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Author Topic: Changing the Distributor  (Read 1955 times)

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  • chfwrench
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  • MG information: '73 red B roadster
Changing the Distributor
« on: October 29, 2018, 01:43:08 PM »
Changing the distributor on a B is fairly easy, especially if you have good mechanical ability.

Unlike American cars, the Lucas (or replacement aftermarket) distributors can only go in one way.  They don't have gears on the bottom, but keys and slots that are different from side-to-sided, so the rotor will always point in the correct direction.  The trick is to reasonably align the number 1 spark plug wire as it was originally.

Also, some aftermarket distributors require reusing the drive (keyed coupling) from the old distributor.  Not a daunting task, but the roll pin that retains it has to be punched-out and the coupling properly aligned on installation to the new unit.  It should be keyed to the shaft, so that becomes easy.

Little tricks:
- Mark all the park plug wires on the cap.  If you don't have the the electrician's numbered bands, a piece of masking tape with the number written in dark ink (Sharpie or gel pen) number works fine.  Make sure the number one plug wire is especially clear.
- Mark the block where the number 1 plug wire is positioned on the distributor. Just helps to get the new distro positioned so the car will start to be able to set the timing with the light.
- Take at least 2 photos of the old distributor before removing it.  One with the cap on (so you can see the #1 plug wire and Vacuum advance pot positions) and the other withe cap off (to see the rotor direction).
- Remove the cap, primary leads (with the CEI you have to also remove the module), the 2-bolt clamp that retains the distributor and just pull it out.
- Remove the clamp from the old distributor (single bolt length-wise across the split side of the clamp) and check the clamp is not bent and the bolt is free and can be adjusted. Repair or replace as necessary.
- Reinstall the retaining clamp to the block with the adjustment bolt still somewhat loose.
- Install the new distributor with the rotor in place.  The rotor should automatically line-up as the keys in the bottom of the distributor should only fit the same way as the old one did.  Determine where the #1 wire would be (that can vary between distributors relative to the vacuum advance pot) and set the body of the distributor so that it is in the same place, either using the photos you took or the mark on the block.

Now, depending on which way you went on the replacement distributor determines what you do next.   I'll follow the 45D4 installation with a Pertronix kit as that's what I'm most familiar with.  The others, being completely different, should come with full instructions, but will only differ in the controls and associated wiring.

With the distributor in place, rotor and cap off, install the Pertronix unit.  A sensor cap goes over the cam that actuated the points on the shaft just below the rotor. 2 screws that used to hold the points in place now secure the reader unit to the plate. 
The kit comes with the wires that replace the primary lead and power source and they have to be snaked through the hole in the lower distributor body the primary lead had gone through.  Instructions are provided.  Make sure in ordering the distributor and Pertronix kit that they match by model.  The kit for a 25D4 will fit the points plate, but the wiring harness is very different and won't fit.  And caps and rotor are not interchangeable.

Fit the cap, wires and vacuum advance line and start the engine.  Once warmed-up, remove and plug the advance line, adjust the idle, fix your timing light and set the timing.  Once set, adjust the idle, tighten the clamp, reconnect the vacuum line* and done.

My personal advice here is a reminder that, with a turbo, the vacuum advance my be hooked up differently or bypassed entirely.  The intake manifold is now under pressure from the turbo, so it won't work conventionally.  With a draw-through set-up (carb before the turbo) it can use fittings on the carb to work pretty much normally, but if you have a blow-through set-up (carb after the turbo), the carb is now under pressure, so no vacuum. 

This could influence your distributor kit purchase, as it may be best to get a fully mechanical advance type distributor, like the one from Crane.  Check and make sure what you have before buying anything.
Moss sells their supercharger kit, so see what distributor they use on that.  Also, sources, like JEGS or Summit Racing carry aftermarket distributors and Pertronix kits for the MGB that may be better suited to your application. 

They routinely work with supercharged and turbocharged racers and hot rods.

Safety Fast!
Art Isaacs
« Last Edit: December 31, 1969, 07:00:00 PM by amgba »
Art Isaacs
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Re: Changing the Distributor
« Reply #1 on: May 12, 2019, 11:38:12 AM »
More on Distributor Replacement

Our first diagnostics was performed in the street outside the Retirement Community where I live.  The B fired up, as usual, and was happy to drive around the property.  After about ten minutes, it died.  So we popped the bonnet, pulled the gas hose off of the filter, and turned the key. 

The problem appeared to be in the ignition system. That could mean coil, electronic module, or distributor.  My car still had an old style electronic module on the wheel well.  We putin a new coil while waiting for the new distributor.  No improvement.   Car would still run for about ten minutes, then die.  The starter would spin the engine, but no sign of life for five or ten minutes when it would come back to life.

When the distributor arrived, my son and I prepared to swap it out.  First- check the timing. Then, following the directions wed received, mark the wires (spark plug and those going to the coil), mark the position of the #1 wire on the distributor, photograph the distributor and the coil wires with cap on and off the distributor, then unbolt and remove the old distributor. The new distributor didnt seem to want to go all the way in, but finally it went home.  A little tightening, then we hit the starter.  It fired and ran, a little grumbly.  The distributor was rotated the timing was back where it had been, and the engine was fine.

 Today was warm enough to run the car. It ran for about ten minutes as I drove it around the property, then died as I was backing up to turn in a tight spot.  But I tried it again in a couple of minutes; it started.  So I drove it around for another ten minutes and decided that it was worth taking the risk. 

First order of business: see if I could get it to the local emissions testing shop to renew the TN license.     But, just to make sure I was paying attention, it stalled when I had to slow down for a car turning off the road a mile or so down the troad.  I pulled off the road, crossed my fingers, hit the starter and away we went. 

 It passed inspection, and drove home very happy.  I have a little work to do on the idle as the gas and the engine drops down to a couple hundred rpm then climbs back up to about 1000). Ill need to drive it a bit more before Im totally comfortable but I think the problem is now resolved.
Thanks to all for the support provided. 

John W Philbrick
Brentwood, Tennessee
American MGB Association
phone/text: 773-769-7084

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