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MG MGB Technical - Plug gap with Pertronix and Lucas sports coil

Hi, I just got a gold Lucas sports coil and want to fit that to my car this weekend. Since I have a new wiring loom which doesn't have the ballast wire in it I figured using the 12 volt coil means I won't need an external one. I also have a pertronix ignition unit on the car. I believe I should open up the plug gap to gain full benefit from the coil but I wasn't sure what the gap should be? I believe from memory (not at home at the moment) I have BP6ES plugs.

Thanks!

Simon
Simon Jansen

Hi Simon, having done a bit of reading on this, I believe most recommendations are that you can open her up a wee bit to 0.030" or so...whether this makes any difference or not, without then adjusting timing and carburettion is very much a subject open to debate. The main advantage as I can see it is that electronic timing corrects for the mechanical wear experienced by distributor parts, and avoids the need to set point gaps.
Curtis Walker

further: I haven't yet convinced myself to plump for Pertronix yet, but am open to convincing!

Finally, how's that car of yours coming along? I still check your website from time to time!
Curtis Walker

Check the resistance of the coil primary before fitting, the labelling on coils is often confusing if not actually incorrect. A 12v coil for use on a non-ballasted system should measure about 2.4 (typical Sport) to 3 (standard) ohms. If it measures about 1.5 ohms then that coil would need a similar value ballast resistance in series with the 12v supply. The Pertronix should pick up the *full* 12v supply, and not the ballasted supply.

The factory used .035 on cars with the 45D distributors. This is because that distributor has slightly better resistance to breaking down than the 25D, which should be kept at .025.

High voltage e.g. Sport coils still deliver greater spark energy even if used with the smaller plug gap. If you go much above .035 you run the risk of breaking down other HT components as it is th plug gap that controls how high the HT voltage goes, not the coil.

You are unlikely to see any benefit from after-market coils (or the Pertronix for that matter), except perhaps if the car is in a poor state of tune. And in fact if you have a rubber bumper car that originally had a 6v coil and ballasted ignition, then changing to a 12v system actually worsens its performance - for starting anyway - under adverse conditions.
Paul Hunt 2

I have ran that same set up and loved it. I opend the gap up slightly more than the book calls for. I played with a larger gap but it actually hurt performance at lower speeds. If you are looking to turn it hard though, go for it. Otherwise stay close to the stock gap and you will be happy. The Coil is hotter and the pertronix is a more reliable spark than points. No other tuning is necessary. If your timing and mixture is right, you are good. You are just improving the spark. As I remember, the sports coil is a non ballasted coil.
Bryan

Curtis,

I was always having problems with my 73 MGB until I changed to the Petronix and hot coil. My car now runs great and it has been totally problem free for over two years. I would strongly recommend going to this setup.

Just my two cents worth.

Robert
Robert Browning

Simon. I have run a .035" gap, using the Lucas 25D4 distributor and a Sports Coil, for something over eight years with no problems noted. Engine seems to run more smoothly with that plug gap than with the factory recommended .025". NGK BP6ES plugs.

Les
Les Bengtson

Hi Simon- I echo Robert above. The 40k sports coil, Champion N9y and Pertronix coaxed me into opening gap to 35, 37, 40, which is where they are now. Lookout though, I'm converting that baby to EDIS and the coil paks allow up to 55K on plug gap! Cheers, VEM
vem myers

Robert, interesting...although for me, I'm having no problems with my "G", so the incentive is not there to splash out a couple of hundred (NZD). Yet.
Curtis Walker

Just an update. I fitted the new coil and adjusted the plugs (to 0.035 as Les mentions above) and it all worked fine. Car still isn't on the road though so won't know until I can really drive it I guess. I have other issues I think. Car starts and idles fine but when you open the throttle she splutters. I will look into that at some point once I get some body work out of the way.
Simon Jansen

Simon. That can be mixture related or distributor related. When you get the car on the road, we can sort it. In fact, I am rather looking forwards to you getting the car on the road. I will begin the restoration of my MGA when it gets cooler and your site provides some excellent inspiration. Both to get my tail in gear and as to the quality of work to be accomplished. I hope do as fine a job as you have.

Les
Les Bengtson

Thanks for the comments Les. I suspect the cause is mixture. When I pulled the plugs to adjust the gap they seemed black and wet but then i only ever run the engine a minute or so just to make sure it's all still working while the car isn't done.

I need to get re-motivated which I am slowly. And I am waiting for it to get warmer! Or rather less wet. A bit more bashing on the front wings and the doors and then I can look at having the windscreen frame rebuilt.

Simon
Simon Jansen

This thread was discussed between 27/07/2007 and 31/07/2007

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