Topic: CB400F stumbles just off idle or when at steady speed

[satch222]

I bought a '76 CB400f last fall. I Drained tank and carbs for the winter and sent the tank to Precision Motorcycle to have the tank sealed and painted. Over the winter I changed plugs, air filter and put a new oil pan on because the original had a crack.

This week I filled her up with non-ethanol, 90 octane fuel and she fired right up. As it warms up however, it stumbles as I'm taking off from a stop or accelerating slowly at low rpm. I believe it's missing at a steady speed as well. If you get on it and keep the revs up it's fine.

I am going to try a hotter plug first off unless someone has a better idea. BTW...The previous owner had the carbs cleaned not long before I bought it.

Ideas?

[Tim2005]

Have you checked and set the points and timing correctly? I've found that too small a gap will cause such a stumble on a 400f.  I'd certainly not try using a hotter plug, that shoudn't be necessary.

[Herr Supersport]

I assume the carbs are really clean and everything stock. My ideas are:
- worn-out/annealed advancer-springs
- worn needles/emulsion tubes
- #40 pilot too rich

My 400f suffered this stumble too when hot. New advancer-springs and needles/emulsion tubes improved behaviour, a light stumble was still there yet.
Tinker with needle position wasn't a success but using #38 pilots with airscrews 2.25 out made the bike ridable.
It's not 100% perfect, for my taste idle is rather lean now.
At least stop-and go is no longer a pain.

[harisuluv]

really just sounds like dirty idle jets.  as far as what the PO did to the carbs before you, just disregard any of that as verified, it's all just speculation.

[Redline it]

quite a bit of things can cause a stumble just as the carb slide starts to move. Before buying anything, check ignition, check for air leaks, are the slided all bottoming out, then  remember where the carb air screws are and make some adjustments to the outer carbs by hand while your riding , sometimes you'll get an idea from that that it could be the problem, they should be very sensitive if you have no leaks and carb are close to synced. High octane gas and hot plugs wouldn't be your best bet. These pistons are tiny and even a little worn out the motors produce awesome power, I'm on one that has 31k on it, it'll still go a hundred, layin on the tank. The stumbling will come and go.  Run it for a while if it is dirty dirty jets they might clean out.

[MoMo]

sounds like blocked emulsion  tubes(possibly pilot jets too, especially if the bike sat for some time with corn gas in it

[satch222]

Thanks for the input everyone. I put some seafoam in the tank. I'll put some miles on it and see if it improves.

[evanphi]

Thanks for the input everyone. I put some seafoam in the tank. I'll put some miles on it and see if it improves.

Seafoam is great, but you will also want to manually inspect and clean those jets/emulsion tubes too.

[flybox1]

Thanks for the input everyone. I put some seafoam in the tank. I'll put some miles on it and see if it improves.

Seafoam is great, but you will also want to manually inspect and clean those jets/emulsion tubes too.

+1
Nothing is better for your carbs than a good hands-on cleaning.
Spray carb cleaner into every orifice, hand clean and inspect all jets, emulsion tube holes.
Make note of your jet sizes, mixture screw settings.  Verify float heights are correct.