Need to check out a slight misfire when cold

[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.


Sent from my iPhone using Tapatalk

[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.

[Mikep328]

Thanks all, I'll make some checks tomorrow.  I don't think it's carb settings/balance -related.  They were disassembled/cleaned/adjusted/synchronized last November and the engine could not run better/smoother at all RPM/loads after it does that short bit of misfire after COLD start.  I'll try moving the leads first and see if the cold pipe switches position.  If not I'll check the plugs; I have a new set but I didn't change them previously because the current ones looked fine when I pulled them at the same time the carbs were off last year. 

[Mikep328]

Did an ignition check - changed wires but no change to cold pipe.  I pulled the spark plug, laid it on cyl head and re connected wire - good strong blue/white spark at cranking speed.

So it might be the carb slightly out of sync though they were carefully bench synced when cleaned several months ago.  But still not sure why there would be poor fueling for 30-40 seconds when cold and then clear up on its own and have no such symptoms on restarts for the rest of the day and idle smoothly at 1100 RPM.

I might pursue it a bit more but since it has no effect on the actual operation of the motorcycle within a minute of cold start up, I won't lose much sleep over it.  However, I do admit I like things to work as they should so I suspect I will continue to futz with it!

[McCabe-Thiele (Ted)]

A bit of a long shot after bench sync of the sliders did you also check/adjust  the choke stop screws  between the carbs. All four need to be fully closed when the choke is fully on, the screws have lock nuts iirc?

[Alexrayz]

Did an ignition check - changed wires but no change to cold pipe.  I pulled the spark plug, laid it on cyl head and re connected wire - good strong blue/white spark at cranking speed.

So it might be the carb slightly out of sync though they were carefully bench synced when cleaned several months ago.  But still not sure why there would be poor fueling for 30-40 seconds when cold and then clear up on its own and have no such symptoms on restarts for the rest of the day and idle smoothly at 1100 RPM.

I might pursue it a bit more but since it has no effect on the actual operation of the motorcycle within a minute of cold start up, I won't lose much sleep over it.  However, I do admit I like things to work as they should so I suspect I will continue to futz with it!

If you let the bike stand for over 3 weeks there might be some ethanol clogging the jets.


Sent from my iPhone using Tapatalk

[Mikep328]

"A bit of a long shot after bench sync of the sliders did you also check/adjust  the choke stop screws  between the carbs."

Yes, I was very careful to be sure they were all adjusted correctly.

"If you let the bike stand for over 3 weeks there might be some ethanol clogging the jets."

Yes, a clogged idle jet is possible but I don't know how a clogged idle jet could unclog 30-40 seconds after the cold start, stay unclogged all day and then reclog overnight for the next cold start.  The fact that the engine idles smoothly when warm and takes the throttle with no bogging at all (it bogged terribly when I bought it last year before the carb overhaul), makes me think all the jets are still clear. 

I suppose it could be synchronization - I carefully bench-synched the carbs during the overhaul but I could check that with the vacuum gauge set.  I'll try to do that sometime this week...

[K2-K6]

Reasonable pointer to that carburettor as the ignition seems to check out.

Syncing status makes no real difference, so wouldn't bother worrying about that, unless its wildly out of kilter.

Non invasive testing, squirt some E-Z-Start or similar into the carb sync port then put screw back in, all before cold start. Then try it to see if it picks up straight off the bat.  If it does and starts on that cylinder, then suspect the idle jet as partially reduced in flow.

Choke of this type works by obviously restricted air flow, interaction on the idle jets is that they then receive an increase in vacuum to pull additional fuel (over and above their normal operating vac range) to give richer initial starting. If they are at impaired it may not get that "hop" in delivery to get it going.

You could also try turning the affected cylinder's air adjustment screw in 1/2 turn from where it is to try cold start like that. Its not to reset that for running, just that it could also confirm the above.

[Mikep328]

Thanks K2, I'll follow your suggestions later this week!

[Mikep328]

  As has been suggested, it appears it is the idle jet.  With the engine at idle, there was no reaction when turning the idle screw on that carb from totally closed to 2+ turns open.  The other three immediately affected the idle rpm/smoothness.   When I overhauled/synched the carbs last year, all of the mixture screws affected the idle as they were adjusted in/out. 

I don't think there is a satisfactory way to clean the passage with the carbs on the bike but please tell me if I'm wrong about that!!

[Alexrayz]

As has been suggested, it appears it is the idle jet.  With the engine at idle, there was no reaction when turning the idle screw on that carb from totally closed to 2+ turns open.  The other three immediately affected the idle rpm/smoothness.   When I overhauled/synched the carbs last year, all of the mixture screws affected the idle as they were adjusted in/out. 

I don't think there is a satisfactory way to clean the passage with the carbs on the bike but please tell me if I'm wrong about that!!

Off the top of my head, I could think only of one thing and I am not sure so correct me if I’m mistaken:

The air fuel mixture screw restricts the flow of the idle jet into the carb cavity. Maybe squirting carb cleaner into the mix screw hole could clean the jet?

Other than that I wouldn’t have an idea. When I was faced with your issue I would just take the carb ramp off for a clean up on the bench.


Sent from my iPhone using Tapatalk

[K2-K6]

Agree with above, but add to that by blowing into the airscrew orifice with compressed air after squirting carb cleaner into it.