81 CB750 no power low rpms?

I have a friend that has an 81 CB750 Custom. It was running fine a
few weeks ago, but now has started acting up. It is really doggy at
low rpms. It starts to clean up a little at higher rpms but still not
like it should be. The bike idles fine and if you are in neutral and
turn the throttle the rpms jump right up. However once you put it in
gear and start to drive, there is no response at low rpms. You can
open the throttle all the way up and it just slowly starts to speed
up. Once you get above 4000 rpms it is a little better. We checked
the basic things - changed the gas, took the carbs apart and cleaned
what we could. 1 of the plugs was a little black but switched a
couple of the plugs around and it looked better.

Anyone have ideas on what could be wrong?

Any ideas are appreciated.

Thanks,
Tom

 >I have a friend that has an 81 CB750 Custom. It was running fine a
 >few weeks ago, but now has started acting up. It is really doggy at
 >low rpms. It starts to clean up a little at higher rpms but still not
 >like it should be. The bike idles fine and if you are in neutral and
 >turn the throttle the rpms jump right up. However once you put it in
 >gear and start to drive, there is no response at low rpms. You can
 >open the throttle all the way up and it just slowly starts to speed
 >up. Once you get above 4000 rpms it is a little better. We checked
 >the basic things - changed the gas, took the carbs apart and cleaned
 >what we could. 1 of the plugs was a little black but switched a
 >couple of the plugs around and it looked better.

 >Anyone have ideas on what could be wrong?

Almost sounds like you have developed a vacuum leak somewhere. Did you
put a gauge on the carbs to see how much vacuum you have at each one? Are
they the same?

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