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MG MGB Technical - 123ignition curve selection

Dear all,

I hear a lot of good things about the 123ignition distributor. It seems very well made, has some interesting features and the MG-version has 16 pre-programmed curves to choose from. On their website it is said that these 16 curves cover all standard MGA/MGB advance curves from the good old days i.e. corresponding with Lucas units 40510, 40916, 40897/41155, 41339, 41370, 41491, 41288, 41290, 41032, 41234/41391, 41692, 41610, 41599, 41600/41643, 41693, 41695 and a nameless one for "stage 2" engines. This is where I get lost - there is too much to choose from, don't know where to start.

I wonder which of the 16 settings I should use for my MGA with MGB engine with LCB exhaust manifold, Piper 270 cam, 1 3/4 SU's with K&N filters - otherwise it is standard.

There is a lot of high quality information on distributors and curves on this forum, but not being an expert - I find the amount of detail overwhelming. I tried my best - honestly- and by deduction (leaving out all low-compression curves, manifold vacuum curves etc) in my opinion the following 4 possibilites remain : a) 40897/41155, b)41032, c) 41610 or d) the stage 2 curve.

My question : Which do you think would be best for use with today's fuel?
I do appreciate your views and expertise!



Huib Berger

Typically with a modified cam you want an advance timing curve that puts lots of advance in at low rpms and not that much more after 2500 rpms.

A cut & paste approach is to use low octane gas that pings/pinks readily. Adjust your timing where you can get some pinging throughout the range. If you get a curve that pings at higher rpms and not a low rpms, then you need a curve with more initial advance and less hi rpm advance. If pinging is an issue at low rpms, then consider retarding the static setting and using a curve that has more hi rpm advance.

I've seen charts of the advance curves. Those charts can assist you in which profile to select. You should be able to feel &/or hear the results of which curve you select. If all else fails, use one of the early curves. Those curve were designed for performance, not emissions. A modified curve with more initial advance however is better adapted to an engine with a modified cam.

The lack of a vacuum advance will impact your fuel consumption in ordinary driving conditions and - to some extent - your part throttle response. Under partial throttle conditions (which is most of the time) your engine can use several degrees more advanced timing.

Barry Parkinson

Hi Huib,

Because of all the modifications, the only way to find out properly, is to set the car up on a good rolling road. In the Dutch MGCC-magazine was an article about someone who adjusted MG's.

It may cost you a couple of hundred Euro's but they will get the most from your modifications. I outdragged a so-called tuned 1950cc MGB with my standard B on a circuit ones, only because the 1950cc was lousily adjusted. After (my) fiddling with the carburettors and ignition the 1950cc was faster but not as fast as it could be after a proper rolling-road session.

If you are not a member please mail me off-line, I'll have a look then if I can find the details.
Willem van der Veer

Willem is right, but there are only certain choices as to advance curves that are preprogramed on the 123 distributor. You can't adjust your own as on a Mallory - or a stock distributor (with some modifications).

You can buy small medium or large pants, but if you are an unusual size you willl have to make do.

Barry Parkinson

This thread was discussed on 23/08/2006

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