for compression.