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MG MGB Technical - Changing from electronic ignition

I recently aquired a 77 roadster, ex California now converted to RHD and with twin SU HIF carbs. Although the 18V engine has been rebuilt, It has the original 45D distributor and electronic ignition which is now 25 years old. Everything seems to be working okay but I have never really trusted EI and may change it for good old points and condenser. My question is which type and number distributor should I use. Am I making a wise move bearing in mind that the carburation has been changed, an unleaded head employed and the emission equipment discarded.
roger lawson

You can always swap out the stock parts and put in a pertronix or other electronics.
Mike MaGee

I see no point at all changing from the factory electronic ignition - if it is the 45DM4 with remote amplifier - to a Pertronix. The factory system was very reliable and the module was used on many different cars and is still used today. If it is still working it shows it is reliable enough, there have been enough reports of problems with Pertronix and other after-market systems to make several users carry points and condenser as a 'spare'!

The problem with changing to points is that there *were* no points distributors for that engine, was originally low compression which weren't used in the UK after the early 70s, and that spec wasn't used in the UK at all. So any points distributor is going to be a bit hit and miss curves-wise. However you could probably get any 45D points-type distributor and change over the points plate to yours, that would mean you would retain the correct (if they are still correct) centrifugal and vacuum advance characteristics for the engine. But then again if the carburation and heads have been changed you have no idea what the curves *should* be. And with any engine after all these years the only practical way of determining the best timing for your set-up is to advance it so it is just short of pinking with any combination of throttle, load and revs. Either that or spend a lot of time and money on a rolling road.
Paul Hunt

Paul, Thanks for your input. The car drives okay but I suspect the timing is slightly out (advanced) since it runs smoother if I block the vacuum pipe. It does not pink nor does it run-on so I think the balance can be found and I'll get it set up by Bob West next week. I was thinking that the response might have been "junk it" and convert to points so I will leave things as they are for now. You mention that the module is still used; can you be more precise. Am I wise to buy a spare?

I am used to fiddling with an MGA and when I converted to unleaded I had problems with running on. I could only cure it by using leaded fuel or Millers VSP. The MGB has a low comp engine yet it pulls quite well and seems quite pleasing.

roger lawson

Roger. The factory used the Lucas 45D4, specification 41599 in the 75 North American spec cars. This is a points type distributor.

As Paul notes, the most commonly encountered distributor was the Lucas 45DE4 or Opus system. These frequently failed and it has been years since I have seen any car with a functional Opus system installed. The 45DE4 has the square box on the side of the distributor body. Lumenition makes a replacement system for the internals which works quite well. The one in my daughter's 77 worked for something over 10 years before causing problems. The factory replacement for the Opus system was the Lucas 45DM4 or CEI system. This is a standard looking body with a black box unit mounted under the coil. Most of these systems seem to have held up well over the years.

The early low compression MGB engines used the Lucas 25D4 distributor to specification number 40916. This distributor, if you could find one, would also be a good starting place for the later NA spec cars. Les
Les Bengtson

Roger - I wouldn't bother buying a spare electronic module given their reliability. I might well source a points plate, and do a trial fit, and carry it as a 'spare', simply from the point of view that it will be easier to get you back on the road if a problem *does* occur than sourcing a replacement electronic module or trigger at short notice. My own experiences of buying 'rebuilt' distributors are not good, the vendors seemingly relying on the fact that most people have no way of plotting the curves to see if they have been sold the correct one. I have to say that the curves of both the earlier 40916 for 18G engines that Les mentions and the slightly later 41290 for 18V (which is similar) have quite different characteristics to the 45DM4s used on later North American engines. And whilst both, like any other spec, can be set to give no pinking, you won't necessarily be getting either the best performance or economy with either. 'Smoothness' is a matter of individual sensitivity, usually disabling vacuum advance adversely affects cruising economy.
Paul Hunt

This thread was discussed between 06/04/2004 and 09/04/2004

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