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Motorcycle Repair/honda cb750 won't start when cold

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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

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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