SOHC.co.uk Forums > CB500/550

Low speed misfire

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peterengland:
Went for a 70 mile run today and the bike was sweet as a nut apart from if I switched it off for 10 mins for a cuppa, then when it starts it misfires slightly in 1st gear at lowish throttle. Thought of pilot jets but then why doesn't it do it in other gears at same throttle opening? As it stands this is  something that I can put up with so I will unless someone has an idea...?

mickwinf:
is it definitely a missfire or a flat spot? Carbs can be a pain to get spot on, i had a 500 that i could never get right.

peterengland:
defo feels like a misfire

K2-K6:
Both would be failure to burn cleanly each whole combustion charge, ignition/too rich/too lean.

If you're confident of ignition and plugs are ok (consider trying another set) and it runs ok higher rpm, then that aspect would appear reasonable.

Couple of things to try;- when you think it's going to do that on restarting, turn kill switch off and open throttle fully wide, then crank it for 5 seconds or so, then try starting as normal to see what effect that has. It should suck sir through it and mitigate if there's any fuel leaked in while standing. That could be slightly sticky float valve or maybe breather pipes not clear that could cause bowl filling to just push a little excess fuel through one of the idle jets to make a cylinder slightly wet. First try to see if it changes.

Another, as it stumbles then pull the choke slightly to see if richer improvement is evident.  The opposite to above and may indicate something to look at.

More detailed;- mark the low speed air screws with a spot of pen on carb casting, then count the turns inward each screw is set at. Record this setting do you can get back to it if needed. Then with it warmed and idling try adjusting each screw (one at a time) to guage if that carb is running clear through its idle circuit. They should each act like a small throttle with revs changing as you move them. One that's either no response would indicate a possible restriction in that carb idle circuit.

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