SOHC.co.uk Forums > CB350/400

400/4 carb flat spot

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totty:
That's typical lean idle in my opinion, either the pilot jetting/adjustment is too lean, or part blocked, or a air leak.

Sometimes accompanied by popping and banging on overrun.

Tomb:
Wouldn't it be easier doing a plug chop at those revs, takes the guessing out of it.

Ride the bike at the flat spot revs as long as you can, then kill the engine - clutch in - shut throttle all at the same time, coast to a stop and take the plugs out and they'll show you how the mixture is. (apologies if I'm teaching granny to suck eggs :-X)

If you can't get a colour on the plugs richen them (screws wound out) for a start point

bigfoot44:
I had exactly the same problem with my CB350f. I finally cured it by changing the main jets to 80's instead of the standard 75's. Modern fuels make the engine effectively run weaker so upping the jet size should combat this. The larger jets are available from David Silver on special order.

K2-K6:
If it runs ok when you get past the flat spot then that suggests the main jetting is fairly accurate (plug coulour in general use will confirm this).

If it idles ok without stopping or hunting, then the slow jets can't be far away.

A question; do you get the flat spot when you pull away from standstill in first gear or is it smooth?

When you have the engine on overun with throttles closed then the engine will obviously be turned by the wheel without much fuel demand to keep it going.  if the idle circuit is below optimum during this phase, then you get the combustion chambers going lean so that to speed up again you then have to open the throttles initially before you get a response (i.e. a flatspot) in that to convert the combustion back to optimum to make the motor accelerate you are relying just on the mains being openend more than really needed for the road speed you want and giving you that "kick" that ends the flatspot.
To blend this more smoothly the idle circuit needs to deliver more, so the advice to open out the idle screws is worth trying but go out by at least one whole turn, the objective is to see if you get any improvement (not just an accurate change) if so then it would confirm the idle jets are responding to adjustment and then let you make a desicion as to what final change is needed.

I know that's a bit of a long winded explanation but hope it makes sense.

NE0:
Thanks for the response guys but I posted the question back in january.

I increased the mixture screws back then and solved the problem.

However, shortly afterwards I took the bike off the road, stripped the engine, replaced all the seals, primary chain, transmission bearings, gearbox parts, forks and a shed load of aother parts!!! and its due its MOT tomorrow at 9am.
The rebuilt motor is nice and quiet but until i get it on the road I won't know if the flat spot has returned.......all being well it's still fixed.

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