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MG MGB Technical - Dwell and points!

I have a 1973 MGB GT C/B. I bought this particular car some 6 years ago and use it every day. It had been fitted with electronic ignition by the PO and a few months ago it died on me. I decided to go back to condensor and points on the grounds that if it packed up again I could adjust the points/fit new ones etc relatively easily. I have however had a few problems with the car missing (and stooping!)and have changed the points and condensor twice within a few months (poor quality parts?). It happened again yesterday and I duly fitted new points & condensor of a different make (Intermotor). It seems to be running OK now and my fingers are crossed. I do have one particular query however and that involves setting the dwell. Am I right in think this is an alterntive method of gapping the points and how do you go about doing it? I have always set the points with just a feeler guage in the past.
Paul Eades
Paul Eades

Correct-setting point gap by the dwell method is exactly as you say Just another( supposedly better )way of adjusting the points. My advice to you is stick to what you know-feeler gauges are fine. Usually if setting points by the dwell method the gap ends up between 14 and 16 anyway if the cam of the dizzy is in good order. It's just not worth the hassle In my opinion anyway To set points by the dwell method you will need a dwell meter, these usually come as a dwell-tacho combination and adjust the gap until you get 60 deg. for a 4cyl. B Cheers Willy

Setting the points via the Dwell angle is more reliable than using feeler guages, but IMHO any points setup is going to be less reliable and more prone to trouble than an electronic triggered ignition system. To set the Dwell angle you'll need an automoticve test meter such as this one from Maplin:

For your car, if it's in otherwise standard setup the dwell angle should be 60+/-3 degrees.

As a point of interest I've just swapped out my old points system for a Britpart electronic ignition kit (Part No. GI95403) which I bought from a Land Rover supplier as they seem to be happier with a much lower profit margin than identical units being sold elsewhere as MG replacements. The system for the Lucas 45D4 distributor is less than ten UK pounds so it's cheap enough to carry a spare and will be much easier to fit roadside than a points system. I've only had it fitted for a week, but thus far I can report that it's every bit as good as the Aldon/Petronix Ignitor I've used previously and reports from
Land Rover owners is that they are very pleased with the units reliability & performance.



Oops, I've just checked and the correct part number for the 45D4 Britpart ignition system is ETC5835K. I beleive they also make/supply a similar setup for the 25D4 dizzy as fitted to earlier cars.

The points are initially set by gap, then checked thereafter using dwell which is much more accurate and convenient than checking gap. As an experiment I ran my last set of points for about 18K (10k change interval remember) and they were still in tolerance from initial setting after all that time.

Gap and dwell are the same thing, but dwell is akin to setting timing dynamically whereas gap is akin to setting it statically. In both cases the dynamic measurement is preferable as it can also reveal problems that the static method won't.

Whilst it is perfectly possible to set the initial gap using dwell alone it does mean keep starting and stopping the engine to do so, or maybe spinning it on the starter with the plugs out, which means the coil lead has to be connected to a plug to deal with the HT. Personally I fit new points and set the gap off-car as it is much easier, particularly on RHD.

Whilst the gap is the same on both 25D4 and 45D4 distributors the dwell isn't, because the shape of the cam differs - 60 +-3 for a 25D4, 51 +-5 for a 45D4, and that is irrespective of what engine it is fitted to.

If the electronic ignition failed *and* three sets of points and condenser since I'd be looking for some other problem. The first step with a stopper or non-starter is to connect a timing light to the coil lead and each plug lead and watch the flashes while cranking. No flashes on the coil lead means a bad coil or bad ignition LT. Flashes on the coil lead but missing from plug leads means the cap and/or rotor is breaking down. Flashes on the plug leads as well means the problem is more likely to be fuel. If the engine cuts out while under way look at the tach first before doing anything. If the momentum of the car is still spinning the engine, but the tach as dropped to zero, then the problem is the 12v feed to the coil or the ignition LT circuit i.e. coil, points, and associated wiring. If the igniton warning light is glowing as well then the ignition supply has been lost from the ignition switch or relay. These steps diagnose everything except the condenser.
Paul Hunt

Hello Paul,

Another possibility of trouble area, is a worn distributor which cause a non-constant fire as the main shaft (where sits points) wobble.


Jean G.
Jean Guy Catford

There have been some issues with duff rotor arms. My mate Ken has had 2 in his midget.
Paul Hollingworth

As a toss up to the group, did the 73 have a resistor in the ignition circut? Some of the aftermarket ignitions do away with the resistor and use full battery voltage, this full voltage will kill points quickly. Ric

I fitted a pertronix to my 71 roadster a couple of years ago and was very pleased with the improvement and it is still OK. A friend with the same model as mine wanted to go down the same road mainly from my recommendation, eventually he found someone on ebay advertising new replacement 45d dizzys with electronics built in. he has not fitted it yet but at a little over
Trevor Harvey

Chrome bumper cars didn't have the ignition ballast resistor, rubber bumper cars did. If you bypass the resistance on a rubber bumper because that is what your electronic ignition instructions say then you must replace the 6v coil (1.5 ohms primary resistance) with a 12v coil (3 ohms primary resistance). But by doing that you lose the cranking boost system and its cold starting benefits, which is probably more use than electronic ignition.

