QuestionQUESTION: Good day Barrie, I am just finishing the semi-restoration of a 1975 MGB. I am experiencing some pretty heavy dieseling after turning off the ignition, I have advanced and retarded timing, set and re-set idle at carbs, have replaced spark plugs with RNY9C type, saw in one of your answers to avoid any plugs with R in them. What is the best plug for 1975 MGB? Book says champion NY9. Also not sure where the run-on-control line hooks int intake manifold, is it right behind the carb closest to firewall? Also manual says to port gulp valve to intake manifold as well can;t find another port. Any advice would be welcomed, I have read every piece of advice on chat lines and still have problem. Starting to get to me a little, please advise. Thank You for your time.
ANSWER: Hi Chet. Plugs for a 75 MGB? I recommend Bosch W6DP or NGK BP6ES. Champion numbers have changed since 1975, so don't try to find N9Y. If dieseling continues, try a harder plug, such as Bosch W5DP or NGK BP7EVX. The numbers go different ways!
Anti run on valve uses a large threaded hole in the intake manifold. Do you have a servo? If so, take a Y piece off that pipe. Most people wire it via an oil pressure switch so it opens if you have no pressure and won't close until you have cranked enough to raise some pressure. You may find that the plugs solve the problem without the need for the valve.
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Does your carburetter have a spring-loaded valve in the throttle butterfly? If so, they tend to stick open and cause running-on (dieselling). Best to remove them and fit plain discs, or you can solder them shut.
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QUESTION: good morning Barrie, I tried the new plugs last night NGK BP6ES, Worked good at first, then the more I ran it the worse it got, I thought tonoght I would try and utilize the Anti run on valve. I do in fact have a servo, I'm not quite sure which pipe I take the Y off of, the intake manifold pipe or the pipe on the servo? In either case both pipes are a different diameter than that off the anti run on. So the Y would have to consist of two different sizes, is that correct? I will also check the throttle butterfly in the carbs.Thanks for the new info. Do you think trying the harder plug is a factor now or not?
ANSWER: Good Evening, Chet
It could well be the plugs, but check the butterflies first. Then try some harder plugs. Then fit the anti-run-on valve with a Y piece at the manifold, not at the servo end. Be sure the tickover is set as slow as possible, no more than 800 rpm.
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QUESTION: Good Morning Barrie, Well last night produced no results, even went as far as hooking up the ARO same thing. Do you think an air leak in the exhaust mainifold can contribute to this issue? Someone brought that up, also the car manual shows an EGR valve, but I cannot locate one anwhere on the car, could that be a factor? Thanks Chet
AnswerHi Chet.
Certainly, an exhaust manifold leak will affect the mixture and could be a contributory factor, but it isn't the main problem.
I did once come across a problem with the emission control stuff. On top of the rocker cover, at the rear, there was a pipe about 1/2 inch diameter. There was a rubber hose attached to it that went to a canister. At the rocker cover end of the metal pipe, there should have been a restrictor that reduced the diameter to about 1/8 of an inch, but it was missing. It caused a problem like yours.
Can you get the car to idle below 1,000 rpm? If not, you must trace the reason for that.
Is it possible that the engine has an internal build-up of carbon? That can cause running-on. Trouble is, you would have to lift the head to fix it.
Do you have twin SUs or is your car a federal spec car with a single Stromberg? If it is federal spec then it may have all manner of extra items bolted to it. I have seen an air pump, an air manifold, a check valve, and a running-on control valve on cars imported from the USA. In this case, my advice would be to buy the twin SU setup from a UK car and dump all that stuff (if the law permits you to do that). Power would go up from about 70 bhp to 95 bhp in standard tune.
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I don't think you have a problem with fuel pressure or the filter.
The fact that the car will idle at 850 says you have the carbs set up about right. My next test would be harder plugs, preferably Bosch W5DC or NGK BP7ES. When stopping, always let the car idle before switching off, do NOT blip the throttle. If the problem comes back, then the top end needs a decoke.
