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MG MGB Technical - Distributor questions

I am rebuilding a 71 MGB with a 25D4 distributor. The vacuum advance unit is froze and broke when I disassembled the distributor. Otherwise, the distributor is fine. I can get a new vacuum advance from Moss, but wonder if itís best to just get a new distributor. I am on a rather tight budget, but donít want to throw good money after bad. Alternately, should I switch to an electronic ignition? If so, is there a way, or some kit, to convert my current distributor? I am not dead set on an all original parts restoration. Thanks for any advice.
JBH John Holecek

John, many folk have reported buying a replacement 25D at Auto Zone or Advance Auto Parts and running them successfully. However others have warned that the dist are assembled/rebuilt from a variety of parts and are highly variable. That said they are paying about $60-65 and Moss gets about as much just for the vacuum advance unit. You might buy one from the auto parts house, confirm that the vacuum advance unit is correct for your application and if necessary use your existing cam plate and mechanical advance springs.
Good luck!
D F Sexton

If you know that the original distributor is OK (including advance weight springs), it's probably best to retain it and install a new vacuum unit. Look up the ID number stamped on the distr. body. You can get the vacuum unit at discounted price from places like Little British Car Company. Also, pertronix Ignitor electronic ignition fits right into the distributor.

Hello John,

If your are on a tight budget, my two cents advice is to buy an used vacuum advance unit. You can find that locally or on ebay very often I saw some for sale.

Jean Guy Catford

John, install a new vac advance unit and if you want, install a Pertronix or Crane electronic ignition kit. I think summit racing carries both at a cheaper rate than Moss, Vic Brit, and others. You can reach them at 1-800-230-3030, and no I have no affiliation with them other than buying thousands of dollars of parts from them and greatly appreciating their professional service.
Jeff Schlemmer

Dennis(or anyone else), If I get a replacenment 25D from an AutoZone and its not my exact flavor of 25D, what effect will this have on the cars operation? What makes the different 25D's different? Can I take the weights and springs from my 25D and fit them in another 25D and get close? Thanks for the great information.
JBH John Holecek

John these are links to various sites which speak at length on distributor curves for the different model MGB's and the correct springs, vacuum advance units and cam plates. May be more than you want to know.
The weights should all be the same in various 25D's, but the cam plate which limits the total mechanical advance, the springs and the vacuum advance units may vary. The combination on these will affect initial ignition timing, acceleration, pinging (pinking).
You can change the springs and cam plate from your existing dist but they may not be original and/or the springs may have considerable wear/stretch which will affect performance.
My own approach has been to rebuild my dist with various components and after installing in the car use a timing light at various rpms to determine advance and thereby construct my own curve.
Hope this may help.
D F Sexton

John - cheap rebuilts using incorrect parts can severely damage your engine. Before you install it check the advance limiting plate, the advance in degrees is stamped on it. My original was 12d, the replacement I got was 19d! You can use your old plate if it's wrong, it's not a part that wears. The springs are more difficult to sort, affect performance & economy, but are less like to cause damage.
Mike Polan

The later model with smog control had the spark retarded for emission reduction. The vacuum advance had a large travel to advance the spark under light throttle cruising conditions. These distributors will kill your performance -- you give it the gas, the spark will retard and it almost feels like you didn't give it any gas at all.

I spent a bunch of time trying to get a late model dist to work. (I was trying to use the factory electronic ignition system) With bushings around the advance posts to reduce the total advance and a weaker springs ( to bring in the centrifugal advance earlier) you can make them work, but unless you like cut and try lab experiments I don't reccomend it. Changing the plate won't change the weights and springs in the advance mechanism, or for that matter, the late model vacuum advance unit designed to only move at very hi (light cruising) vacuum.

Once you get a distributor with good bushings, so the shaft doesn't wobble, and an advance curve that suits your engine ( the earlier the distributor the better) then install the electronic ignition for set and forget reliability.

By the way the optical shutter systems seem to be more tolerant of shaft wear/wobble than the orginal point system.


Barry Parkinson

Hi, John - Another thing to watch for is the advance built into the vacuum unit. It will three numbers separated usually by dashes or dots, stamped on the vacuum housing, that indicate the amount of vacuum where advance begins, the amount where it ends, and the total advance (crankshaft degrees). Several are available, such as 5-13-10, which would obviously give different results than 5-8-3, or 7-13-5.

best of luck - - Alec

Alec Darnall

If you are rebuilding your engine, the final specification of the engine will determine what the distributor should be. Your 71 distributor was optimized to provide the proper level of emissions control required for that year. Performance suffered somewhat as a result. If you are building a high compression engine to the earlier specification, you need a distributor that will work with that engine, nor your previous engine. The earlier engines used a Lucas 25D4 distributor to specification number 40897. The ideal would be to find one of these distributors in rebuildable condition, then send it off to John Twist, at University Motors Ltd. for rebuild. The bottom line is that your engine specification will determine what distributor you need and this is not an area to skimp money on. The wrong advance curve can damage your new engine. Thus, a hundred dollars on a good distributor can save you the thousand or more you spend on rebuilding the engine. Les
Les Bengtson

Even if it doesn't damage your engine, it can make a strong engine, weak.
If you are interested in playing with the advance curves you can buy a dual point Mallory aftermarket distributor. These are designed to allow adjustment of the total spark advance, as well as the timing of the advance. You can also buy a version with vacuum advance.
You take out one set of points or both, as applicable, and hook up an electronic system such as the Crane 700.
i.e. in my opinion, the main advantage of the aftermarket distributor is the ease of tailoring it to your engine.

Ironically, it is easier to adjust the spark curve for a performance engine than a stock one. Because the performance cam reduces the compression at lower rpms the spark advance can be set to come in full by 2000 rpms or less. The initial advance is also greater, and the additional advance less.

The vacuum advance is important for the modified engine because the spark advance that is optimum for part throttle is substantially more than needed for full throttle. I never understand why performance distributors often omit the vacuum advance. For racing only - ok - but for street and performance a vacuum advance is needed.

I have on the shelf, an after market knock sensor system which electronically retards the spark as needed. Some day I'm going to try installing that with the distributor advanced a little too much, and see if it (by sensing knock and retarding the spark accordingly) will act as an automatic spark advance system.

Barry Parkinson

I don't know why, but Jeff and I are from Minnesota.
Is that why we agree so often?

Install a new vacuum unit.
A old mechanic friend of mine, a collegue at the MB dealer, called them 'spark modifiers'
But, be sure to check the mechanical advance. They have a habit of sticking on the shaft.
With the unit in the engine, turn the rotor back and forth. If it won't turn, then it needs to be repaired. Out of the engine, hold the shaft, and turn it.
There is a screw holding the cam unto the shaft, and you must remove the breaker plate, and remove the return springs. Be sure to mark everything, to get it back correctly. Or you will be 180 degrees out of time.

Yes, use a Pertonix!
I have that, plus the 'Flamethrower coil, and the 'Second Strike'
Which you don't need unless you need a rpm limiter, and the multiple spark.
Starts much quicker, idles better, timing never changes. And it's mostly hidden.
What could be better?
Safety Fast
Dwight McCullough

WOW! Thanks for all the good information. I am going to have to digest all this. I never knew there was a whole world of MGB distributor choices.
JBH John Holecek

This thread was discussed between 10/03/2005 and 14/03/2005

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