pre ignition

I have tried the advanced search , pre ignition, engine run on, engine fires when turned off, no luck, its as if these pommy cars don't suffer from pre ignition, well I must have an odd one as mine does, is there a cure other than ignition off car in gear and let the clutch out bang crash shudder, shudder . saltiga ( I checked how later MG do it and may be I could fit their solution to the problem, has any one tried it???)

Do not know about MG.
However, Leaning fuel mixture and reducing idle speed can help the problem considerably.
The engine running hot can also contribute to this problem.

OH!!:
Just ease the clutch out as you shut the engine down and hold other foot pressing firmly on brake pedal-
Stops it every time--:highly_amused:

Saltiga, Keoke has mentioned some of the causes. Unfortunately it is a combination of eveything. I happen to believe that an overly lean mixture which contributes to excessive heat helps to contribute to it. Also an overly rich mixture which tends to leave unburned fuel causes it as well. In my opinion, whatever contributes to heat will contribute to a run-on condition. An inefficient cooling system, lean carburation, too much advanced timing, too hot of a spark plug, too fast of an idle. Too much carbon in the cylinders, (helped by to rich of a carb mixture). I just tuned up my Healey last week. Sometimes in the past it would run-on and it felt a little sluggish on the first outing this year. Looking at the plugs I decided that my carbs were alittle lean, so I richen them just a tecsh. My points were closed up almost half of what the specification is to be. The timing was at about half what it should have been. Probably at about 3 or 4 degrees before TDC. My idle was tending to be too slow so that at times I would pull the choke out just to increase the idle. Which of course richened the mix. So I put in new points and condenser, new plugs, and set the timing at about 10 degrees BTDC. That brought my idle up to about 1000 rpms. That's too much for me. So instead of adjusting the carbs, which i have said i already richened them a tesch, I backed my timing off a wee bit with the advance/retard micro adjust wheel on the distributor till the idle dropped to between 800 and 900 rpm. I know that by doing this I will have no risk of pre-ignition. The car runs great, the water temp is between 170 and 180 F. and their is no run-on. I'd say a super good tune up is the best defense against run-on.

Low octane fuel and the fact that the standard flywheel weighs 28lbs doesn't help either.

I agree with Derek J-
same problem disappeared fueling the car with 100 Octane fuel -or a mix, 50% 100 oct and 50% 95 oct Fuel -also work for me
?? Probably 95 Oct Fuel have less octane than declared ??

saltiga said:

I have tried the advanced search , pre ignition, engine run on, engine fires when turned off, no luck, its as if these pommy cars don't suffer from pre ignition, well I must have an odd one as mine does, is there a cure other than ignition off car in gear and let the clutch out bang crash shudder, shudder . saltiga ( I checked how later MG do it and may be I could fit their solution to the problem, has any one tried it???)

Click to expand...

Just to be exhaust port retentive about it, I believe you're describing 'run on' or, sometimes, 'dieseling.' Both my BN2 and BJ8 will do it when hot if the idle is set more than 800RPM (as Keoke mentioned). Mixtures will burn hotter as you lean up to stoichiometric--the theoretical point where every oxygen molecule will burn every fuel molecule--then it will burn cooler if you lean further (called 'lean of peak' operation in aircraft). I don't think a Healey will run very well LOP anyway, but a too rich mixture, inadequate cooling, higher compression and timing too far advanced will contribute to run on as well.

I don't like the 'stop it with the driveline' approach, but if nothing else works ...

Thanks for the info, I will try these one at a time and see what happens, the MG B has a vacuum bottle from the inlet manifold , on turning off the ignition switch a solenoid is momentarily activated giving a vacuum to the float chamber (which are sealed, vented when running), so as when the engine is winding down there is no fuel flow from the main jet , no fuel ,no detonation. It actually works. Saltiga

Saltiga, while you're at it make sure the two intake manifold vent tubes are functional and venting surplus fuel. If these lines are clogged all manner of pre-ignition and run on problems may occur making diagnosis difficult. The vent tubes pull a little vacuum, so slipping on a vacuum gauge to their openings should confirm this. The front vent tube is easily accessible, the rear not so much - watch out for the exhaust down pipe. Good luck, GONZO

As the late great Henry Manney said: There will always be an England because when the key is turned off it will continue running on.

My old MGB and Midget used to suffer run on especially on hot days ( which we do get here in Britain sometimes). Both were in excellent states of tune. I solved the problem on both with a solenoid controlled air admittance valve connected to the inlet manifold common rail. With the ignition on the valve is closed with it off the valve opens and lets clean air into the manifold. This was years ago and they used to be available as a Ford part I think !!This is a Rover one . Hope that helps.
https://www.johnrichardssurplus.co.uk/car-parts/rover-blmc/rover-anti-run-on-valve-adu9535l.html