SOHC.co.uk Forums > CB350/400
Need to check out a slight misfire when cold
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:
--- Quote from: Mikep328 on June 17, 2024, 12:42:40 PM ---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! ;)
--- End quote ---
If you let the bike stand for over 3 weeks there might be some ethanol clogging the jets.
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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...
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