Motorcycle Repair: CB77 too much advance, pivot pins, cam shaft

Question
I cannot set my timing as per your book (which is awesome!)

If I set the timing at F or even T, then at full advance I am well past the two marks.  If I set it back to not exceed the marks, then it is left of T and will barely start and run poorly at low speed.

It also bogs down off the line badly unless the timing is at least advanced well past F, which again leaves me way past the two marks at full advance.

The top end is new, the advance springs replaced, sprocket weights have black rubber intact.

Electronic iginition.  Stock carbs, pistons, cam, etc.

I suspect its the advance sprocket, but it seems to spec.  Is there any kind of fixed advancer that can be fitted?  I see reference to one option in the book.

Thanks for all you do to keep these bikes on the road!

Answer
Jon, did you set the cam timing correctly? It is easy to get it off about a half-tooth and the cam timing runs late. I have found that if you loosen all the rocker arm tappet screws first, then tighten the right side screws by hand, just enough to lock the camshaft/sprocket in the dead horizontal position, then the cam chain will generally line up okay and cam timing is correct.

Check your compression readings. If they are down about 150-160 on a fresh top end, then the cam timing is late. Normal compression readings should be right around 175 psi.

The other factor that can affect the ignition timing is a loose point cam shaft, which runs inside the right camshaft. If you have side play, often around .005-.007" the wobble sets up some ignition timing problems.

Sometimes the camsprocket advancer weight pivot pins wear, as well as the holes in the weights, so there is some extra slop in the whole system which increases the total spark advance excessively. Make sure that the sprocket, itself, isn't loose on the rivets which hold the whole assembly together. I assume you have a CB advancer and not one from a CA installed. The length of the weights is longer on the CAs, then the CB series to increase the advance curve speed, along with lighter springs.

I have found that #140 main jets help perk up these bikes, as well. I had 100 of them made up at Keyster and have a few left for sale, if you need them. Put the needles at the #4 slot, too. I think I have mine all the way up in #5, but it depends on your altitude and humidity, etc.

Bill Silver