Motorcycle Repair: cb77 spark advance, cams and followers, electronic ignition system

Question
Bill,
I have a 1962 cb77 with 22k miles that I have been restoring.

I've had the bike running for a while, but it has always had sluggish off the line performance. I adjusted and synced the carbs, tried both points and electronic ignition. However, I have been unable to synchronize the timing on both cylinders. The timing light always shows one cylinder more retarded/advanced than the other. So, I dismantled the top end to get at the spark advance unit thinking the springs were worn (they were). I found worn cams and followers. The plate on the cam sprocket was brazed in two locations. I've replaced all the parts. However, before putting the motor back in the bike I want to make sure that I have not altered the action of the spark advance unit.

I have heard that originally the pins that locate the weights within the holes that limit travel on the cam sprocket had a band of neoprene on them. There is no neoprene on the three used units I have inspected -- one of which is now in the motor. All three units had a couple mm of slop between the weight and the center boss of the sprocket. I attributed this to the missing neoprene allowing for a greater range of movement. I tightened the springs so that the weights were tensioned against the boss. Was that the right move? Will this adversely effect timing? Also, is it possible to overtighten the spring action?

Thanks for any insight or help you can provide.
Alex  

Answer
Alex, the ends of the weights usually have a layer of rubber bonded to the tips to soften the motion as the weights reach the outer edge of the holes where they travel. When the weights are left to move excessively, the total spark advance will exceed the two II advancer marks on the rotor. The first one is 45 degrees and the second one is 48 degrees advanced. With today's gasolines, I like to keep the total advance closer to the first mark. The electronic ignition system can be fooled a little if the points cam is loose in the end of the camshaft and/or the whole backing plate is a little off center with the centerline of the points cam. The slop between the camshaft and advancer shaft is as important for the electronic ignition system as it is for the points, even though it is magnetically triggered. The cam sprocket does need to be evaluated for both condition of the advancer bar inside, which engages the weights, the condition/fit of the weights, amount of travel determined by the weight end stoppers and the spring tension on the weights. You can overdo the spring tensioning, to the point where the spark advance is coming out later than specified. Normal idle speed is 1,000 rpm and spark timing starts at 1200 rpm. If you put too much tension on the return springs it may impede the spark timing curve to a certain extent, causing a flat spot off idle.

Check your spark timing so that neither side exceeds the advance marks. That is the most important parameter. If one side is at the second mark and the other is at the first one, then that is probably about as close as you can get it. Check the amount of shaft play for the advancer shaft fit in the camshaft end. Hopefully, you have the correct shaft in the camshaft. There are two different shoulder sizes for the advancer shaft. I recently opened up a motor that had the big hole/small shaft mixed together. There was so much sideplay that the engine wouldn't run.

I have restoration guide CDs available from my website.

Bill Silver
www.vintagehonda.com