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MG MGB Technical - 1979 MGB Ignition Problem

I own a 1979 mgb roadster and on my way home from work last Friday she suddenly started running rough… hesitation etc. On Monday, she started up fine and after a blip on the throttle settled at about 1300 rpm as the automatic choke kicked in. However, once up to temperature she would not idle (~ 900-1000 rpm), making negotiating traffic lights quite a balancing act! The symptoms have not changed since then… starts ok, but once up to temperature runs like a dog. . Temp is showing normal, i.e. needle almost vertical, half way between C & H (I have a 180 deg thermostat fitted). Lots of hesitation when pulling away from junctions and I could ‘feel’ the engine missing when cruising at 60 mph and ~3000 rpm. Talking to friends at work, we’re pretty sure it’s an ignition, not fuel, problem. I’ve checked the obvious things, plugs (and leads); dist cap contacts and rotor etc., and nothing seemed untoward. A couple of the contacts in the dist cap are slightly pitted however. So, we figured it could well be the condenser (points look ok too) and this is where it gets interesting as my ‘B’ has an electronic amplifier fitted! However, closer inspection showed that the amplifier is not connected to anything, the left hand (i.e. towards the rear of the car) cable (red & blue, I think, in a black sleeve) is nicely tucked away under one of the cable looms, with the right hand wires (white & brown) similarly stowed. All the amplified appears to be doing is providing a base for the coil bracket! The distributor does have a condenser fitted and this is connected to the coil (upper right-hand connection, with the coil pointing downwards). There also another capacitor/condenser (blue in colour) which is connected to the lower right hand coil connection. This blue capacitor is then ‘earthed’ via a largish blade connector to the coil bracket on top of the amplifier. I can only assume that the amplifier failed at some point and rather than replace it (they’re very expensive), the previous owner went back to a conventional ignition system.

Any thoughts and/or advice would be most welcome… and thanks in advance!
A R Mumford

If you pull the distributor cap you should see the points if it has, indeed, been converted back to a points type ignition. The points might need cleaning and regapping to .015" to cure your problem. Don't be too certain that it is not a fuel problem,however, as the Zenith-Stromberg carbs are known for a host of ailments. RAY
rjm RAY

AR. The Lucas electronic ignitions were of two types, the 45DE, or Opus system, and the 45DM, or CEI system. It sounds like your car may have had the CEI system installed at one time. If, now, you have a points type distributor, you will need to determine if it is a Lucas 25D4 or a 45D4 type, both of which were used on the earlier MGBs and the 45D4 is available aftermarket. The points, condensor, rotor, and distributor caps do not interchange between the two models. A photo of the dizzy would help.

Good idea to check out the distributor and make sure the points are clean and properly gapped as Ray suggests. If the points are pitted, that may be part of the problem. You, also, need to check out the wires and the plugs. There is information of checking out the wires on the MG portion of my website, www.custompistols.com/ under the tech articles.

As Ray notes, the Z-S carb, if you still have one, can be a problem, especially the rubber diaphragm which can deteriorate over time. The water choke mechanism, also, can go bad causing the car to run excessively rich. This will cause the plugs to foul and can give the indications you describe.

The catalytic converter can plug up which can cause a restricted flow of exhaust gasses giving similar indications to what you describe.

Check the intake manifold for leaks by either spraying some carb cleaner around all of the joints (while the engine is running) or by using an unlit propane torch around the joints. In either case, increase in the engine speed indicates either a bad (loose) connection or a bad gasket.

Let us know what you find out.

Les
Les Bengtson

Be nice to see a pic of the IGN mess, sounds like an aftermarket system -" All the amplified appears to be doing is providing a base for the coil bracket! ". It also may or may not have all or part of one of the OE Lucas systems in place, and/or a points distributor, and it is not unknown for idjuts to crosswire all of this.
The blue capacitor is radio noise suppression.

All that said, the symptoms are of excessive richness. If it still has a ZS (only ZS commonly have autochoke) then either a bad diaphragm or the autochoke falling off the side of the carb are the two prime suspects. The 3 screws holding the choke should be checked immediately; if any are missing they are very special screws of two different lengths and an unknown thread form and an odd tapered underhead section. A too long screw in the wrong hole runs into the float, causing mayhem. I get them from Moss or TRF, but they may not show in the catalogue listings.
If the diaphragm hasn't been replaced recently get one. It's either bad or about to be; lifetime is variable by what sort of fuel you get and the application of any sort of cleaning or starting fluids = bad idea. OE diaphragms with real gas lasted 10 years, repro with panther piss might only get 6 months.

Very rich mixtures are lethal fire hazards if it still has an operational catalytic converter. And the loose autochoke will leak liquid fuel right on top of the red hot cat.

FRM
Fletcher R Millmore

I'd agree with richness.

You can tell if it is an ignition LT problem i.e. points, coil primary or connections by looking at the tach, these usualy cause the tach to jump about with any misfire. No jumping is usually HT, carbs or mechanical e.g. valves.

Condensers or more correctly capacitors connected to the coil terminals and especially if they are blue are interference suppressors. An ignition condenser should be under the cap with points.

The colours on that amplifier don't sound like either of the factory systems. The 45DE4 'Opus' system was self-contained i.e. all in the distributor and had white, white/blue and white/black wires going into it. The later 45DM4 module *was* remote i.e. bolted to the inner wing but had white and white/black wires going to the coil and black and yellow red wires coming from the distributor trigger. There are many after-market electronic ignition systems of various types, it's more than likely one of those, failed (well well) and the owner couldn't be a*sed to remove it and the coil and refit the coil.

The contacts in the cap *will* be blackened, as will the curved edge of the rotor, as there is a gap between them and the spark has to jump it, with the usual results.
P Hunt

If the Stromberg carb is original, I'ds say the choke mechanism is long past it's "use by date" Barrie E
B Egerton

This thread was discussed between 03/03/2010 and 05/03/2010

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