SOHC.co.uk Forums > CB350/400

Need to check out a slight misfire when cold

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Mikep328:
My CB400 starts easily with the choke when cold BUT for about 30 - 45 seconds after the cold startup, it runs on three cylinders; #3 exhaust pipe remains cool.  After the 30-45 seconds that cylinder begins firing and the fast idle RPM jumps up.  This only occurs on COLD starts.  Any other start during the day it fires instantly on all cylinders. 

Can't see how it could be an ignition issue since it begins firing on its own after 30-45 seconds and the engine runs perfectly under any throttle/load/RPM conditions once it is past that cold start 30-45 seconds misfire.  It acts as if there is no fuel initially in that carburetor and then it starts running as the bowel fills.  But that doesn't seem likely either since all float levels are at spec...

Any "first thing to check" ideas?

taysidedragon:
Try a new spark plug in faulty cylinder or could be plug cap. If not that it could be a sticky float valve or a dirty pilot jet.

Alexrayz:
Hey there!

Could be multiple things, mainly an imbalance between this cylinder and the other ones.

Carburetor wise it could be:

- Bad air fuel ratio at idle = play with the mixture screw on the side of the carb acting up. Mark where it was before you started playing with it, otherwise you’ll get lost and miss a reference point.
Turn by 1/4 turn increments while idling. You’re looking for the best performance, meaning for when the engine idles the highest. If the RPM falters that means it’s getting out of stoichiometry.

Bad sync = The slides don’t all have the same height meaning not the same amount of fuel+ air at any time, maybe more prominent with the choke on. Follow the synchronisation procedure. If all the gauges read the same depression all good.

Fouled jets (ethanol gum up) = Requires taking out the carb ramp and dismantling and cleaning which sucks.

Bad fuel inlet valve or float height= ditto

Spark plug wise:

Bad gap or electrode wear.

Checkup :

Check plug for not chocolatey color: If black or sooty —> Too rich
if white or pale —> too lean

Word of caution: Anything that involves fiddling with the carburetors can be finicky, counter intuitive and possibly worsen the performance if you’re not careful or experienced enough, talking from experience: I had to have a mechanic reset everything because I messed around without proper method and knowledge.


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McCabe-Thiele (Ted):
Mmmmm could it be spark plug related - plug only working correctly once warm - have you tried switching the plug into a different position?

Plus as said by TaysideD - easier to eliminate before taking carbs off.

K2-K6:
The 2 & 3 HT leads can be swapped to see if that transfers the fault, without any other changes. Worthwhile as first step to check those without cost or much time.

Plug cap possible, I've had that before that took a while to find with a fault internally to plug cap that wouldn't start on cold, but chimed in usually within about 90 seconds.

If you can't get it to move over with the above (and previous suggestionst o switch the plug)  then consider carbs.

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