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Constant misfire ??????
#53
(26-08-2024, 10:39 PM)John Cornforth Wrote: 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).

Hi John, having tried just about everything to get the engine to run properly this is my last resort,ref the needle changes. I'm at a point where I simply cannot throw any more time & money at what's been a very very expensive mistake in not rebuilding the engine myself !!!!!
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Messages In This Thread
Constant misfire ?????? - by Bluebird7 - 13-07-2024, 07:45 PM
RE: Constant misfire ?????? - by Nick Salmon - 13-07-2024, 07:56 PM
RE: Constant misfire ?????? - by Bluebird7 - 13-07-2024, 08:30 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 13-07-2024, 08:33 PM
RE: Constant misfire ?????? - by Ivor Hawkins - 13-07-2024, 08:37 PM
RE: Constant misfire ?????? - by Bluebird7 - 13-07-2024, 09:04 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 13-07-2024, 09:08 PM
RE: Constant misfire ?????? - by Bluebird7 - 13-07-2024, 09:26 PM
RE: Constant misfire ?????? - by Robert Leigh - 13-07-2024, 09:39 PM
RE: Constant misfire ?????? - by Bluebird7 - 13-07-2024, 09:49 PM
RE: Constant misfire ?????? - by John Mason - 17-07-2024, 09:55 AM
RE: Constant misfire ?????? - by JohnD - 17-07-2024, 10:31 AM
RE: Constant misfire ?????? - by Colin Morgan - 17-07-2024, 10:53 AM
RE: Constant misfire ?????? - by AustinWood - 17-07-2024, 11:09 AM
RE: Constant misfire ?????? - by John Mason - 17-07-2024, 11:47 AM
RE: Constant misfire ?????? - by bob46320 - 17-07-2024, 08:59 PM
RE: Constant misfire ?????? - by Charles Levien - 20-07-2024, 09:18 AM
RE: Constant misfire ?????? - by AustinWood - 20-07-2024, 10:52 AM
RE: Constant misfire ?????? - by Howard Wright - 20-07-2024, 11:13 AM
RE: Constant misfire ?????? - by Bluebird7 - 21-07-2024, 09:25 PM
RE: Constant misfire ?????? - by Charles Levien - 23-07-2024, 05:46 PM
RE: Constant misfire ?????? - by Tony Griffiths - 23-07-2024, 07:39 PM
RE: Constant misfire ?????? - by Bluebird7 - 23-07-2024, 09:31 PM
RE: Constant misfire ?????? - by Ian Williams - 24-07-2024, 04:29 AM
RE: Constant misfire ?????? - by John Cornforth - 24-07-2024, 08:16 AM
RE: Constant misfire ?????? - by Bob Kneale - 24-07-2024, 10:20 AM
RE: Constant misfire ?????? - by PeterN - 28-07-2024, 01:22 PM
RE: Constant misfire ?????? - by Bluebird7 - 14-08-2024, 04:55 PM
RE: Constant misfire ?????? - by Tony Griffiths - 14-08-2024, 05:11 PM
RE: Constant misfire ?????? - by Nick Salmon - 14-08-2024, 05:15 PM
RE: Constant misfire ?????? - by Bluebird7 - 14-08-2024, 05:16 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 14-08-2024, 06:40 PM
RE: Constant misfire ?????? - by Bluebird7 - 14-08-2024, 10:41 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 15-08-2024, 09:03 AM
RE: Constant misfire ?????? - by NBS - 16-08-2024, 07:59 AM
RE: Constant misfire ?????? - by Bluebird7 - 16-08-2024, 09:57 AM
RE: Constant misfire ?????? - by `Gordonhbm - 16-08-2024, 12:49 PM
RE: Constant misfire ?????? - by Bluebird7 - 16-08-2024, 02:50 PM
RE: Constant misfire ?????? - by Malcolm Parker - 16-08-2024, 03:26 PM
RE: Constant misfire ?????? - by `Gordonhbm - 16-08-2024, 03:46 PM
RE: Constant misfire ?????? - by Steve Jones - 16-08-2024, 03:58 PM
RE: Constant misfire ?????? - by Bluebird7 - 16-08-2024, 04:01 PM
RE: Constant misfire ?????? - by Bluebird7 - 20-08-2024, 04:25 PM
RE: Constant misfire ?????? - by Reckless Rat - 20-08-2024, 05:32 PM
RE: Constant misfire ?????? - by Ivor Hawkins - 20-08-2024, 09:56 PM
RE: Constant misfire ?????? - by David Stepney - 21-08-2024, 09:31 AM
RE: Constant misfire ?????? - by Bluebird7 - 26-08-2024, 04:13 PM
RE: Constant misfire ?????? - by Alan - 26-08-2024, 04:27 PM
RE: Constant misfire ?????? - by Bluebird7 - 26-08-2024, 05:15 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 26-08-2024, 05:15 PM
RE: Constant misfire ?????? - by Bluebird7 - 26-08-2024, 08:33 PM
RE: Constant misfire ?????? - by John Cornforth - 26-08-2024, 10:39 PM
RE: Constant misfire ?????? - by Bluebird7 - 27-08-2024, 08:57 AM
RE: Constant misfire ?????? - by Ivor Hawkins - 27-08-2024, 09:10 AM
RE: Constant misfire ?????? - by Bluebird7 - 27-08-2024, 09:32 AM
RE: Constant misfire ?????? - by Alan - 27-08-2024, 10:05 AM
RE: Constant misfire ?????? - by Ruairidh Dunford - 27-08-2024, 10:54 AM
RE: Constant misfire ?????? - by Bluebird7 - 27-08-2024, 12:10 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 27-08-2024, 12:12 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 27-08-2024, 12:22 PM
RE: Constant misfire ?????? - by Chris KC - 27-08-2024, 12:40 PM
RE: Constant misfire ?????? - by Ruairidh Dunford - 27-08-2024, 01:08 PM

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