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MG MGB Technical - 95 Octane fuel tuning

Hi folks. I've recently gone from 98 octane to 95 unleaded here in NZ. I've retarded the dizzy half a turn to cope with the lower thump juice, yet the old girl over-runs terribly. She's a 71 engine. It pulls great, starts readily and never misses a beat. It's just the awful overrun at engine shutdown. Any suggestions please? Thanks
R P Shoebridge

RP, I think you will find its the lower octane thats the cause. Here in the UK my B run on occationally until i found an outlet that sells good old 4 star leaded (nearly a hundred octane i believe). It does'nt help if the idle speed is high. I used to dip the clutch before i turned it off as this brings the revs down slightly and eliminated most run on, but not all. Theres a bit in the archives about this problem and would also suggest a visit to Paul Hunts website 'Bee and Vee'. Hope this helps. Chris.

Running on can be problem with 95 octane fuel. Can you get the Shell V-power 99 octane in NZ - it's very good, and if combined with Castrol valvemaster Plus even better !
Miles Banister

Try advacing the timing as far as you can go without causing pinking. A little bit can have a large impact. Not sure if they measure the same on both sides of the pond but the best we can get over here is 92 Octane. Both our "A" and "C" have the timing set quite a bit more advanced then the factory specs with no problems. If I back the "C" up by only a few degrees it will start to run on quite a bit.

Otherwise there is always, put car in gear, put foot on brake, turn off key and let out clutch.
John H

John - no the RON in NZ (and I believe it's the same in the UK) is different from the rating system used in the US. A quick google / wikipedia will turn up lots on this subject.

To the original poster - hello fellow kiwi! My 71 BGT and my sister's 70 BGT both run-on with anything less than 98 Octane. These cars were designed to run on high octane - particularly the high compression engines. In my experience it's really not worth saving a few cents a litre - let her drink the good stuff!!
Curtis Walker

Running on is a real pain but you can't do more than set the timing to just pink and no more under heavy load. Make sure that the idle speed in not more than 800 rpm as this will greatly increase the risk of running on. Your car doesn't need decarbonising by any chance does it? Make sure also that the engine is not over hot.

I'm really not for stopping the engine by putting it in gear and releasing the clutch as this winds up the transmission and does the UJs no good at all.
Iain MacKintosh

I found that while running the 45DCOE type weber on my B there was no tendency to run on..NONE AT ALL.
But when switching back (after 18 yrs on the Weber) to the factory type HIF SUs the runing on has returned. So how does this happen to occur??? No other adjustments at all were made to the motor other than replacing the carbs and of course the intake manifold.
When the Weber was installed, it stopped when the key was turned off. Now it's just got to give me a putt or 2 at shutdown.
Also, it started much better in cold weather with the Weber, but I attribute that to the accelerator pump in the Weber which is lacking from SUs.
Wayne Hardy

Miles - BP98 Octane available, but not in the country town where I live!

John H: I thought one retarded the dizzy. But having done that the problem is worse. I'll try advancing instead. Thanks.

Curtis: I agree, when it's available. What about Avgas? That's 110 octane isnt it? Or Racing fuel, 110 Oct with lead? But where from?

Wayne: I have the original SUs. No chnce to swap out with Webers.

The verdict: I'll see if I can store the 98 octane, and advance the dizzy too. Thanks to all.

R P Shoebridge

In California, at least, buying 100 octane for your MG is against the law. Racing fuel (off road), and aviation fuel have different tax rules --- it's a question of who gets the money.

The same for burning jet fuel in your Mercedes.

Dan Robinson

Dan, they aren't talking the same octane as we have in North America.

Any stock MG will run just fine on 89 octane R+M/2.

If it doesn't it isn't a sign that it needs higher octane, it is a sign that it needs the combustion chambers smoothed and decoked.
Bill Spohn

Ray, you may not be able to run as much advance as you used to but I was going by your comment that you had retarded it when you changed fuel. You may just have went further back then required.

In the seventies, many an american car was fitted with a solenoid that would drop the throttle closed when the key was turned off to stop run on due to hotter running engines and lower octane fuel, all due to emission laws.
John H

John H: I understand that later MGBs were also fitted with an anti-run-on device, like a solenoid. Perhaps this is a "feature" of SU carbs? I don't know. I'm definitely not a fan of gearbox/clutch based anti-run on solutions. I fear clutch or engine damage.

Nevertheless, 98 Octane is only 75km away (one way).
Cheers from NZ
R P Shoebridge

Just my 2 cents, here. I try to tune my car for whatever fuel is commonly available, and to how it runs at speed. Not how much it runs on after I turn the key off. New cars have spoiled us with their fuel injection. Just letting the clutch out will stop the engine, there's not much horsepower going into the drivetrain at that point. My car tends to run on somewhat if it's hot, and most times it runs backwards. I don't know how many times people have told me to check my timing. How can it be timing if the ignition is turned off?

What a good point, how can it be timing with the engine turned off, hence no plug spark.

Also how can gently stalling the engine, at the same time as turning the ignition off cause any issues. Actually driving the car must put a lot more stress on the drive train components.

