Welcome to our resource for MG Car Information.



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 - Engine Run-on

Today I refueled with the cheap stuff (87 rating -US).
I found that when I turned the engine off the engine would diesel/run-on for 2seconds.
Other than that the engine runs fine and seems to have the same power as with higher octane fuel.

I have read Barneys page on this topic.

My question is; I this something I should worry about if I want to continue the cheaper fuel?
I could retard the timing to make it go away, but I would prefer not to mess around with it, since the engine is running good currently.
(you know what they say 'Never touch a running system').

Have you experienced something similar?
Gonzalo Ramos

Hello Gonzalo,
I had the same problem and I found using the higher octane fuel helped a lot, so why use the cheap stuff!, and over here im paying 1.26 a ltr, and no im not welthy, vin
Vin Rafter

If you are not getting any spark knock,(pinging/pinking) with the lower octane fuel it will not be a problem. Run-on can be caused by a fast idle speed, try lowering the idle a bit. You can put the car in gear and let the clutch out just enough to put a slight drag on the engine when you turn the key off. That will prevent run-on. (That's what I do.) With a little practice it becomes the normal shut down procedure.
Ed Bell

Another trick to try is to let the engine idle briefly (15-20 seconds) before you shut it down. Run-on is usually caused by hot spots, often carbon built up, providing ignition. Idling the engine lets the hot spot(s) cool down. That said, run-on is more annoying than harmful.
Octane is all about detonation. Higher octane makes it harder to ignite the fuel, which is why it helps reduce run-on. But tt may actually make a lower compression engine harder to start. It doesn't produce more power, burn cleaner, etc. All that marketing hype is just BS to get you to buy it. Unless you have a higher compression engine you don't need it. Save your money and buy stuff that will benefit!
G T Foster

As GTF states once you have enough octane more WONT help. Octane does not create more power in any way by itself. It allows a higher tuned engine to create more power through changes to (mostly) compression. If your car runs properly on 85 octane buying higher octane is a waste of money. The primary causes of runon are too high of an Idle setting and engine running too hot.
R J Brown

OK, thanks for the feedback. I will try to lower the idle but it is already low. The car will idle nicely at 1000rpm when warm (after 10mins drive), but will turn off when I come to a light any time before the 10mins if I don't pull the choke slightly (fast idle).
Gonzalo Ramos

Idling speed above 800rpm is almost certain to result in run-on.
Barry Bahnisch

Sounds to me like you may be running lean. Lean engines run hotter. Hot engines run-on. You might want to check that mixture. Another cause of lean is air being sucked in around worn throttle shafts. A way to check for worn shafts is to spray some starter fluid around the throttle shaft bushings. If the engine speeds up it sucking air around the shafts. Just some thoughts.
G T Foster

I turns down the idle down to about 800rpm and it doesn't run on any more.
I would be interested however to make sure I am not running lean.

The story is as follows;
When I got the car the PO said is was running strong, in fact it was running SUPER-rich and galloping. Black smoke and sooty spark plugs.
I rebuild the carbs and used the SU-book method to adjust the jet height. When I thought I got it right using the method of the lifting pin, it it turned out to still be running rich.
I was not successful at the standard method described in the book or on Barney's side, which involved lifting the pin and until the engine speeds up just a bit and then slows down again. Once I have done this on both carbs, I synched and the engine was definitely running rich!.
So I closed the jet a couple of flats until the engine sounded good and synched the carbs. Then I went for a drive. After I check the spark plugs. 2 cylinder had rosy spark plug (I understand this is how they should be) and the other 2 cylinders they were very sooty. So I closed the jet on the rear carb 2 more flats and checked again after driving. Now they are all running 'rosy/pink' with very fine white deposits (I read this is a normal for a normal operating engine).
I will try to post some pics.
Maybe I am running lean, but the engine seems to run pretty good otherwise...

Is there any other way to check if the mixture is right?

Gonzalo Ramos

I don't know about "rosy/pink". I shoot for medium to dark brown. If you are seeing fine white powder you are almost certainly too lean. Some people like to use a "color tune" It screws into the cylinder and lets you view the "flame" when the car is running. Other like to use an O2 sensor. I believe there is info in the archives on this technique. I just use my SU adjustment kit, my ear, and the "lift the pin" method. Unless you know the history of the carbs I would still check those throttle shafts.
G T Foster


I had a reddish tint on my plugs a few weeks back and posted pictures of them. I was told by the guys that it was the additives in the fuel.

Steve Gyles

This thread was discussed between 13/05/2010 and 15/05/2010

MG MGA index

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