There *are* duff rotors out there, they tend to be the ones with a dome-headed rivet inside the base circle of the part of the rotor that fits over the dizzie shaft, see and click on 'Rotors'.

I know someone who bought a distributor off eBay complete with Pertronix-type trigger (blue, complete with suspect rotor) and it was absolutely hopeless. The positioning of the trigger on the advance plate was way out and the spark was occuring when the rotor was between two cap contacts. Fortunately retro-fitting points and a condenser at least allowed him to use the distributor, so it wasn't a dead loss. Another problem is that the rotor itself contains the magnets for the trigger, so when the rotor fails you have to get one of the same type, and be sure the magnets are in the correct position to fire at the right time. It's just not worth the risk.
Paul Hunt

"The positioning of the trigger on the advance plate was way out and the spark was occuring when the rotor was between two cap contacts."

Whenever I've fitted electronic ignition units to B's I've always had to adjust the timing. I would guess that's down to more efficient triggering as the need has always been to retard the ignition.

Those distributors sold on eBay appear to be stock units fitted with the Britpart ignition kit. The rotor supplied with these kits is identical in profile to the original (certainly to the Lucas item that was fitted to my '73 car). I had to retard the timing by around 4-5 degrees when I fitted it last week and so far the car has performed faultlessly and the reports back from the Land Rover boys are that these units perform very well and I don't suspect many of them are too gentle with their vehicles!

At 8.10 for a complete set, it's no great expense to carry a complete replacement Britpart kit anyway. I'll never return to an archaic points system, they're just too much hassle.

Indeed, The electronic conversions all seem to throw the timing off. From personal experience, I've seen where the rotor was so far from the distributor cap terminal that it burned a hole right through the cap! These units are not centered on the point plate and require you to compensate for this by adjusting the timing. I cut a hole in one cap and ran the engine with a timing light connected to the nearest plug wire. To my horror the spark was occuring almost 3/8" from the terminal. RAY

Many thanks for all the comments everybody - very interesting! Just to clear up a couple of issues. I replaced all the 'consumables' from the plugs to the coil (unballasted 12v)just to eliminate all the possibilities there. As a matter of interest, the points were fitted by my local garage as it was in for another job which I don't have the facilities for (or read that as lack of ability!). Anyway, when I looked in the dizzy there was oil coating everything, so I guess he must have been a bit enthustiastic on the points fibre pad. In any event I removed those points & condensor, cleaned all the oil off the dizzy and put a new set of points plus condensor on with just a bit of grease on the pad (2 days ago). The good news to date is that it hasn't missed a beat and I have just done a round trip to Swansea this morning (120 miles)and it flew. So fingers crossed. Paul
Paul Eades

"I would guess that's down to more efficient triggering "

Not so, its the same issue of the trigger point shifting relative to the advance plate, but most of them don't move it as far as the one I referred to. Some movement (in the correct direction) is probably a good thing as I don't think the trigger point is exactly where it should be with points. You can tell this from the burn pattern on the rotor. Under changing vacuum advance the trigger point is moved relative to the rotor, which is why rotors generally have a wide contact and there is burning across most of it. Ideally this burn pattern should fit within the rotor arc and have a small clean patch at each end. But I have noticed over many years on many cars that it is always offset slightly to one side, going right up to the edge of the rotor. One hopes that it it doesn't go too far past the edge of the rotor, or you run the risk of getting misfires at one extreme of vacuum or the other. This shift of trigger point only occurs with changing vacuum advance, not centrifugal.

BTW, should be *grease* on the felt rubbing pad of the points, not oil. Oil goes down the centre of the shaft (rotor removed) to lubricate the centrifugal advance mechanism. But I've found oil bridging my points in the past and it has been running perfectly. Quite probably aids spark quenching, HV switch-gear often uses contacts in an oil bath for that reason.
Paul Hunt

Unfortunately Britpart don't supply a unit for the 25D distributor only the 45D
Iain MacKintosh

Similar units for the 25D distributor are available on eBay. Item number: 120374753788 Seller: simonbbc

I fitted an electronic ignition because the quality of the ignition consumables was lamentable. I had to retard the ign by a few degrees to get it run without pinking.
Stan Best

I changed points at the beginning of last year and although the supplier was Unipart it also had 'TURKEY' stamped on the box. At the time I wondered whether that was the country of manufacture or a comment on its quality. They are still in spec after a year so maybe the former, but it's early days yet!
Paul Hunt

Mike, I ordered one off e-bay so we will see how it goes in the roadster. I already have a Magnetronic system in the GT and it has been fine for years.
Iain MacKintosh

It's happened again!! I was coming to working this morning when the MG suddenly lost power. I managed to nurse her the half mile or so to work, but this is getting ridiculous now! Bearing in mind I have changed everything from the coil to the plugs I am at a loss to understand what is going wrong. I will go out at lunchtime to investigate.
Paul Eades

Two things to check - make sure the earth wire for the distributor base-plate is in good condition and firmly connected. Also check the earth wire to the fuel pump, you may be suffering from fuel starvation due to a poorly running pump. You'll find the earth wire runs to the offside (RH when viewed from the rear) rear number-plate mounting bolt and it's at this point that corrosion can build-up and prevent a good earth.