Charles Goozee

Timing affects combustion temp. Retarded timing makes the engine run hotter. To much heat, glowing carbon, or the sharp point on the head casting between the valves getting very hot causes the run on. This is why they tend to run backwards, the fuel ignites too soon from the heat and drives the piston down before it reaches TDC. The early emmision controled cars had the timing retarded so they would run hotter which reduces emmisions but causes lots of other problems.
John H

One way of eliminating run-on or dieseling is to open the throttle just after you switch off. This has been discussed at length elsewhere as some are concerned that it may result in neat fuel washing the oil from cylinder walls, however as SUs do not have an accelerator pump this may not be much of a problem.

I have two identical engines, one in my MGB and the other in my Magnette. Same head mods, cams, compression, and one runs on and the other doesn't....

You live in a beautiful spot, Ray, only perhaps a Deux Cheveaux would be more appropriate than a B!

David Overington

Slip of the fingers - "Chevaux". Apologies to francophones out there.
David Overington

The Anti-Run-On Valve (BMC Part # 12H 4295) fitted on the 1973 and later models as its purpose is to apply such a strong vacuum to the chamber above the fuel in the float bowls that the fuel cannot exit the fuel jets when the ignition is switched off, thus preventing the car from running on. When the ignition is turned off, the ignition switch energizes this solenoid-actuated Anti-Run-On valve in order to close it, and then the oil pressure switch releases it after the engine has stopped and oil pressure has fallen. The Anti-Run-On valve is open when the engine is running, allowing fresh air to be pulled through the adsorption canister, clearing it of the vapors that have expanded into it from both the fuel tank and the carburettor float bowl chambers, then through the rocker arm cover and tappet chest into the induction system to be consumed in the combustion chambers. The rocker arm cover (BMC Part # 12H 3252) of the North American Market 18GJ, 18GK, and 18V engines is equipped with a restrictor tube in order to prevent the fresh air that is being drawn in from overly diluting the fuel-air mixture and causing lean running. The size of the hole in the valve cover restrictor tube has been restricted so as to not present a significant vacuum leak to the induction system. If you enlarge the hole size, then you will adversely affect the tuning at idle speed. This may result in the loss of smooth idle, or an inability to idle at the correct low engine speed. It will also result in an overly-rich mixture off-idle, the vacuum leakage remaining relatively static and not increasing proportionally as the throttle is opened. Much the same happens with a genuine vacuum leak, i.e., at the intake manifold gasket. This Anti-Run-On system can be readily retrofitted onto 1970 through 1971 18GJ and 18GK engines as well as onto the 1972 18V-584-Z-L and 18V-585-Z-L engines, all of which have the necessarily modified fuel tank (BMC Part # NRP4), adsorption canister (BMC Part # 13H 5994), nonvented oil filler cap (BMC Part # 13H 2296), nonvented fuel tank cap (BMC Part # BHH 1663), and restrictor tube equipped rocker arm cover (BMC Part # 12H 3252) as standard equipment. Conversions of earlier engines will need all of these items.
Steve S.

Thanks again for the tips everyone. Chris82 suggests depressing the clutch to assist. I'll try that too.

Steve S has the issue sorted, but that much additional hardware would make the solution more painful than the cause.

Akaroa is a lovely place but David Overington, wash your mouth out with soap! This is a lovely French town, but NZ is a Colony of good old Blighty! The Union Jack flew here in 1840 before any Tricolour did, so British Cars, especially MGs RULE! OK! Besides, the Deux Chevaux is so un-cool. Ask Clarkson!
R P Shoebridge

Pinking can be cured on low octane simply by retarding the ignition, but that makes running-on worse, as does a high idle, but a low idle can be very rough with a tendency to stall. I have found even the higher 97 and 98 UK octanes are significantly worse in both respects compared to 4-star, although some eras of MGB were always worse and pinking and running-on than others. I tried an after-market anti-runon valve but it made no difference, until I plumbed it in similar to the North American valve which is 100% effective.

FWIW only a very *small* vacuum needs to be applied to the float chamber vent to suck the fuel out of the jets, I was amazed just how small. By simply applying vacuum to the top of the overflow pipe and leaving them open at the bottom I wasn't sure it was going to do anything at all. It did - but it still ran on although completely differently to before. I rapidly discovered that there was so much vacuum that it was sucking fuel out of the float chamber and feeding it into the inlet manifold, and that was enough to keep the engine running! I had to put a restrictor on the vacuum pipe between the valve and the overflow pipes. You can see just how small the vacuum needs to be by sucking gently on the overflow pipe with the engine running. Now I can have a higher idle for smoothness and no stalling, and only very occasionally get a slight and apologetic cough as I switch off in any conditions.

Paul Hunt 2

relay plse need a simple wiring diagram for fitting a relay to headlights,main beams only

Trevor - you should probably have started a new thread with an appropriate subject, but you might like to have a look at and click on 'Lighting' and 'Uprated headlamps - Relays and Fuses'. If you only ant to fuse the main beams you will only need the relay in the blue/white circuit.
Paul Hunt 2

This thread was discussed between 05/02/2008 and 14/02/2008

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