CB750. Carbs good Plugs good. Fuel good. Points good. Adjusted Valves (0.002 intake; 0.003 exhaust. Wont start

Question

Bike has been running well. Doing a seasonal tune-up. Discovered plugs were carboned up. Have completed compression test, valve clearances, points inspection, carburetors in good condition. Why won't it start?

Answer 1

Have you tested your fuel relay/sensor? If your sensor and/or relay is bad, your fuel delivery system will not function. I know nothing about the Honda CB's, but I did have a feul relay go bad and have experienced the frustration of tearing apart carbs and everything else trying to figure out why one of my bikes suddenly would not start and stay running.

Answer 2

Your bike does not have a fuel relay and while I can't remember the valve clearances, but your single overhead cam engine is certainly going to be different than the double overhead cam 1100.

What model is your bike?  I had a '69 as well as a '75 F.  I believe the 4 pipe '69 ran 110 main jets and I know the 4 into 1 F came with 105s.  It smoked and sucked gas...got something like 28-32 mpg.  I lowered the jet needles one notch which got the mileage up to around 42-43mpg.  In altitudes above 4000' I had to switch to 100 main jets.  

If your plugs are coverd with dry soot, then you're running a too rich mixture.  If the black stuff is more oily, then you're probably looking at rings or valve stem seals.  If it smokes blue all the time, it's rings if only on start up, valve stem seals.

The 750's had a mediocre ignition system and I had to keep fresh plugs in both of them.  Once I had over 5,000-6,000 miles on them, I'd get high speed missing.  Try brand new plugs and set them to maybe .026".  If it runs, then you'll know what the problem was.  Even if that's not the problem, consider Dyna coils to get a more powerful spark, you'll like them.  

Get a spark new plug, stick it in one of the outside plug wires and lay the metal part of the plug on the engine.  Crank the engine over and you should see a spark, if not then you need to check the ingition system further.

One thing about the points.  Did you file them, check the clearance or otherwise mess with them?  Even the tiniest bit of oil from a feeler gauge will kill your spark.  Use the old trick of closing the points and sticking something like a white business card between the contacts and pull it out.  Keep doing so until the paper stays clean.  Even something like a grocery bag will work.  I think the points would be the first thing I'd check were it me.

Answer 3

May need to replace the plug wires, check the coil, check the timing, replace the fuel.

Answer 4

well mine used to eat plugs....try a new set but pour a little fuel in each bore before starting...

Answer 5

Valve clearances sound wrong.  I gave my shop manual to my brother with the 1100F, but I am fairly certain the exhaust was 0.004, intake was 0.007.  these were the "tight end of the band, which I always felt gave the best performance.

I did have plugs giving spark, replaced them, and the bike started right up one time.  So I would look there first, if you are sure the valve gap is correct.