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1981 CB750C running on 3 cylinders.

John Luke

CB750 Enthusiast
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I bought this bike a few weeks ago and now have her running. She runs on cylinders 1-2-3 but cylinder 4 doesn’t run. I found this out by checking the exhaust on the 4th cylinder and I can rest my hand on it forever without burning it. So I removed the spark plug to see if I had spark. It sparks but seems to be doing it intermittently. Compared to cylinder 1 it seems to be not sparking at the right times. How would I fix that? Does that have something to do with timing?
 
Most people never set the valves on them due to the requiring of shims to reset them. The valves also tend to close up due to the use of unleaded fuel. Meaning they eventually burn. Do a compression test pretty quick otherwise you may jump through many hoops only to find out it never will run right regardless of how much work you do. A LOT of people do that.............

If you do not ground the plug really good you will get intermittent spark. If you have then check out the wires, caps, etc. Bear in mind that one plug not firing right affects the other in a pair, they are linked, one fires backward. The ground path is through one plug to the head and back out through the other plug backward.

Timing has nothing to do with it.
 
I have had the air cut off valve leak and the cylinder runs way rich at idle up to about 4k rpms. It runs so rich the plug can't fire at idle. My symptoms showed if I could get it above 4K rpms suddenly the bike took off and ran great! But it struggled around idle and up to about 4k rpms.

I found this helpful:
https://www.motorcycleproject.com/text/aircut_valve_more.html

Basically, he says to plug the air cut off valves. The air cut off valves were meant to reduce backfiring on hard deceleration.

I'd check compression as AMC49 says too check valve adjustments too. Good luck
 
An aircut valve leaking on either side of the diaphragm or at an o-ring gets leaner rather than richer. Only by activating (compressing the spring) a normal working one does it get richer. That shuts off one of the idle air bleeds to richen up, a leak adds the two bleeds to each other plus the added variable air leak to get leaner than f---.
 
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