Constant misfire ??????

[John Cornforth]

Hi again

I use a GG needle in a 1 1/8 inch SU (Prewar weighted piston with damper type) and the car runs very well. Sooty plugs can be the result of a too-rich idling mixture even if the running mixture is OK. Adjusting the jet height predominantly changes idling mixture strength, has a small effect on light throttle cruising and almost no effect on the top end.

I can't help feeling that from your previous descriptions changing SU needles is barking up the wrong tree. An engine will run on a range of Air-Fuel ratios from about 10 to 1 up to 16 to 1. OK, it may not give best power or economy, but won't actually misfire assuming the ignition system including plugs is up to scratch. You really have to be very rich or very lean to give a noticeable misfire or surging and kangarooing.

To lean out the mixture enough to cause a misfire you would either need a considerable restriction in fuel flow (noticeable when sustained power is needed) or a pretty serious air leak which would make idling near impossible. Another cause of rough and erratic running is fuel boiling / vapourisation if the float chamber gets very hot for some reason.

I'd suggest setting the idling mixture for fastest idle and then weaken just slightly. Then try a fresh set of spark plugs in case the existing ones have become fouled (clean them and keep as spares).