Motorcycle Repair: 1974 cb 125 was running now not, cam bearings, compression stroke

Question
I bought a CB125 (1974) the other day. Changed the oil, cleaned out the carberator and hotwired the ignition (no key). After a while I got it to run, ran up and down the street a couple times, topped it out and put it back. this morning I did this again, and on the way in I heard a faint schreeching or whizzing sound. A couple backfires and the bike was no longer working. Also I could not get it to crank again. I retightened the spark plug, which was loose, recleaned out the carb and now when i crank it (via running really fast and hitting it into second cause there is no kickstarter lever ) the engine turns over, and sometimes while sputtering over i hear this popping sound. My friend thinks its a backfire but its more clicky and goes on even when I can hear the exhaust.

Im wondering if my cam bearings are going out. How can i test for this? I am very novice and this is my first bike, but forums mentioned these cam bearings. My friends think Im nuts, but after that whizzing, i dunno. The oil in it is a quart of 10 -40. with a splash or two of Seafoam.

Answer
Cody, these engines do have issues with worn cam bearings in the head. You can check the play by turning the engine up to the T mark on the flywheel when the valves are both closed (compression stroke) and then grab the little bolt on the end of the camshaft that hold the spark advancer in place, then try moving the end of the cam side-to-side/up-down to see how much play there is. Any play in the camshaft equates to changes in the point gap, which changes the ignition timing.

It sounds like something has seized inside. The piston might have seized if the ignition timing was too far advanced and/or the carb jetting was too lean.

If the camchain tension is too loose, the timing chain might have jumped a tooth on the sprocket. Best thing to do is to turn the motor over to the T mark again (compression stroke) and then check the valve clearances, by removing the valve/tappet covers and sticking a .002" feeler gauge in the space between the valve stem and the rocker arm. If you can't feel a bit of play on the rocker arms, then the valve clearances are too tight and the valves are not closing enough to build up good compression. Compression checked with a gauge should be about 160 psi.

Here is what stuff looks like inside the engine...

http://www.cmsnl.com/honda-cb125s-s1-usa_model411/partslist/

Please be aware that this engine's ignition system is powered by a 6v battery. If you have a dead/dying battery in place, the voltage to the ignition system is borderline and often causes misfiring and stalling at idle. The charging system is a permanent magnet type, so if you spin the motor up fast enough it will start to generate some excess voltage to the system, but the battery is the storage point for the voltage and keeps the charging system from over charging and burning out light bulbs and other electrical components.

Be sure that the point faces are clean, that the gap is set to be about .012" maximum on this engine and the points just OPEN at the F mark alignment on the rotor, just before the engine comes up on the compression stroke again (T mark).

For the engine to run, you have to have compression, fuel/air at the right mixture, spark at the right time and no mechanical tie-ups internally.

Bill Silver