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MG MGB Technical - Engine run-on

Short of finding 100 octane. What suggestions are there to eliminate engine run-on. Killing the engine by stalling in gear with foot on brake does not seem the best way of stopping the motor from turning over.
Lee Sheldon

First try retarding the ignition slightly - then get an anti run on valve from any other car in a wreckers yard and wire that into the ignition circuit and attach it to the inlet manifold so that when you turn the igniition off it opens the valve, letting lots of fresh air into the manifold and killing the engine.
Chris Betson

Lee,

First do a tune up -- change plugs (points and condenser if you have them)

FYI -- Go to your local airport using airport transfers sevenoaks and chummy up with folks at a hanger or service provider --- then ask if he'll sell you aviation gas. It's 100 octane or better and it is the last of leaded gas -- albeit low lead.

Also, it's anywhere between 50% to 100% more expensive than gas station gas.

Off we go into the wild blue yonder,


glg
glg gimbut

I have successfully cured hot weather run-on on an A series (midget) engine by increasing the plug gap and the Valve clearances by 3/1000 of an inch in both cases.
Hope that helps.

Martin

Lee,
You could also try an octane booster - a number of suppliers sell lead substitute together with octane booster - bit on the pricey side depending upon how many miles you do though. I use Millers and forgot to add it when I filled up once - it was only when the engine started running-on that I remembered
Richard
Richard Atkinson


Chris,

I am not familiar with anti run-on valves. Are there specific automobiles that use them? My GT is a 1970 and am not familiar with the later B's; did they use this valve?

The car has been recently tuned and timing set at factory specs. The engine is slightly modified with a mid range Datsun steel cam, no smog head,and no change in compression ratio. Since the car is over 25 years old it is no longer subject to California smog requirements. All smog equipment has either been removed or inoperative. (hope the gov't busy bodies are not reading this!!)

The engine is strong and and smooth in all speed ranges.
Lee Sheldon

Hi

I suggest you try a colder spark plug (eg BP7ES).
I put them in my GT, and it has helped with run on and pinking, although it has made the engine a little grouchy before it has warmed up.

BTW, a tip for the 'thrifty' ones amongs us...
I was in the breakers today and noticed that some Metro's / Rover 214's (small UK hatchback) have what appears to be an anti run-on valve fitted (by the brake servo).

Don
Don

Hi Lee,

I find keeping the idle speed down in the 500-750 rpm range helps eliminate run-on...

cheers
mike donaldson

Try checking the idle speed if it's too high you will have runon. Carbon deposits in the cylinders head will also cause runon especially after high engine speed driving.

Cec
cec

I had the same problem on my 78 B and eventually fixed it (mostly) by doing the most comprehensive tune-up I could muster. Albeit I was trying to fix other problems and encountered the over-run along the way, but it turned out that colder plugs (as per Don from Cornwall) and near-perfect timing and carbie setup stopped the problem completely.
I was already using a lead substitute system (Flashlube) to run premium unleaded (I think around 98 octane) but have since been able to disconnect that and rely on a well-tuned head with the right valves, etc. to still use the unleaded. I've been checking my mechanic's advice against a great book the guys here recommended, "How to Power Tune MGB 4-Cylinder Engines" by Peter Burgess and so far all my recent "adventures" with the B have been improvements rather than stabs in the dark.

Dave
David

Have a look here http://www.mgb-stuff.org.uk then spanners, fuel, running-on.
Jean-Marc Thély

Octane boosters and dumping air into the manifold did nothing for mine, which is what led to the link in the previous posting, which has completely cleared it. YMMV.
Paul Hunt

This thread was discussed between 24/03/2003 and 27/03/2003

MG MGB Technical index

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