I've been running on 2/4 cylinders. 1980 CB750

hellhound1

hellhound1

Do No Harm. Do Know Harm.
So I found out after I put on the new exhaust and fired the bike up that only the right 2 cylinders are firing. I figured this since those two headers were hot while the other two were cold. What could be causing this? poor head job, valve clearance? I checked and I have spark on that side, but apparently no firing.
 
Carbs could be fuel starved on that side, maybe a blocked fuel tube that feeds those two. Maybe a weak spark, bad coil, wires or condenser. Plugs could be shot. Are you getting any oil blow-by on that side? Compression check might be a good idea. My money is on the carbs though.
 
I don't think it should be coils - isn't it a 1/3 2/4 split on the coils? I could be wrong.

How are the plugs on those 2 cylinders? Are you working with 4 fresh plugs?

Compression, Fuel, Spark - confirm all three at all cylinders.

Compression - do a compression test. Basic test - record the PSI on each cylinder - they should be 150+ PSI I think at least and all about the same +/- a few PSI

Fuel you can confirm by dropping the bowls off the carbs and seeing if the floats work and allow fuel to come. Clean your jets etc. to make sure it's getting from the bowls into the cylinders.

Spark - check the spark on the plugs - pull the plug, ground it against the engine case with the lead attached and kick it over - is there a spark?

Check your valve clearances as suggested, as per the service manual. Do a static timing check with a test lamp and your points, again as per the service manual.
 
1980 doesn't have points and Tim is almost correct, the coils are 1.4, 2.3 so it narrows things down to carbs or mechanical. (I think the firing order is 1324, sound about right to me?)
Carbs are easier and cheaper than pulling motor out to do top end, although you can remove valve cover to check tappets. They use 25mm shims if I remember right
 
Well it seems to be a combo of reasons. - pulled the plugs to find out that the 1 and 2 aren't in the greatest shape and I did a compression test.

3,4 - ~150 psi

1 - 30 psi
2 - 70psi.

I have a spare motor on a bare frame with about 25,000 less miles so I think I'm just gonna take my time with that one and rebuild the top end.
 
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