Motorcycle Repair: honda cb100, ebay auctions, stock muffler

Question
hi bill.  i'm thinking of buying a cb 100, either 1973 or 1975--there are two
that i'm looking at.  i'm new to bikes, and especially to something this old and
wondering if getting parts will be a problem in the future.  mostly i'm
interested in these bikes because they're pretty cheap, because i assume
they'll be easy to work on (i'd like to learn) and because they get great gas
mileage.  any thoughts?  thanks!  daniel smith

Answer
Daniel,  The early CB/CL/SL100-125s had one-piece cylinder heads which gave problems locating their camshafts properly. Excessive clearance builds up in the cam bearings which are incomplete on these versions (changed in 1976 to a two piece head). If you find a low miles bike that has really be cared for, it may be okay for a while. Grabbing the end of the camshaft by the spark advancer retainer bolt and moving it side to side will reveal the amount of sideplay in the cylinder head/camshaft interface. Loose cams equal ignition timing irregularities because the ignition points cam is on the end of the camshaft. Any wobble causes the point gap to change, which changes the ignition timing.... not a good thing.

There are a lot of engine spares on Ebay auctions coming from Asia. The plastic side covers seem to be scarce due to their tendency to become brittle, which causes cracks and the locating pins to break off. Otherwise, the basic design is pretty sturdy. For a street-bike the CB125 is better as they had disc brakes for a few years. Many of the engine parts interchange between the 100 and 125s, however the wrist pin size is smaller on the 100s, so you can't easy bore the engine out to 125 without using a special piston with smaller wrist pin holes.

The engine series is pretty sturdy overall. I modified a 1974 CB125 into a production roadracer and won the 125cc championship out here in Calif. The engine would turn over 11,000 rpm and top speeds with stock muffler and 24mm carb were into the low 80 mph bracket. As long as I had it tuned properly, it was very reliable. I had to hard-chrome the camshaft journals to lessen the wear issues of the racing cam. POWROLL in OR, has a lot of experience modifying the older engines and can convert the cylinder heads to needle bearing support which solves the loose camshaft issues.

Bill Silver