Topic: Cb400F stumble

[MADCB400F]

Hi all, need some tuner help, I have a CB400F with a 466 kit, webcam stage II. The carbs have 98 mains, 38 pilots, air screws about 1 1/2 out. They have been synced on bike, new plugs , timming correct(electronic), vales correct. I have a stumble between 1 and 3000 rpm. Need advice to correct. Thank you.

[jessezm]

Try raising the needles one position maybe?  You're running pods, I presume?  I'm watching closely, as I have almost the same set up but haven't fired her up yet.  Have you checked to make sure your advance mechanism is functioning properly?

[MADCB400F]

Yes running pods, will try the needle, thanks

[mlinder]

Try stacks instead of pods.

1 to 3 grand on a 400 is usually mostly idle circuit, really. Needle position doesn't really affect this.

I recently had some issues with an XS400 that could not be run with pods. Didn't matter what the jetting was.

I have seen pods work on your particular carbs fine, though.

What do your plugs look like?

Try the airscrews at 3/4 out.

[jessezm]

With the open exhaust (I'm also guessing that's yours in your avatar picture?), I wonder if the pilot jet might need to be a little bigger?  I was told by Rick Denoon that the trick to running pods on the 400 is dropping the pilot to #38 and raising the needle one notch, but with all that extra breathing i guess I wonder if it's starving a bit for fuel.  I'd try the needles first (even though they are a pain to change!), then maybe turn in the mixture screws another quarter turn, and if that doesn't work, maybe think about bigger pilot jets.  Just guessing...  Plug chops would probably tell you more of course.

[jessezm]

Ah, and mlinder seems to have some ideas--maybe go further in with the mixture screws as he suggested.

[mlinder]

With the open exhaust (I'm also guessing that's yours in your avatar picture?), I wonder if the pilot jet might need to be a little bigger?  I was told by Rick Denoon that the trick to running pods on the 400 is dropping the pilot to #38 and raising the needle one notch, but with all that extra breathing i guess I wonder if it's starving a bit for fuel.  I'd try the needles first (even though they are a pain to change!), then maybe turn in the mixture screws another quarter turn, and if that doesn't work, maybe think about bigger pilot jets.  Just guessing...  Plug chops would probably tell you more of course.

That's for 1/4 to 3/4 roll-on richness, not 0 to 1/4 roll on, which is about all you should be at below 3 grand on these little fellas.

[Tim2005]

I'd do a full throttle plug chop first, 98 mains are huge, with pods on my 460 I was running 85s. When you're sure the top end is correct work down through the rev range with jetting.

Also you could try advancing the ignition 5 degrees, should help too.

[MADCB400F]

Here is what the plugs look like so far

[MADCB400F]

Since the plugs seem to look ok, does the slow jet control the gas at the rpms I am stumbling at, if so would it be better to try a 40 slow jet or adjust the needle, man pulling these carbs apart is a real pain in the @#$, thanks

[jessezm]

Did you try turning the mixture screws to 3/4 out instead of 1 1/2 as you had them?  I'd give that a shot before pulling the carbs to change out the pilot jets.

[mlinder]

You say you have an electronic ignition (which one?), so if it's working correctly, you aren't even in advance yet when this is stumbling.

Please try 3/4 out turn on your airscrews.

[MADCB400F]

Thanks all, so far this morning, I double checked the valves, readjusted a couple, advanced the timming about 1-2 degrees (Dyna) it is working fine. Resynced the carbs. Set the air screws to one turn out, anything before or after that makes the stumble worse.

[mlinder]

So does that mean it's still stumbling, or not?

Which dyna?

[MADCB400F]

It is the Dyna S, yes it is still stumbling, I also tried using a 40 pilot today and that seemed to be alittle worse even after trying diffrent settings on the air screws.

[Rick4004]

Try turning the choke on a little bit. Does that make the stumble better or worst. Better means you are lean, worst means you are rich.

Does the stumble get better or worst as the engine heats up. Better means you are lean, worst means you are rich.

Check the float levels, that can have a big impact.

You don't say if you are running the stock coils, but if you are I would greatly recommend an upgrade to some Dyna's or equiv.

The stage 2 cam might be a problem too. Haven't had any experience with that particular cam, but generally speaking the wilder the cam the harder it is to get the tuning right thru the whole rev band.

Rick Denoon