Topic: cb750 drops a cylinder when accelerating

[kurtkarram]

I have a 1971 cb750. Bike idles well on all 4 cylinders. When accelerating, it feels at about 2k rpm, I lose power. It feels like im dropping a cylinder.

I checked the plugs and all are brown except for #4, which is white.
I have just finished the 3000 mile tune up (including carb sync). Right now I have a new spark plug boot on order for #4 as the screw connector snapped on the inside when I attempted to cut 1/4" of the coil wire off and rethread.
Could a bad spark plug cap cause a lean plug? (weak spark, incomplete combustion, lean mixture) or wouldn't my plug be fouled because of unburned fuel? Anything else that could cause this while I wait for my cap to come in?

[Tracksnblades1]

Sounds like you may have found your lean carburetor. #4 maybe too lean to accelerate.
Acceleration usually requires a richer mixture while accelerating than steady state operation.
The 3 light brown plugs indicate a richer mixture than your white one. Unless #4 is wet washed clean white.

[ekpent]

Really clean plug usually means no spark or fuel so good places to start. Easy to check for spark in a dark place like garage etc.. Putting a wet rag on the header pipes after running will also show a bad running cylinder. Sounds like a new plug cap or four is a good thing in your case.

[Don R]

 You can swap #1 and #4 plugs or caps to see if a problem stays there or follows the part. Unburned fuel will wash off a plug but it should smell gassy or appear wet.

[Tracksnblades1]

You can swap #1 and #4 plugs or caps to see if a problem stays there or follows the part. Unburned fuel will wash off a plug but it should smell gassy or appear wet.

+1

Some are having trouble with knock off NGK’s..
I’ve had really good service with the ND plugs. Mine are from the 80’s though..

[HondaMan]

Could just be a plugged mainjet in the one with the white plug?
Also possible: bad coil for that one. When the coils start losing their inside enamel insulation (now 50 years old) they can short one plug or the other as the frequency (RPM) increases. This happened to my own K2 at about 126k miles, the 2-3 coil. The #3 would still run above 2000 RPM, but the #2 would cut out.