MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - New restoration running on!

I just finished a complete restoration of a 58 roadster with a 3 main 1800 engine and it wants to keep running. The engine has been completely overhauled and timed with a strobe light at 20 deg before TDC at 1000 rpm, as per John Twist. I replaced the distributor with a spare and same results. One of the past threads said that cured run on problem. It is not too hot and I am at wits end. I have two other A's and have never experienced this problem. Any suggestions? Thanks for your help!
John Progess

John, there are some hot spots somewhere in the cylinder head to make it run on. Sharp points of metal, or coke. Or you have a very high CR. Does it run on if you start it from cold and stop it immediately?
Art Pearse

Hi John,

Ive got the same year car with a 64 early B motor. Stock distributor with electronic ignition.
I really havent had run on issues though.

When you rebult the engine did you have the machine shop remove the point in the heart shape of te head?

This is a known hot spot that is usually removed to help prevent run on. It looks like Art has covered the basics on CR. If you run high test In my opinion it seems to help. I notice also if your Idle is set over 1000 rpm on a car thats runs on it seems to make it worse. If you have a compression gage what is your pressure reading? Maybe the head was skimmed to far? Just throwing a few things out there for you to look at! Good luck with it!
Steven Devine

John

I have not seen the John Twist set-up video. I prefer to run my engine at 3500rpm with the vacuum disconnected and set it up to 33 degrees. I accept whatever that gives me at 1000 rpm.

Steve
Steve Gyles

So , any way to stop it without dismantling the engine?
I also have the problem, when hot.
Edward
Edward Wesson 52TD

A water-spray "decoke" ?
Art Pearse

I have just solved this problem on my 3-main engine which is recently remanufactured. (I am running it in a ZB Magnette using "MB" needles in the original H4 carbs.)

I found that the fuel levels in the two float chambers were unequal and one jet tube was higher than the other to compensate. By correcting these inequalities I was able to achieve a slower idle speed around 800rpm and the running on has reduced to virtually nothing.

Andy
Clacksman

Art,
I have not tried shutting it off when cold. I will try that tonight. I can get it to idle down around 700 rpm, I just checked the timing at 1000 rpm. Everything in the engine is fresh so I don't think there is any carbon yet. I did not do anything to reduce any sharp areas in the head. I think the CR is stock but not sure. I will also try so premium gas. Will keep you posted!
John Progess

I seem to remember there is something on Barneys site about cleaning the internal connections on the control box. Unless you fitted a new one, might be worth having a read as it would be a very simple fix.
Good luck, Graham
g Victors

Another thing in the combustion chamber that could be responsible is the spark plug. Have you tried replacing with a known good set? ..................Mike
m.j. moore

Something simple to try John. After a run, let the engine idle at its 7/800rpm for a good 30 seconds then as you switch off the ignition, press the clutch down.
I find the premium, non-ethanol fuels help here too.
I think it may also be worth trying the next richer SU needles to give cooler running with ethanol fuel.
Pete
P N Tipping

Hi Pete,
Apologies for going off thread, but woyuld be interested to know which fuel is non-ethanol here in the UK? I had thought government policy meant all fuel was at least 5% ethanol.
Thanks, Graham
g Victors

I had a lot of trouble with running on with both of my MGA's. Someone told me that idling above 800rpm will cause it. I made sure the that I kept idling speed below this and cured the problem! My daughter's Dolomite Sprint was chronic (a common problem I understand with Sprints, perhaps more so in this country?) but a new (expensive) coil cured the problem in that case.
Barry Bahnisch

Open the plug gaps by another 5 thou
J Bray

Any news here from the original poster? Wondering if youve made any changes that helped.
Steven Devine

As I understand it all the premium grade fuels, except Shell's V-Power, are still ethanol free. On these my "A" has really got some - "Thrutch"!
Pete
P N Tipping

Is that the same as "grunt"?
Art Pearse

A fairly common problem on all the B series engines - normally minimised by adjusting the minimum advance and richest mixture consistent with good running. I would be looking for a CO figure of 5% at idle ( around 800 rpm on a standard engine).

For those engines that fail to respond to adjustment, simply floor the accelerator as you turn the ignition off - the engine will stop dead.
Chris at Octarine Services

Art - the answer is of course yes; an expression we used as students in the 60's. But our MGA's don't grunt do they..?
Pete
P N Tipping

Hi to Dr. Betson, it's good to see you back!
dominic clancy

Hi Dominic - thanks!
Chris at Octarine Services

This thread was discussed between 11/06/2013 and 16/06/2013

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.