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MG MGB Technical - Spark Plug Gaps

I just put new NGK BP6ES plugs in 1974.5 mgb roadster. The previous plugs (Champions) had been sitting in the car for 15 years, but were curiously working fine. The car seems a bit smoother now, but not that much.

Which leads me to wonder whether altering the gap a bit will improve things more. I set the gap at .22--which is what my original MG Workshop Manual says it should be. My ignition coil and wiring are all stock original.

What gap are most people running or recommending?

Jeremy
photos.yahoo.com/jeremysmgb

P.S. I have read a number of posts by people with sport ignition coils talking about this; but I'm hoping to hear from some people with original coils.
J. Palgon

Jeremy - I have always run 0.025" with a standard coil and had good results. A larger gap will cause the ignition voltage to be higher (up to the limits of the coil), which will result in a hotter spark. Cheers - Dave
David DuBois

Standard gap with a standard coil is .025" which I've always run without problems. There may be a case for a wider gap with a high energy system but the whole system needs to be capable. When the gap is widened there's more chance the energy will leak elsewhere rather than jump the gap. That will cause a misfire and the moment it has done it once then its found a new route it will use next time rather than fire the engine. This might be across the cap or rotor, down the leads etc. It may also be an internal short in the coil and hence the start of a major fault. The Champion in our lawnmower is now in its 20th year without cleaning or gapping. Doesn't do a lot of mileage though:-)

Rich

Modern engines from the factory are running plug gaps of .050 to .060". The double platinum plugs will not need touching in 100,000 miles. By that time the gap will be even greater.
With electronic points and an ignition system in good condition, the MGB will run well with the wide gap. I've found cold running and start up are much better with the wider gap.
The ignition system only develops enough voltage to jump the gap. A hi compression engine requires higher voltage, (the more air is compressed, the better insulator it is) and of course a wider gap requires a higher voltage.
The higher the voltage, the more challenge to your entire ignition system. Electricity is not selective, it will take the shortest path to ground. That may be inside or outside your distributor cap, from a wire or at the plug, or at the coil.
I'm using a stock cap, electronic points, upgraded plug wires, and a higher voltage coil. It has all worked very well, except when the heater valve spilled water on the cap. In short order I had spark jumping from a plug wire to the distributor body. I now carry an extra distributor cap just in case.
Barry
Barry Parkinson

Now ya'll got me wondering if i should change my gap, I'm running .035 using NGK plugs, Lucas Sport coil, (40k volts i believe) with Ignitor instead of points, in a 73 B. Was running sweet before it went in the paint shop but still looking for that little extra edge. Anyone with this type set-up, what are you gapping your plugs at?
LUVMYMG

'59 TwinCam, Lucas sport coil, Pertronics ignition, .032. '64 B stock everything, .30. I think one would be splitting hairs as you get into the "how tight should the feeler gauge be?" argument. Frankly, I don't think it matters much for everyday driving as long as its in the range of .025 to .040 and all else is in proper working order. jm2c
Paul Hanley

Thanks for the input guys. I just noticed that I said I was running a .22" gap--clearly a typo. I meant .022". As I mentioned before, the Workshop Manual said this was the correct gap. However, most people seem to think the gap should be no lower than .025". Do you think I should bother to pull the plugs out again and regap them to .025"? Or is the difference so minimal that it doesn't matter?

Jeremy
J. Palgon

The standard coil's an excellent unit and around 24kV. Not many sports coils do a lot better and I'm doubtful of 40kV claims other than with a modern full electronic setup. There's a test somewhere on the web I'll post if I find it. Even the Lucas sports coil gave only a bit more voltage and you pay for it by hammering the points with more current.

The old servicing guides used to reckon it took another 1kV per 1 thou of gap to keep firing as the plugs became worn. The 25 thou standard gap no doubt has a margin in hand to allow for wear between servicing and wear was reckoned at 1 thou per thousnad miles, so, a 30 thou should be no problem when new but may stop firing when the plugs foul or get a little worn.

Once it misfires the coil can be ruined as the energy still tries to go somewhere and will soon break down a new path to ground (and is the reason for taking off the coil low tension lead when cranking compression tests etc so it doesn't operate if there is no path to ground for a spark). imho you're far better off starting with a new coil and never causing it to misfire than risking odd misfires later on trying for a negligable improvement from a bigger gap.

Rich

http://www.mgcars.org.uk/electrical/body_coil.html

Rich

This thread was discussed between 19/06/2005 and 20/06/2005

MG MGB Technical index

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