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MG MGB Technical - Distributor Setup for LC Engine

My 25D4 #40987 distributor needs a new vacuum advance capsule at minimum. It's on a low compression (LC) engine. Is there any advantage to having it recurved for the early LC specs (such as 40916)? Here are specifics.

1. 1965 roadster, positive ground. Aftermarket stock style coil.
2. Engine is an unknown 18V with LC pistons and the big valve head. ).030" overbore. All other stuff is mostly 1965.
3. AUD-135 HS-4 carbs (need rebuild)
4. Cam ground to 1965 specs.
5. Compression pressure around 140 psig
6. Stock exhaust.
7. Current timing at 15+ deg BTDC dynamic.
8. Little timing scatter although its hard to get the engine to idle below 900 rpm. Probably worn carbs.
9. Dwell at 60 deg with no waiver.
10. No pinking or run on. But, the vacuum advance may not be doing much given the leak I detected using a mouth vacuum pump.
15. Full mech. advance appears to come on about 2000-2400 rpm. About 20 deg advance observed for total of around 35 deg.
16. Gas mileage okay. Engine starts fine and revs good up to about 4000+rpm. Less responsive above that. Good low rpm pull.

I think the early LC distributors had less vacuum advance (16 deg vs. 20 deg for HC) and higher mechanical advance but peaking at 4200 rpm vs. 2200 for HC.

Obviously fuels have changed a lot since 1965 so the LC curves may be out of date. I'm hoping someone has experience.

PS - I'm not after peak performance. But, since I need a new vacuum unit and possibly a rebuild I'd like to get the best early stock setup.
Robert McCoy

Robert - The place you want to go is Advanced Distributors at: Jeff will make your distributor like new, complete with a rebuilt vacuum advance unit. Cheers - Dave
David DuBois

A leaking vacuum capsule will upset the mixture as well as the advance curve.

Personally I think *all* the curves are out of date with today's fuels, but short of putting the car on a rolling road and spending a lot of time determining what the ideal curve is for your engine and your fuel, and trying to determine that with vacuum advance is a lot more difficult than if you have no vac advance you are always going to be poking and hoping. The fact you don't know what spec your 18V is makes it difficult to say which standard curve would be best, i.e. the original or an alternative, and an 18V with a 65 cam is a bit of a hotch-potch. From 1967 onwards USA curves were progressively more about emissions than performance and economy anyway.

Do you mean you have a 25D4 40897 and not a 40987? the 40897 was for the 18G-GH HC engines from 62-70, I don't know what the latter was for, but it has a very different curve to the 40897 or indeed any MGB distributor. It peaks at 8.25 degrees at 1000 rpm, which is very different to the figures you give, whereas the 40897 peaks at 20 degrees at 2200 rpm which fits in with your figures. The early LC distributors like the 40916 had quite a different characteristic with 24 degrees at 4400.

The bottom line is that you can make any curve work with any engine just by keeping the advance short of pinking at any combination of throttle, revs and load. It's just that some curves will be wasting more energy as heat instead of performance than others.
Paul Hunt

Paul, I meant to type 25D4 #40897. The standard HC distributor for 62-70.

Most recommendations I've seen in the archives are to keep total advance to around 32 crank deg. and if you can get it fully advanced before 3000 rpm.

This car had a lot of PO bodging (I think that's the correct word). It's made things interesting through the years.

Robert McCoy


Do exactly as David recommended and send it to Jeff! I have a somewhat similar setup to yours, and was on my third distributor, Brit-tek's Eurospec model, but could never get it to run quite right. I finally decided to send my original dead dizzy to Jeff to have him rebuild it, and all I can say is WOW! I dropped it in, without touching anything else on my car, and it really brought my beast to life for the first time in 5 years. Absolutely, without question, the best $100 I ever spent on anything automotive!

Robert...quick comment...140psi compression sounds like HC not LC....mine is a much later car, 1977 with a LC engine in excellent shape, no oil use or burning/but no emissions crap and twin HS4s/Eurospec etc...compression runs 125psi across the board.

Steve, thanks for you're advice. I think I'll send it off this winter when the MG is in hibernation (I live in NH not CA now). I've heard nothing but good reports on Jeff's work.

PJ, Thanks. My engine has the deep dished pistons which give a nominal 8:0 CR. One of my manuals lists pressures for that set up as about 130 psi with 160-170 for the HC version. So your readings are definitely closer than mine. I'm guessing the differences between us may be to due: 1)my overbore of 0.030" which increases CR (most likely), 2)carbon buildup, or 3)gauge differences. I do use some oil as my valve guides are worn and the engine has about 60k miles since rebuilding. Does the Eurospec work for you?
Robert McCoy

Don't want to hijack this thread but I have recently installed a pertronix ignition and also hooked up the vacuum advance that hasn't been hooked up since I bought the car 10 years ago. The car starts and runs smoother but I can't seem to get the same power I had with the points. The first thing I noticed was that the timing was off with the new electronic ignition ( I did drive the car with the vacuum advance hooked up prior to going to the new ignition). The archives have a lot of great info but I find that as I am getting older I can't hear strange noises which means that I can't hear any pinking. This being said, are there any other ways to tell if the engine is pinking other than listening. I have played with the timing and have it a little better power wise but still have a way to go. Any ideas?

Brian Smith

I have just found a web site that provides Lucas distributors by number and what their applications are. It turns out that the distributor in my MGB GT is from a Land Rover (45D4 #41630). Now if I can just find a way to tune it properly for the "B" that would be great.

If you are wanting to check your distributors original application the here is the link.

Brian Smith

Brian, The Pertronix units are notorious for having misalighment problems inside the distributor. I installed one a few years ago and the firing point was at least 20 degrees off between the cap and rotor. Eventually the spark was jumping so far that it burned a hole in the cap! RAY

Thanks for your comment Ray. I just came in from making a few more changes and I think that I have the engine running a whole lot better. As I mentioned in the previous posting, the 45d4 distributor that has been in my "B" since I bought it 10 years ago and never had the vacuum attached, is actually for a Landrover. Now this car ran really well and had a lot of power previously but in an attempt to increase fuel mileage I fitted a vacuum hose and then the electronic ignition and things started to get really messed up. So today I thought that I would disconnect the vacuum hose and plug it and try it that way. The car runs better, idles better and just generally feels better so I will not be hooking up the vacuum again until I find a distributor meant for the MGB.

I found the chart I provided a link to most helpful and will use it in the future. I had an old 25d4 distributor in my shop and wasn't sure what it came out of but have found that it was originally out of an Austin America!!

Brian Smith

This thread was discussed between 10/09/2008 and 14/09/2008

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