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MG MGB Technical - Spark plug gaps

I've just fitted my 72 B with magnetronic ignition and a Mega spark 3 coil . Does this mean I can open up the plug gaps a bit more for a bit more oomph- if so how much? Cheers, Martyn
Martyn

A higher voltage coil means that you can run with a larger plug gap. The actual voltage generated will be limited to that required to jump the gap. The downside of running at a higher voltage is that it stresses the distributor rotor, cap and wires with the potential (no pun intended) for breakdown of any weak components. Unles your engine is modified such that a fatter spark is benficial, popular opinion is that the standard gap is OK.

Regards,

Barry
B.J. Quartermaine

Thanks, Barry- I've carried out a few mods- k+n filters + richer needles, tubular manifold and free flow exhaust- the car is quite lively perhaps i'll just try a small increase seems a shame not to!
Martyn

Martyn
I run .050" gap and find it makes a noticeable difference especially in cold start up situations.
The down side is that if you have a weak link in your ignition system you will probably find it as you widen the spark plug gap. The coil voltage only goes to the level of voltage necessary to jump the gap. More gap means higher voltage. Higher voltage means the spark can jump further at the spark plug or elsewhere. Electricity/voltage always takes the easiest way home.
Barry
Barry Parkinson

The Barry's are both right on the money. If you open the gap, consider carrying a spare cap with numbered wires already attached, and definitely, absolutely, without fail, carry a spare *tested* rotor. Leastways, I would!

Best wishes - - Alec
Alec Darnall

Martyn-
Carry an extra set of breaker points as well. High-output coils can be hard on them.
Steve S.

Many thanks for all the comments from across the pond- I use the car daily so I don't want to affect reliability - I think I'll just use a 'generous' standard gap. Martyn
Martyn

The previous posts are right on.

Since changing over to a higher voltage coil I've
been shorting-out rotors through the distributor
cam-spindle. I've been going through 2-3 rotors
a year. All different brands.

With the stock coil, I've never - ever - had a rotor
short-out on me.
Daniel Wong

Daniel, if your rotor has a brass rivet holding it together, you can drill out the rivet and replace it with epoxy. That will prevent the shorting issue with the new rotors. Very few are available without that rivet, but I know the brand "OEM" do not have it!
Jeff
Jeff Schlemmer

What problems would be associated with using a high voltage coil and pertronix electronic ignition. That is the set up I have and I am running with a .035 gap on the plugs. Can I safely go wider or leave it alone?
John

.035 was standard for rubber bumpers. As has been said opening the gap further will increase the maximum HT voltage throughout the system and could find a weak link somewhere else. OTOH under marginal conditions i.e. poor state of tune and cold, damp weather it could make the difference between starting and not starting. But OTFH (On The First Hand) rubber bumper cars used a boosted coil votage when cranking which gives a significantly higher maximum HT voltage anyway, over the running voltage, making a sport or similar coil superfluous.
Paul Hunt 2

Thanks Paul,
I think I'll open them up to .040. It has been getting a little harder to start now that the cooler weather has arrived in New England. I wish I knew about the boosted coil in RB cars, I wouldn't have bought the sport coil, it does look good though.
John

My experience is that if the wires are good, cap & rotor too, that a .050 gap is very reliable. My big problem came with carbon core wires that began to fail and sparks started jumping everywhere except where they were supposed to.

As a practical matter the stock ignition system will operate the .050 gap. Starting under cold low voltage conditions could prove a problem though.

Barry
Barry Parkinson

John. I have run a standard .035" spark plug gap on my 68 BGT for a number of years. It was the standard gap on US cars of that vintage using similar coils and wires. The idle seems to be better at that gap than it is at the factory .025". I might mention that I, too, am running a Lucas Sports Coil, mainly because I did not know as much "back then" as I do now.

Paul's comment about the "boosted" starting capacity of the RB cars is due to their running a "6 Volt" coil, a coil designed to be run at 6-8 volts when the ignition switch is in the "run" position. When the ignition switch is in the "start" position, another wire, which provides a full 12 V from the starter solenoid, "over charges" the system, allowing easier starting when the emission controlled RB engine is started. This system requires a wire from the starter solenoid to the coil and a resistance wire/ballast resistor in the normal run circuit (white wire).

While it would be possible to set you car up with such a system, I do not believe it is necessary. Set the plug gaps at .035" and check out Paul's website, "The Pages of Bee and Vee" for information on setting up and tuning the carbs.

While I currently live in Arizona, where winters are not severe (I do, however, have a photo of snow on my 79 LE in my driveway here in the Mesa area), I used a 61 MGA as my primary daily driver in Wyoming. Many of the troops who had served both in Alaska and Wyoming felt the winters in Wyoming were more severe. (I, later, served in Fort Yukon, Alaska which had as severe a winter as anywhere I have ever been. But, it was eight miles north of the Arctic Circle and does not reflect duty in the southern part of Alaska where my troops had served.) The reason I used the MG as my primary trasportation was that it would start when most of our American cars (a Mustang, a Camero, a couple of Corvettes, etc.) would not start.

It a vehicle is capable of being started, an MG with the carbs properly tuned and with the ignition system in good working order, will do so.

Please take some time to visit my website, www.custompistols.com/ where I have some tech articles on the ignition system. David DuBois has some tech articles on the fuel system there and others have contributed articles which may be of use in the future. These articles, combined with the information on Paul Hunt's website should allow you to make your MG fully as reliable as any car on the road--and better than most cars of its vintage.

Les
Les Bengtson

This thread was discussed between 04/09/2006 and 07/09/2006

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