1977 CB750 Super Sport Need Help Diagnose

[Daddyoo]

1977 Honda CB750F (Super Sport)

The bike has been sitting for several years—part of that time outdoors—but is now garage-kept and in generally decent “toodle-around” condition.

I’ve been working on getting it running properly for the past six months. It does start and run; however, after about five minutes, it begins to emit smoke from the exhaust. Upon inspection, I noticed that cylinder #2’s intake manifold doesn’t get hot like the others.

• Spark is good on all cylinders (1–4).
• Compression readings: #2 at 120 PSI; the others between 115–120 PSI.
• A borescope inspection of cylinder #2 showed minor pitting on the wall, which I was told is not serious.

When I cover the throttle body intake on cylinder #2 while the engine is running, there’s little to no suction and no change in idle speed. In contrast, covering the other three carburetor intakes causes the engine to bog down or stall as expected.

The carburetors were professionally rebuilt ($550) , with extra special attention to carb #2. Bike runs extremely smooth now after the rebuild. However, the same issues persist—no suction at the #2 intake and smoke (bluish-white) from the exhaust.

I’d greatly appreciate any diagnostic advice or insights from anyone who has experienced similar issues. Specifically, I’m looking for guidance on what to check next to get cylinder #2 firing properly.

Thank you for any help or suggestions.

 

[Loki]

I too have a 77F1 which had been left for a long time in a damp shed, long story short when I took off the head for a top overhaul as I had an oil leak from the head gasket I discovered a badly corroded valve seat which fortunately could be recovered with a recut. Guessing this valve was in the open position during storage hence the corrosion on the seat, all the other valves were easily lapped in with fine grinding paste without any trouble.

Loki

 

[dirtdigger]

Check valve clearances especially the non running cylinder. I would be doing a leakdown also. pitting is probably causing the smoke but never know. The supersport engines are known to where out the valve guides, especially the exhaust. If number 2 exhaust isnt hot then its not firing. If you have spark and compression then you are missing fuel. Just because the carbs are rebuilt doesnt mean it didnt suck up some debri and plug the jets. Spray some carb clean in cylinder 2 see if the engine picks up. If it was my bike I would be pulling carbs and checking/cleaning jets first after verifying valve clearances, leakdown test, etc. How does the spark plug look?

 

[hinksy]

My 77 F2 had been last tagged in 92 , it sat for a while , 32 yrs . There is a good chance the valves could look like mine did . I also bought a brand new 77 in australia when they came out and the valve guides failed within 3 mths . Seems it was a common problem on this model