Topic: my 75 CB400F is revving out of control at idle... help!!

[ianxshiver]

thats pretty much it. i'm new to fixing motorcycles and i'm not sure what could be causing this. the throttle cable seems to be returning fine.

could it be that i am missing a leaf spring in one of my carbs? or would that have nothing to do with it?

thanks!

[flybox1]

what have you done to your bike up to now?

[stickfigure]

Could this be anything choke related?

[magconpres]

Could it be an air leak?  I know on on of my two strokes, a leaking crank case seal caused the bike to rev to the moon.

[ianxshiver]

well, the update is that i synced my carbs tonight and there was one that was way off from the others, and now it isn't revving crazy anymore in neutral. however, when i rev it and it comes back down, its sticks around 2000rpm until i pull on the choke to give it less air, then it comes back down to 1200. but the problem seems to solve itself when i put it into gear also, so i don't know if i should worry about it or not.

its entirely possible that there is an air leak somewhere.

[betterthanurs916]

Sounds like a classic vacuum leak on the downstream side of the carbs to me...I would check your manifold boots for cracks/proper seating.

[ianxshiver]

downstream... do you mean on the intake side of the carbs or the airbox side? or could it be either?

[magconpres]

Between the carbs and the engine.
A leak there would introduce more air into the mixture after the carbs had already done the mixing, resulting in a leaner condition than the carbs are tuned for.  the engine runs a bit hotter and faster when the mixture is lean.  Sometimes that causes more air to leak in, making a leaner mixture, making the revs higher, etc. 

[SanDogDewey]

well, the update is that i synced my carbs tonight and there was one that was way off from the others, and now it isn't revving crazy anymore in neutral. however, when i rev it and it comes back down, its sticks around 2000rpm until i pull on the choke to give it less air, then it comes back down to 1200. but the problem seems to solve itself when i put it into gear also, so i don't know if i should worry about it or not.

its entirely possible that there is an air leak somewhere.

The 400F doesn't have vacuum operated slides. They are mechanically linked to the push/pull throttle cables. If they are hanging up, there is something mechanical keeping them from coming back down to idle, or you have some slack/slop in part of the linkage. Are you sure you got them synch'd equally at 1200 rpm idle? She should sit there and purr nicely at idle. While holding the throttle closed, can you move any part of the thottle linkage?

[magconpres]

I don't think the slides are hanging up. 
I still think he has an air leak.  The fact that putting a load on the engine lowers the idle or puts on the choke both suggest to me the bike is running lean.

Ian, you may want to spray some carb cleaner around the manifolds while the bike is running.  If you have a leak, the carb cleaner will get sucked in and the idle will change (should increase iirc).

[Kevin]

Ian,
Check to make sure that the little ball and spring that lock your choke lever in the off position are still in place and functional.

[ianxshiver]

new problem- after i gapped my spark plugs, i put them back in, started up the bike, but when i pull on the throttle the bike just shuts down. what did i do wrong?

[Kevin]

Have you checked to make sure that the petcock is open and that you have gas in the tank?

[thrutheframe]

Have you checked to make sure that the petcock is open and that you have gas in the tank?

been there done that... I spent a whole afternoon dressing up the wiring on my kz 750, only to have it REFUSE to run aftr I was done.  luckily my brother, who has NO mechanical ability whatsoever happened to ask, "does it have gas in it?"  ....

DOH, it was bone dry

[ianxshiver]

haha jeez, its entirely possible that i just didnt turn the gas on. i guess i'll try again tomorrow.

[strynboen]

hey my 550 did the same thing...tighten that vire that pull dovn. in dobbel  push ,pull. trottel handle ,as san dog said erlyer.and renember that spring(trottel return) in mittel of rack that pulls dovn trottel also

[dave500]

sort your ignition fully before you tamper with the carbs,,this is my standard reply.