SOHC.co.uk Forums > CB350/400
Need to check out a slight misfire when cold
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:
--- Quote from: Mikep328 on June 18, 2024, 12:24:37 PM --- :( 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!!
--- End quote ---
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.
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K2-K6:
Agree with above, but add to that by blowing into the airscrew orifice with compressed air after squirting carb cleaner into it.
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