Topic: 75 cb750f no fuel to #4

[Joe71k1]

I have a 1975 cb750f. I just rebuilt the crabs with k&l rebuild kits, I also replaced the entire ignition system with the dynatek s. The coils are bran new dynatek coils and all new plugs and wires. The bike is timed, and I’m getting spark to #4 but it’s not firing. I know this because when I pull the plug wire off it changes nothing but when I hold it close to the plug it’s definitely arcing. When I pull the plug there is no carbon or residue build up at all!! It’s perfectly clean. The float height was set set with a mic and the float bowl has fuel in it and like I said all new jets, needle, and gaskets in the carbs. Air screw is 1.5 turns out. anyone have any ideas??? I’m lost here.

[MJL]

How do you know it's not firing? have you tried spraying gas in the carb as an experiment? Have you pulled the carbs to double check that one is't clogged somewhere?

[Joe71k1]

I know it’s not firing on all 4 because I can hear it.. I have a 71 cb750 to compare it to. also it’s the only cylinder I can pull the plug wire on and it changes nothing... and the plug is perfectly clean after running the bike for a decent amount of time while all the others have some residue

[seanbarney41]

I would try perking that one cylinder up with starting fluid.  If it doesn't you have a spark problem, if it does but quits again when the starter fluid runs out something still wrong with your carbs

[ekpent]

 The best way to tell its not firing is to touch the header pipe on that cylinder after running and see if it cold or only moderately warm compared to the rest. Wet rag should sizzle. I'm sure you have checked for good spark by laying a hooked up plug on the outside of the engine and looking at the spark while running. If its fuel related then about the only things would be a stuck/jammed float or the float needle being stuck up inside the holder which is doubful.

[Jerry Rxman Griffin aka MuthaF'er]

Does the float bowl have fuel when you open the drain? If yes, have you rechecked the operation of the float, the fuel level, is the slow jet still screwed in, etc?