CB400 SuperFour, flat spot.

[ducatilover]

The CB400 seems to be very flat in the mid range, with a bit of hesitancy near 10,000rpm. Anyone know of a common problem before I start to pull it apart?
Will be checking the intake rubbers, plugs, leads etc. Or could it be the shitty old fuel in the tank?
I know it's a 400 four, but, it shouldn't have a torque trough like it does. Pulls cleanly above 10K and tapers off right up the top as expected, pulls fairly well in the low mid range, about as well as one would expect.

[AllanB]

Sounds like you are on to it. If the fuel is over 6 months old, dump it and replace with fresh fuel. Plugs then the others you mention.

I'd do one at a time, ride and see what happens ie fuel - ride, if still shitty, plugs, ride etc. Do it all at once and you'll never know what it was if it improves.

[ducatilover]

Sounds like you are on to it. If the fuel is over 6 months old, dump it and replace with fresh fuel. Plugs then the others you mention.

I'd do one at a time, ride and see what happens ie fuel - ride, if still shitty, plugs, ride etc. Do it all at once and you'll never know what it was if it improves.

I'll dump the fuel, checked the plugs, the burn on them is spot on. Drained the bowls and cleaned them, clean anyway, checked for air leaks, none, coil resistance on primary and secondary were within spec.
Could be running on crappy fuel, so that's the next step Great excuse for a ride...a very long one. I imagine it needs a few hundred KM put on it just to be sure....

[2 wheeled worx]

try when you are riding the bike to get it to this flat spot and pull the choke on slightly and see if this helps if it does wil have a lean issue maby air leak bad fueling if it dosent help it is a rick problem ie needle to rick or floats or just a dirty air filter or bad fuel or poor spark

[ducatilover]

try when you are riding the bike to get it to this flat spot and pull the choke on slightly and see if this helps if it does wil have a lean issue maby air leak bad fueling if it dosent help it is a rick problem ie needle to rick or floats or just a dirty air filter or bad fuel or poor spark

I found the problem Who ever had the carbs off last had not tightened the clamps between the head and carbs enough. Stripped the carbs anyway and she runs great now