CB400/4 Poor Acceleration and running

[PB1968]

CB400/4 Poor Acceleration and Top End

Hi, I’ve just bought a 1976 CB400F. The general condition of the bike is good however there is a problem. Poor acceleration and top end performance.
The engine revs freely and appears okay up to 4000rpm about 40mph but then appears to bog down and takes an age to reach 60mph. The max speed is 70mph but acceleration thereafter is non-existent. Before I go to work can anyone offer possible explanations? I have orders carburettor overhaul kits from Dave Silver in anticipation of the carbs being at fault. However would these symptoms indicate an ignition problem? Any advise would be greatly appreciated. Ta Paul

[Tomb]

I would have thought a good general service would be the first place to start, points' gaps, setting up ignition timing, valve clearances, carb mixtures, plug gaps and checking they're the right grade, and their colour at all throttle openings.

Fuel flow from the tank? higher end revs/power being poor can sometimes be lack of decent fuel flow to the carbs. Float heights being wrong can cause the same feeling.

Are all pipes getting hot, could be a coil breaking down, if the bike is new to you, you might not notice if you were dropping a cylinder at higher revs.

I'd have gone through the whole bikes set up before ordering in carb kits that you may not need.

[mickwinf]

above advice is sound, if it revs freely when stationary but not when under load it could be fuel starvation, i had similar problems on a 550 which turned out to be a faulty float needle, the little springy thing had siezed open thus not letting enough fuel through. good luck!

[K2-K6]

Agree with the aboves but would check the timing and advance/retard mechanism first.

Also consider a compression check to see what you've got.