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MG MGB Technical - Running on

Hey Folks,

there are a few issues remaining on my 73 B. The first is that it seems to operate a little hot, about 1/2 way between the "N" and the "H" on the gauge. With an infrared thermometer I read about 255 degrees, (F), at the hottest point on the back of the head. Do I have a problem?

The second is that the car "runs on" after shutoff. I read that this is a common problem with mildly tuned cars. Does anone have a "fix" for this issue? (High-test?)

Are the 2 conditions related?

Thanks, Tom, enjoying his ride!
Thomas McNamara

Tom,

I don't think half way between N and H is overheating. 255F is overheating if it represents the coolant, but I suspect it doesn't. What do you get with the infrared on several places on the radiator?

Overheating will contribute to running on, but I'm not convinced it is overheating.

Yes, more octane may cure it. Also, getting the idle as low as practical may help. Yes, tuning, such as higher compression, can contribute. If the ignition timing is off, this can also contribute to running on.

Until you get it sorted, I would turn it off while in second gear, and ease your foot off the clutch just as it is about to transition from slowing down to running on.

Charley
C R Huff

Hi Tom

I can't help regards the overheating but we run a 76 BGT. I use Castrol Valvemaster additive and use a high octane fuel when filling up. When I switch off, I push the clutch pedal down putting the engine under load and keep it depressed as I turn the key. I don't get run on when I do this.

Hope it helps!

Martin.
Martin Clegg

This is usually caused by a design fault in the combustion chamber. There is a small protruberence in the kidney shape of the head between the intake and exhaust valves. Grinding this down a bit will help to rectify the problem. Also, as mentioned, the lowest possible idle will reduce the tendency for run on. My engine is highly modified and doesn't like to idle below 1,000 rpms. I, too, engage 2nd gear while letting out the clutch when shutting the engine off to prevent run on. RAY
rjm RAY

Ray,

That isn't the case on a 73 B which will have the later design of chamber without the sharp protuberance.

Run on is normally due to a combination of advanced ignition and weak mixture causing the normal carbon deposits in the chamber to glow red hot.

A richer mixture and retarding the ignition slightly is all that is usually needed to stop the running on.

In bad cases a "decoke" achieved by spraying water into the carbs with the engine running will clear enough of the carbon to stop the running on.
Chris at Octarine Services

Hi Thomas!

Easy fix, 73 MGB came with a Anti Run On Valve BL part # 12H4295, MOSS part # 367-110, which was part of the emission controls system,re-connecting / re-Installing if missing should solve the problem. I have the originals still connected on my 73 and 77 Roadsters.

Hart

Hartley Mayers

Hey Folks, the running on seems to have stopped after fillig uo with high test! OUCH!! Thanks for all of the input. Tom
Thomas McNamara

Chris has it right. Spray water in the carbs while running will clean out the carbon. I also have used (ATF) automatic trans fluid. I squirt it in till the motor falters and then turn it off. Let set overnight and start it up in the next day.
WARNING.
Cover your exhaust pipe with a sack or some tough cloth. It will spew out black gunk on start-up.
If when you pull a cylinder head and notice clean streaks across the piston tops you have had a water leak from the gasket.The steam cleans it off. I always use the oil method prior to pulling a head as it leaves the carbon mostly all soft . Makes for an easy clean up.
Sandy
SANDY SANDERS

From what I've heard (from engines, not comment) 73's are one of the prime candidates for pinking and running on on unleaded fuels. Mine got so bad after the change from leaded I had to fit an anti-runon valve. At a hill climb a few years ago it was very evident that era of engines were clattering their way up the hill, mine included, whereas earlier and later were not. Over-advanced ignition timing certainly affects pinking, but retarding it to stop that causes higher combustion chamber temperatures as more energy is being convcerted to waste heat, which makes running-on worse. I don't know about the clutch loading the engine, I've heard people say their idle slows down when they push the clutch pedal down but neither of mine or ones I have worked on do. Others say to *stall* it on the clutch, or floor the throttle as you switch off, both are which are horrible. Whilst I use Castrol Valvemaster as a protection against valve seat recession, it is only the 'basic' product, the more expensive one with the supposed octane booster made no difference at all to my engine. And for that matter neither does 98 or 99 octane unleaded fuel over 97, but 95 is dire in the extreme.

If Thomas has a US spec engine that would make it low compression, so pinking shouldn't be a problem, but US cars from 1973 (18V672/673) did have an anti-runon valve as standard. If any of the emissions plumbing, or the valve itself, isn't functioning correctly then the anti-runon feature will be effectively disabled.

Whilst half-way between N and H isn't in itself overheating (that is only when it is steaming or boiling) it certainly sounds too warm for me at this time of year, unless New York is experiencing a heat-wave. But that could be a gauge problem as much as anything else, an infra-red thermometer needs to be pointed at the thermostat housing or the radiator header tank by the inlet to get an idea of coolant temperature. It also needs to be pretty close as even with a 10:1 aspect ratio of distance to focus you don't hav to be very far away to start reading ambient from the surroundings which will depress the true reading. High coolant temp is caused by the engine generating more heat than it should, or the radiator not being able to dissipate as much as it should. The former can be caused by any number of things such as retarded timing, weak mixture, the latter by blocked areas in the radiator, insufficient circulation caused by corroded pump vanes, and all sorts.
Paul Hunt 2010

This thread was discussed between 05/04/2010 and 09/04/2010

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