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Misfiring and unable to start once hot

rory Cosgrove

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If there was/is a problem with the choke giving a rich mixture then you should have lots of smoke just before it cuts out as the engine drowns. If there is the opposite problem, i.e. a fuel supply problem (blocked jet, blocked line, fuel pump failing when hot etc) then after turning over and not starting, whip out a plug and see if its wet with fuel. If it's dry then fuel supply could be an issue. However, by the sound of it, you get smoke when its finally starts, probably not a supply problem, but could look at plug anyway. Also when it turns over and doesn't start you could whip out a plug and with the lead attached check if there is a spark when the outer part is touched to earth with the engine being turned over (you need two peaple for this). You need to see a really good spark, because when the plug is in situ and under compression a spark is harder to produce, so a failing coil could produce a spark, but not at enough voltage to work in situ.

Edited by Sparepart
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The tiniest bit of muck or residue from old fuel can make a needle valve stick. It needs to go up and down with virtually no pressure from the float or your finger. Get a rebuild kit and change it if you are unsure if it is sticking or not. Personally I would just swap it for a new one and then you have ruled it out of the equation. Rebuild kits are not expensive 

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Take off the air filter. When cold you should see the choke butterfly is flat across the main inlet tube. As the engine warms up the butterfly should rotate and open up the inlet, eventually becoming almost vertical, fully opening the inlet. If this does not happen then the fuel/air mixture will gradually become too rich as the engine warms causing rough running and eventually the engine stalls, usually with a strong smell of petrol in the engine bay.

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