what the cause of run on

lately my car wants to keep running a bit after shut off. any cures to correct? it doesn't happen every single time, but it seems to be increasing to everytime recently

for your 66..

magic.

you can fiddle with timing, fuel, wheel bearing torque, and sun visor angle. each will have the same result. nada.

I do not believe you can make minor adjustments to timing or carbs and get a "fixed" situation. might be ok now, bit not tomorrow. pissing on the headlights might work as well. (get the icea I've tried a lot of things and am not optimistic...<G>)

on the later cars there was a faulty electrical circuit that kept feeding juice to the ignition, they fixed it with the aniti0run on valve that shut off fuel.

I have found on my 73 that I do not get runon when shutting down after a spirited run, i.e. not fussing in 20mph traffic. if i am in traffic, it will usually run in. I just put in 1st, hold the brake, and let off the clutch. sounds nasty but it works and no harm done.

Also, if the idle is high (mine is set at about 800, it varies form 600-1100 depending on what it wants to do--an MG is a lot like a cat in that respect<G>). If I pull, in and let it return to idel for `15-30 seconds, it will usually turn off OK. If not, pop the clutch.

Mac's right about the idle. If it is over 800 the chances for run-on are greater. Retarded timing will also contribute to it. Speaking of cats, I wish I could keep mine off my car, at least she hasn't pissed on my headlights................yet.

Carbon in the combustion chamber and low grade gas will cause it. Timing I would think advanced would make it even worse. Maybe other folks have some better ideas.



Dwight

Is run-on a bad thing? Meaning, will it hurt the engine in anyway?

Attempting to save money, I started to use regular gas about two months ago. The result was pretty excessive run-on. I went back to Premium and while my wallet said "ouch", my MGB says "yum". Now I have very little run on if at all.
Rob

rbowe Wrote:

It is caused by the petrol auto igniting with out the assistance of the spark plug. Heat and pressure alone. Other wise known as knock, or dieseling. Not especialy good for the engine, not instantly fatal either.
High octane Fuel is all about knock, or auto ignition, prevention. It has nothing to do with power. High octane fuel is less explosive than low octane, it needs the spark. High octane is associated with power because it is used in high compression performance motors. Higher compression means more pressure and more efficient combusion and more power (just as in a super or turbo charged car)
You can also fiddle the timeing a bit to help, put in a lower temperature thermostat (there were a 82 and 87 degree C versions). Install an antirun on valve, found in post 74.5 American MGB's with all that anti polution gear on them.
Possibly you've got carbon build up in your cylinder heads.

Hi Sam,

With the weather getting hotter dieseling will become a problem if you’re using lower octane fuel. The dump the clutch trick is fine if you don’t mind replacing the clutch more often!

This may seem counter intuitive, but as you turn off the key… press the accelerator pedal all the way down.

Steve

You've already received some good advice but wanted to share my success story.

I had a dieseling issue and searched the posts for answers. There were plenty of possible reasons and only a few of them were "easy" to me, a newbee. I first switched to higher octane gas but that didn't fix it. I was dreading checking the carbs, changing the anti-run on valve, installing a diode etc.

Then I did what one of the posts said and checked all the vacuum lines. Low and behold one of the lines to my carbs didn't have a hose clamp. Installed it yesterday evening late and drove it tonight. Got'er nice and hot and shut'er down and not a bit of run-on. Man was that nice.

Checking your vacuum lines is very easy and little if any money to repair. It worked for me.