Topic: cb750a two cylinders only?????????

[fondofhondas]

got a 78 cb750a no power. stalls. wont idle. think only two pistons are firing? two exhaust pipes feel cold while the other two are hot after running. plus a knocking at the bottom end of the engine? if you could figure exactly what this is (Guessing a carb thing) how much should this work cost? oh...had a tune up, sat for year and a half, I bought it. 3,000 miles on it.

[BobbyR]

I have the same bike and I think you may have have the same problem I had. The cylinders fire in pairs, #1 and #4 fire together and the same for 2 & 3. If one of those pairs of cylinders is out it woud be ignition. You have two sets of points. I had the wire to the coil chafe and shorted out 2 & 3. If you have a pair out just pull one of those plugs out and hold it against the engine block and check for spark. The engine block is part of the circuit so a bad plug or plug wire on #1 can prevent #4 from firing.

[fondofhondas]

bike only has 3,000 miles on it. guy sold it to me as a running bike? he was suprised that I never got it running. both CENTER pipes are cold. two outer pipes are hot? wires are brand new and so are the plugs?

[MJL]

Check plugs, clean carbs.

[TwoTired]

Center pipes are 2&3.  Check that the points for that coil are operating and don't have an insulating film (oxidation) preventing electrical contact.

[OldSchool_IsCool]

You can check the points with a test light or volt meter.
- pop the right cover off the crank (right side as you sit on the bike)
- the 2.3 points on the right side (yellow wire)
- clip the light or meter across the point set (points spring and any good ground will do)
- key on, kill switch on, good battery installed
- plug wires disconnected from plugs (don't want her to accidentally start!)
- slowly rotate the crank by hand or by kick starter
- the light should turn on/off or meter jump from zero to around 12v
- if this doesn't happen, then you have a wiring problem, mis adjusted points or as Lloyd suggests, gummed up points faces that won't pass electrons.

While you are at it, look through the hole at the top of the points plate.  The light/meter should transition when the F mark by 2.3 lines up with the casing mark