Flat Spot Above Idle

[tom400f]

Guys and girls

The carburation on my CB400F seems pretty spot on: starts easily; idles fine; pulls like Thomas the tank Engine

Except in one respect. There is an annoying flat spot just above idle that gets in the way as you are pulling off. I need to use more revs and clutch slip if I want to avoid it.

Valve clearances; ign timing; I think these are fine. Idle screws are set to standard position and float heights likewise. What do I do to diagnose and experiment?

Thanks 

[Bodd]

Under your name it says you are an SOHC Pro so I would think you would be best placed to answer this yourself 

The pilot jets control the idle and low rev range, if everything else is set right and the carbs are balanced I would suspect a blocked pilot jet, or two.

[exvalvesetdabbler]

I had similar problems after replacing all of the brass bits with OEM items.   I am running with about 1 1/4 turns on the pilot air screw.  The problem has been more noticeable now they are watering down the fuel with ethanol. Aside from the fuel quality issues these bikes need to run a little rich around idle because there is no kick down or throttle pump. When you open the throttle, sometimes too quickly the vacuum drops away a little and less fuel is drawn up through the metering system so the bike tends to stumble.  Up to about 1/8 throttle this is the pilot circuit.


With my bike, running the idle mixture at 1 1/2 to 2 turns I can feel it holding back if I try to maintain constant speed
at about 2000 rpm in a low gear.  Try giving it 1/4 to half choke and see if it gets better.

Dave

[tom400f]

OK my haste - turned out to be ignition - points gap on 2&3 plus timing was somewhat advanced 

Might look at electronic ign  ...