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MG MGA - Spark Plugs

A number of years ago I rebuilt my engine. As part of the rebuild I had to shave the deck slightly and it increased the compression ratio to 8.8

At the time I also installed a VPN 11 fast cam.

The car also has electric ignition and a petronix ignition coil.

These are photos of the Autolite 64's I've used for the past 9,000 miles, 8 years.

The car has had a tendency to diesel after shutting down and I just felt like it was running too lean, so I enriched the mix slightly.

Last weekend I switched over to Autolite 63 for no other reason than its been 8 years and they are referenced by Barney.

So I'm curious. Given the changes to my motor should I be running a hotter plug.

Is there a summary of plug that fit the MGA with their relative hotness.

What do you all think of the coloring of the plugs?

Thanks,

Tysen


Tysen McCarthy

Looks good from here!
S

Tysen
your plugs look ok to me, maybe on the weak side of perfect if anything, I personally wouldn't go any weaker.

I use NTG BPR 6ES on my car and they seem fine although I do run a tuned up MGB 1950cc engine.
The number 6 describes the heat range and the lower the number, the hotter the plug runs.


So if my plugs looked like yours I could try fit a set of cooler plugs which would be BPR 7ES but I would probably try to richen up the mixture first as this could also cool things down.

If the plugs run cooler by either of those options, you may find that the "running on", or dieseling as you call it, could be much reduced.

You could also try slowing the tickover a little bit as this can also reduce "running on".

Colyn


Colyn Firth

Not fuel quality? I find my sprite runs on if I don't use high octane petrol (gas). I can virtually get out the car and shut the garage door while it runs on (well, slight exaggeration for effect)
Graeme Williams

"I use NTG BPR 6ES on my car and they seem fine although I do run a tuned up MGB 1950cc engine.
The number 6 describes the heat range and the lower the number, the hotter the plug runs."

So presuming that Autolite follows the same numbering format Autolite 63 would be hotter than the Autolite 64 I was running.

So I'm probably going in the wrong direction.

Graeme I'm running the highest octane I can get here in the States without going to racing fuel.
Tysen McCarthy

I found this online Tysen which may help you decide.

It looks like it is the last number on the Autolite plug type that indicates how hot it runs.

More interestingly, it seems to say that Autolite numbers run the opposite to the NGK in that lower numbers are cooler and higher numbers hotter!

(Unless I have misread it!)

Check out
http://www.briskusa.com/spark_plug_cross_reference_heat_range_chart

Also useful is that it explains how a cooler plug is able to take heat away from the combustion chamber by clever design of the central ceramic insulator.

Hope this helps

Colyn
Colyn Firth

I'm with Colyn. Look a trifle weak to me, but not an area of expertise for me. Also, like Graeme, I cut out the run-on with the use of 98 octane - just one of the pluses other than price.

Steve
Steve Gyles

Champion's numbering system also runs contrary to NGK's.
Dave O'Neill 2

Usually a fast idle or low octane cause a run on. What's your car idling at?
S

NGK numbering system is backwards from most everything.
Generally, for every rise or drop in NGK numbers, like 6 to 7, there is at least one in between spark plug from Champion or Autolite. Sometimes 2 or more in between. Do not trust spark plug converter websites, as they will give you something like 10 different Champ plugs (for instance) for a NGK6. Wrong, all wrong. You need a hotter plug for around town versus blazing along on the highway. A racing car with high compression and high rpm's needs a much cooler plug for instance
T Grey

I spent years worrying about running on until I read a 1950s original MGA road test and that said th car ran on so I stopped worrying too much.

Paul
Paul Dean

Running on is usually caused by some carbon in the combustion chamber that is glowing enough to ignite the mixture even after the ignition is switched off.
So getting the car to run cooler in the chamber can help prevent it, cooler plugs, richer mixture, and generally improving the cooling system etc.

If you can wait until you decide to decoke the engine and in the meantime try slowing the tickover or just live with it.

On my previous Mk II I used to either stall the engine by lifting the clutch in gear after switching off or alternatively, floor the throttle which also works by letting fuel mixture cool the chamber.

Cheers
Colyn
Colyn Firth

There are many areas that can cause running on.
They all relate to having a "hot spot" in the combustion chamber.
They can include an overheated spark plug tip, a sharp end of a spark plug thread, an overheated edge of worn exhaust valve, glowing carbon in the head etc.....
It is not a fuel problem. No fuel will ignite without an ignition source.
The only real solution is that used by the factory in the MGB.
A bit complicated with vacuum being applied to the carburetter bowls when the engine is switched off. This draws the fuel away from the jets.

Mick
M F Anderson

The car is/was set to idle at 900 rpm. (that could stand a double check)

I'm thinking that increasing the compression ratio to 8.8 contributes to the dieseling issue.

I'll give the new plugs a chance to get a patina and check them.

After that I'll play with the mixture.
Tysen McCarthy

Two completely diffferent problems should not be confused.
"Running on" is fuel being ignited by a hot spot in the combustion chamber after the ignition is switched off.
"Dieseling" is detonation, also called "knocking" or "pinging" or "pinking" and is a rapid fuel explosion instead of controlled burning of the fuel. This is caused by a mismatch of the compression ratio and the fuel octane (assuming that the ignition timing is correct).
But just to complicate things a combustion chamber hot spot may ignite the fuel before the spark plug fires. This is not dieseling as even high compression ratios do not ignite gasoline fuel without some ignition source.

Mick
M F Anderson

Thanks Mick,
I hope I have not been confusing the issue by my assuming that Tysen was talking about running-on.

The term "dieseling" for " pre-ignition isnt used so much here in the UK. I have always called it "pinking" as I think that word best describes the actual sound that it makes.

Tysen, your compression ratio shouldn't be an issue here, my engine has a 9.3 to 1 ratio and doesn't run on.
I would slow the tickover to around 600 and see if that helps, it may be all you need to do.

Colyn
Colyn Firth

Mick,

Thank you for the clarification.

The issue I'm handling is running on after ignition is switched off.

Its still doing it with the new cooler plugs, but not as bad.

I'll check the color on the plugs after a few more drives and then pursue it with enriching the mix and then idle speed.

I'll report back
Tysen McCarthy

Tysen,

What temperature is your engine running at? Mine would run-on/diesel (in the USA) when the engine temperature was hitting 220 dF.

Gene
Gene Gillam

Gene,

Mine runs a pretty reliable 195 F
Tysen McCarthy

Tysen,

Cool enough...good luck, these kinda niggling little problems sux.

Gene
Gene Gillam

Tysen, I would say the colour looks about right. Extended nose spark plugs are designed to keep clean when subjected to slow driving in heavy traffic but are also prone to causing pinking and running-on. NGK recommends B6ES for MGAs, not BPR6ES which is used by many MGA drivers! The B6ES is not an extended nose plug, so the tip runs cooler and is less likely to cause running-on etc. The B6ES is much better suited to 'hard driving' on the open road but more likely to foul in slow moving stuff. Racing spark plugs are not extended nose!
Lindsay Sampford

Just to reinforce what Lindsay has said here is a comparison of the B6ES (NGK recommended for the MGA and pictured centre) and the BP6ES (same as BPR6ES but without a built-in suppression resistor), with an original Champion N5 on the left and as recommended by the factory.............Mike


Mike Moore

This thread was discussed between 10/06/2016 and 24/06/2016

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