Motorcycle Repair: Honda CA110: Wont Run, displacement engines, exhaust stroke

Question
I have a 50cc CA110, SN C110-459968. I believe it is a 1968 model. It was not running when I got it, but it was complete. I can't even get it to cough. It's getting fuel and has a good blue spark. Things I've tried: pull starting to spin engine faster (didn't try any ether), taking exhaust off to reduce backpressure, cleaning tank and carburetor (every port), points look clean and begin to open at F mark, plug is gapped about .026", valves working (haven't checked clearance, but no excessive lash). Why does the plug fire on every crank rotation? Is this an emissions reducer for the exhaust stroke? I don't have a battery in the bike. Does there need to be one? I assume not since I have good spark. I have compression. The book says standard is 121 psi. It sure doesn't seem like that much out the spark plug hole, but it's only 50cc so I wouldn't think there'd be much. Even if it was low, I'd think I'd get at least a puff. Your thoughts? Thanks.

Answer
Nik, the valve clearance is .004" (TDC compression stroke) and critical on small displacement engines. Do check it for compression before you pull out all of your hair trying to get it lit off. When these bikes sit for years and years, often the piston rings stick in the ring lands of the pistons, causing big compression losses. If one of the valves was open for a long time, there could be rust built up on the valve seats, causing loss of compression.

For these engines to run you need:
Compression
Spark at the right time (F mark) The ignition is self-powered magneto, which will fire every stroke, once on compression and once on the exhaust stroke (called wasted spark).
Fresh Fuel, properly metered in the idle and main jet circuits.

Magnetos need clean points and a good condenser. You might be having condenser problems, which requires a flywheel puller and a soldering gun to resolder the condenser lead back to the low tension coil. Also be sure that the magnet inside of the flywheel is clean and shiny.

The carburetor idle jet MUST be clean. You may be seeing fuel on the plug from cranking and cranking with the throttle open, which draws fuel from the main jet, but the idle jet needs to be open for low speed running.

The battery is required to run the brake light, horn and neutral light, primarily, but if you run the motor with dead/no battery the charging system can pump excess voltage into the harness, blowing out the light bulbs. The battery acts as a reservoir and damper for the charging system.

Spark plugs can get easily fuel fouled when you are in these situations, so keep a couple of spares on hand.

Due to the pushrod design, you can take the top end off the engine in about 10 minutes, so it might be a good idea to dissect it and see what the innards look like, if you have low compression values.

Bill Silver