Motorcycle Repair/Honda CB77 Valve timing
QUESTION: I have set up the timing on my 1961 CB77 as per the Honda manual IE, right piston at TDC ('T' mark lined up on rotor )dot on cam sprocket and line on points cam both at 12 oc. The problem is both inlet and exhaust valves are still slightly open so should I undo the tappets so the valves are closed and then set the tappets to the correct clearance or is there something else wrong ?
Hope you can help.
ANSWER: Rob, the cam timing setup is on right side overlap stroke, NOT compression stroke, which would seem more intuitive. Both valves are slightly open at TDC overlap stroke. The intake valve opens 5 degrees earlier so the split overlap comes up away from the T mark if you left the valves adjusted and used the camlobe centers to find themselves equally. You don't want that to happen.
What I do is loosen all the valve adjusters, then use the right side adjusters to center the camshaft on the lobes using the rocker arms. Just turn them down finger-tight until the camshaft flats are dead even with the upper cylinder head surface. With the cam held in place that way, you can string the camchain over the sprocket and secure. If it seems to be "too short" use a few flat washers and nuts to pull the head downwards in a few spots to compress the gasket and secure the cylinders and head in a full down position. This should allow you to connect the camchain link and then you can finish up the assembly. Don't forget to adjust the valves back to .004" on the appropriate compression strokes.
I hope that clears up your questions and concerns about the CB77. I have a 1961 CB77 here, as well. Great, unique bikes!
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QUESTION: Hi Bill,
I still have a problem with the timing on my CB77,I have set the timing on the overlap stroke as described above but when I rotate the engine to check the left side the timing marks on the cam lobe and sprocket are at 3oc (1/4 of a turn out) with the piston at TDC.I have removed the chain taken the head of and reset the cams reassembled with timing marks all lined up but still cant get the left hand side to line up, what have I got wrong?
PS. Do you cover the timing of the CB77 in your books, if so which one and how can I get A copy ?
Rob, I have a CD full of PDF files which can be ordered from my website contact page.
You are overthinking the cam timing setup. You only have to set up the right side at TDC overlap.
This is a 180 degree firing engine, not a Brit 360 twin. It doesn't fire every stroke. Right side fires, then left, then right side coasts, left side coasts, then starts over again. There ARE rare 360 firing Type 2 engines in the 305 range, but are seldom seen in anything other than CYP77 Police models for the most part.
Just set the right side cam timing as suggested and the rest takes care of itself. When you rotate the crankshaft away from the RH side timing setup the left side is also going to be on overlap stroke. Next half turn puts the RH side on Compression, followed by LH side compression strokes on one more half turn.
When installing the right side camshaft, be sure that the points cam tang indexes with the camsprocket so that the index mark on the point cam is at 12 o'clock.