Motorcycle Repair: honda cb750 wont start when cold, honda cb 750, honda cb 750 kz

Question
I wrote to you in October regarding my 1979 Honda cb 750 kz dohc, since then I have checked everything you said and bike still won't start.
I have put new coils, ht leads and caps on, ignition timing is ok, Compression on all 4 cylinders is 135 psi, plugs are getting wet so petrol is getting through, choke is working ok, valve timing is ok, I have taken the air box off and cleaned out the carbs again. I have put the carbs back on without the air box, battery is good, turns over ok but still won't start. It does not even fire once. I have noticed that petrol is spitting out of the carbs sometimes, especially No 1 carb but I assume the valves are seating ok because the compression seems ok. I am baffled by this and the next step for me seems to be to take off the head and check the valve seating but is this necessary if the compression is reading 135 psi? What do you think? Dave.

Answer
David,

Have you observed the spark intensity at the plugs? Are you aware of the "spark units" which control the spark timing and advance? These degrade over time and can fail, although they seldom go down at the same time.

I don't know if you know the repair history of the bike/engine, but 135 psi readings across the board are about 40 psi too low. Honda specs are normally about 175 psi for most 4 cylinder machines. When all compression readings are the same, but equally low the indication is that the camshaft timing is off one tooth. Normally, valve clearances are checked and adjusted one valve at a time, using a removal tool to get the shims out and back in again. Someone may have not had access to the valve shim tool and removed the cams to change the shims. Bit of a stretch to consider that, but it is a possibility. I would certainly check all your valve clearances and the actual camshaft timing. With the valve cover off, you can also check the condition of the tensioners and adjust them as necessary.

http://www.repairthatmotorcycle.com/Specifications/honda-cb-750.html

I would consider testing the spark with fresh spark plugs, as yours probably are fuel-fouled now. But, my primary concern is your low compression readings. DO check the compression with the throttle held wide open, KILL switch in the OFF Position for most accurate readings.

Bill Silver