SOHC.co.uk Forums > CB500/550

Cold misfire

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peterengland:
Start 550 up from cold and it misfires badly for exactly 1 mile then it's perfect after that and doesn't do it again until next cold start, pulling out choke has no effect on misfire and all headers seem to be the same temperature. New plugs, points and timing set. Does occasionally have a hanging idle, but not all the time.

McCabe-Thiele (Ted):
It's my experience that cold starts put pressure on the plug sparking process. When the engine warms up the plugs spark with less effort.
My suspicions would start with the cheaper easy bits first, check condensers, plugs, plug caps, leads & finally coil.

K2-K6:
Try starting it and running for 30 sec, switch off and check pipe temperatures then to identify which one is cold.This should help narrow it down.

I had one with internal failure (on a car engine) of plug cap, it wouldn't fire cold but as it warmed the cylinder came to life and ok until next cold, rich mixture, starting then same again as here.

Problem was inside the cap which had HT separated in line but ran fine when cylinder was warm and mixture normal (off cold start system) took me a while to find it though!

peterengland:
I have done this and headers are all the same temperature

McCabe-Thiele (Ted):
Cold engine compression check - are all the same?

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