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MG MGB Technical - Engine Run-On (dieseling)

This more of a question that a request for help. My rebuilt MGB 3-main has a problem with engine run-on (Dieseling) after the ignition is turned off. The carbs are tuned correctly, as is the timing and the car is fitted with a pertronix distributor and coil and uses NGK BPR6ES spark plugs gaped at .032". The compression is 165 PSI. The problem exists only when I use regular grade (87 octane) gasoline and does not occur when I use premium gasoline (93 octane).

My question is: why does the octane rating of gasoline affect the run-on condition? I wonder, should I consider a cooler sparkplug, e.g. BPR5ES?

Frank Grimaldi
Frank Grimaldi

The MGB is prone to do this. My car stopped doing it after the fast road head went on. Openeing the throttle used to stop it, but it's rough and unpleasant when it happens.
Stan Best

Use the better quality gas but the plug range mon NGK goes the other way so try BPR7ES not the 5
jim soutar

Usually it's pinking that's worse with the lower grade, and if you retard the ignition to deal with that the engine runs hotter and then Dieselling gets worse. I found Dieselling was significantly worse when changing to unleaded, even 97, 98 and 99. Idling was rougher on unleaded but increasing that to smooth it out also made Dieselling worse. 70s engines seem to be worse with their slightly higher compression and a knife-edge protrusion into the combustion chamber between the valves. Lyndsay Porter recommends grinding this back slightly, even though it lowers the compression a bit. I could live with the one or two coughs before unleaded but not with the shuddering that carried on for several seconds. I hated flooring the throttle as I switched off, so bought an after-market valve that Tees into the servo vacuum hose and it did ... absolutely nothing. So I plumbed it in similar to how the North American anti-runon valve is, which sucks the fuel out of the carb jets as you switch off, and at last a peaceful switch-off. Subsequently I tried attaching the valve direct to the inlet manifold, with the early PCV valve port, and that also worked.
PaulH Solihull

Paul,

would you please be more specific. What kind of valve are you referring too? I understand that you screw the valve into the PCV port on the intake manifold, but does the valve have an electrical or vacuum connection and, if so, where does it connect?

I already have a late model MGB PCV valve installed on my 3-main engine, but it is there to create a vacuum in the crankcase to help prevent oil leaks.

Frank Grimaldi
Frank Grimaldi

Someone once told me that higher octane fuel is actualy slower burning? Is this why we do not get the 'explosive' pinking that we get when using lower octane fuel?( old jaguars used to burn exhaust valves because jaguar insistent on high octane, so fuel was still burning on its journey out of the exhaust port?) Running on in your case may be that there is a little hot spot, for example a sharp edge in the combustion chamber,it simply ignites some spare fuel. Remember the old 'Glow plug' engines of our youth in model airplanes, we got them hot with the battery first, then once engine running disconnected, but the engines kept running so long as there was fuel. If the hot spot was your plugs, going colder might not necessarily stop the problem, you might soot up also. Puting load on the engine will stall it. But certainly, the valve that Paul mentions should quit it. Mike
J.M. Doust

Frank The early MGs tend to run-on if the idle speed is over 800 rpm. Even if everything else is OK. Aim for 700-750 if you can but up to 800 is usually all right. Denis
Denis4

Frank - the after-market valve is an electric valve that closes when you apply 12v to it, and opens when 12v is removed. Thus it is intended to be powered off an ignition supply, to open and close a port into the inlet manifold. When the ignition is switched off the valve opens allowing copious amounts of air to be drawn into the inlet manifold, diluting the mixture beyond the point at which it will ignite. Also makes a useful immobiliser.

Higher octane resists compression ignition better than lower octane, hence timing can be advanced with it. With lower octane, if the ignition occurs too much before TDC, some of the mixture explodes from the additional compression from the part that is burning normally, and the sound of the explosion is what you hear as pinking. I'd be surprised if the mixture were still burning as the exhaust valve opens, that implies the timing is way retarded from what it could be, and would be converting large amounts of the energy in the fuel to wasted heat rather than forward motion. The more of the burn that occurs as the piston is going down on the expansion stroke, the more is wasted.
PaulH Solihull

Paul,

Thank you for your explanation. One more question: Would you please tell me the make and model of the valve you are referring too or at least the name of this type of valve and what it is normally used for. Also, if possible, a source in the USA.

Thank you,

Frank Grimaldi
Frank Grimaldi

Most '70s Bs, that were shipped to N.A., were fitted from the factory with what Paul is describing. It's called an Anti-Run On Valve and allows air to enter the engine's intake manifold when the ignition is switched off. This lean mixture reduces the probability of the engine running on. The 3 main and early 5 main heads have a kidney shaped protuberance, between the valves, that tends to get very hot during combustion. When I rebuilt my engine, I ground this point down so that the running on would be eliminated. The later heads have this modification incorporated from the factory. These early engines were designed to run on premium fuel and using regular fuel will cause the results that you are experiencing. RAY
rjm RAY

The factory anti-runon valve dates from 1973, it was the charcoal canister that was added to California cars in 1970, the rest of the US in 71. It's not really feasible to retro-fit the factory valve to earlier cars as it was intimately linked with the rest of the emissions system.

The aftermarket valve is ADU9535 used to be available from the MGOC, is apparently available here http://www.ebay.co.uk/itm/MG-TRIUMPH-ROVER-ANTI-RUN-VALVE-ADU9535-NEW-AUSTIN-/170691454176 both in the UK. LBCarCo references it here http://www.lbcarco.com/parts/valve-anti-run-on-ADU9535.htm but there is no other info, perhaps no longer available. There may be eBay sources in the US.

You can see how I connected it to the inlet manifold here http://www.mgb-stuff.org.uk/fueltext.htm#solution2
PaulH Solihull

This thread was discussed between 12/09/2011 and 16/09/2011

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