cb 400 carb or electrical problems?

[Dylanbrown]

Hi there

My name is Dylan. Im new to the forum.


Ive got a cb 400 with an interesting problem. The symptoms are :

when the bike just warms up it runs perfectly for a while then starts missfiring and spluttering. If i leave it for a few min i can get another short blast of power then back to missing. In the rain or early in the morning it runs well. On hot days it runs terribly, on hot days its just a pinch faster that a honda cbx 250.



The bike starts every morning no problem, straight to idle no choke needed.

Any ideas?





Thanks in advance


Dylan

[Superjack44]

How long have you had the bike ?
Do the checks in the easiest order first
First thing I would check would be the plugs, are they the correct type .
Second I would make sure the tank breather pipe is clear lift the rear of the tank the breather pipe is the smaller of the two disconnect it and try and blow through it, if you can reconnect it and then try and blow through it back into the tank if you cant there is a blockage in there that has to be cleared before looking further, I used a piece of bicycle bowden brake cable to insert it up the pipe push and twist at the same time till you meet the blockage then continue twisting and pushing until it clears the blockage it worked for me.
Third If everything is ok up to now turn your fuel tap off then open the carb bowl drain screws and see what comes out if there is any water/crap comes out then you may have to remove the carbs for a good clean .
Thats a bit more time consuming.

[Dylanbrown]

Hi there


Ive cleaned the fuel tap and replaced the fuel lines. Il do a plug check and have a look at the carbs hopefully today.

Will keep you posted.

Thanks

Dylan

[Bigdave982]

That's a fuel issue from the sound of it.

I'd check the pilot screws on the carbs make sure they're right and equal.
A colour tune is a very basic way of checking they're running about the same fuel ratio.
And a balance is always good, it's shocking how far out bikes get!