I had originally thought that if my motorcycle was running on both cylinders, then I shouldn’t need to manipulate the ignition timing. If both cylinders were firing, what could possibly need to change?
I could not have been more wrong. After bringing my ignition timing and points clearance into spec, the CB350 runs much much better. It purrs through first and second smoothly instead of rattling anemically.
Here’s what I’ve learned: A motorcycle engine works by tightly controlling a combustion process. The precise timing of delivery of fuel, air, and spark determines how the engine will run. The electricity for the spark is provided via two electrically-conductive strips of metal that control the opening and closing of the ignition circuit. These strips are placed within a few hundredths of a millimeter of a slightly-oblong metal ring that sits on the end of the camshaft. This ring is called the points cam. The strips are called the points. As the engine turns, a chain on the crankshaft attaches to the camshaft, turning it and the points cam on the end of it.
As the oblong end of the points cam spins, it will briefly brush into each of the points. This (or more specifically, the moment when it STOPS touching each of the points) causes a spark to jump across the gap at the other end of the circuit connected to that point. You guessed it, that’s the spark plug.
There’s also a mechanical device called the spark advancer that works only at higher engine speeds, causing the spark arc to take place a little earlier in each rotation of the engine. The spark advancer spins with camshaft, and has weights attached to it that move outward in proportion to centrifugal force and are returned to their original position by springs when engine speed falls . The weights put pressure on the points and cause timing to be altered. This component is necessary because there is only a limited amount of time to deliver and mix air and fuel in the cylinder before the exhaust stroke begins, and higher engine speeds necessarily shorten that period of time so the spark must be delivered earlier to provide better combustion, or in other words, the spark timing must be advanced.
Adjusting ignition timing should really be done at the same time that you adjust valve clearance. Both adjustments require you to drain the oil from the crankcase, remove the stator cover, and manually turn the crankshaft with a wrench. Both adjustments involve changes to components underneath the points cover. Also, you should really do valve and spark adjustments before messing with the carburetion and exhaust of the engine,because if the engine doesn’t breathe and spark properly, then no amount of other tuning will make it run properly.
ANYWAY. the actual operation took about two hours total. Measurement of the points clearance was simple. The factory service manual says to measure at the point where the gap is greatest, but it doesn’t say at what point on the crank that should be. I just left my 0.30mm feeler gauge at the gap and turned the crank until it went in, and then kept re-inserting it while turning the crank to determine when the gap interval was over. Looks like the greatest gap is about 90 degrees after firing. Both sides were within 0.3-0.4mm.
I then figured out how to connect my timing light to the points, and used it to determine that each spark plug has a complete circuit until about 60 degrees in advance of the “F” mark on its cylinder, and then the circuit is broken. The light comes back on (spark!) around the F mark.
I found that my left cylinder was about advanced by about the same amount of crank distance as between spark fire and top-dead-center. This is probably about 15 degrees. The right cylinder was retarded by the same amount, which makes sense. I adjusted the breaker plate using a screwdriver while holding the crank at F, and then checked again at LF. All in line.
I put the stator cover back on, filled the case with oil, and took the bike around the block. Man, what a difference. So smooth, so much more power.
I realize that I forgot to re-check my points clearance again after adjusting the spark timing. Because I put some Marvel Mystery Oil in this fill-up to see if it helps with the oil burning issue on the left cylinder, I’ll be changing the oil again soon anyway, and I’ll just re-check them again at that time.