I just went out to have a look under the bonnet and checked the points (which are correctly gapped). Also checked for loose connections. I started her up and she fired up no problem. Initially she ran OK, but I noticed that when she warmed up she started missing again and there was a bit of a twitch on the rev counter (LT problem?). This has been something of a constant throughout this saga but I am at a loss to understand it, as I have changed the points and condensor 3 times since the new year. She seems to run fine for a while (days / week or two) and then starts playing up again. Oddly enough I never had this problem when I had electronic ignition (until it died!) but it has been a recurrent problem since I went back to points and condensor.

Paul Eades

What did the tach do the moment the engine cut but was still being spun by the forward momentum of the car? That is crucial. If the tach is still registering the LT side of the ignition is fine (including the earth wire Mike mentions), which points to HT or fuel.

If the tach has dropped to zero while the ignition is on the the engine still being turned over by the momentum of the car it is ignition LT. If the ignition warning light is on as well then the 12v supply from the ignition switch has been lost.

If, when you turned the igntion back on again and it started, the pump chattered away like mad then it is the fuel pump or its connections. If it only clicked once or twice then the carbs are full so it isn't fuel, which only leaves HT. That needs a timing light to diagnose when it won't restart, if it flashes on the coil lead when cranking but not regulary and consistently on one or more plug leads then it is the rotor or cap. If the coil lead is irregular or not flashing as well then it is the coil or possibly condenser.
Paul Hunt

Interesting thread this chaps,

I have an old 74 Gt Ive been re-commissioning and have an odd problem where the the engine runs great for 10 minutes then goes flat and wont take load just backfires, when left to cool it restarts lovley but quickly goes lame!

tried fitting spare britpart kit and same problem drove lovley for 10 minutes then same again,

changed coil a few times, leads, new cap etc etc

The engine is sound and just rebuilt,

The carbs are a little worn but should do just to run it up and down outside the shop!

have fitted new fuel pump and replaced feed lines all through, checked fuel pressure 3 psi constant.

fitted said britpart igntion kit and also tried spare, I have used them on a few mini 998s with success so would suppose it would be ok on the B?
andy davies

What happens with the fuel filler cap off? I.e. could be fuel starvation if a non-vented cap is fitted instead of a vented. If the fuel pump chatters like billy-oh the next time you turn on the ignition and it runs OK then the problem is fuel starvation.

Could also be heat related ignition problems, backfiring in the exhaust is often caused by intermittent ignition where unburnt fuel in the exhaust ignites the next time a cylinder fires. But having read the previous posts you should have monitored what the tach and a timing light does when the problem occurs and eliminated or confirmed that already.
Paul Hunt

I don't generally subscribe to chasing down electric problems by part replacement BUT this story reminds me of intermittent (and eventually total) ignition failures on both my '72 & '73.

Check the connection in the white wire circuit near the fuse box. There's a one-into-two wire sleeve connection. In both cases the aluminum connector corroded and the circuit (naturally) suffered. In one case, when I went to replace it, the old sleeve connector crumbled into dust in my hand.

As the connector is covered by its own rubber sleeve, this fault is essentially invisible.
John Z

Aluminium? Copper originally, and should have a clear sleeve (at least they *were* clear originally, gets a bit murky with age but should still slide off).
Paul Hunt

All the rubber covers on the wiring harnesses on both the '72 & '73 have plenty of carbon black. The connectors look silvery, but the one that crumbled seemed to have some green in it, which would indicate at least some copper.

In any case, that particular dual connector led to intermittent ignition problems on both cars once they reached 25+ years. It was a few years ago, but I think on the '72 when I examined it, the connector was split and easily broke apart when I went to replace it, and it was the connection on the '73 GT that actually crumbled.

I, too, went through points, condensors, wires, plugs and coils on the '72 before I sorted it. When it occurred on the '73, I was better prepared.
John Z

Any sense that the problem occurs on longer runs? I had a problem with fuel starvation caused by a faulty fuel cap - the problem only occurred on longer runs. Also, I recently had a problem with the engine dying on me when warmed up (see archive) - turned out to be the points in the fuel pump.

Richard Atkinson

John - I was thinking of spade connectors on the fusebox that do have two wires in them in some cases. I realise now you were talking about bullet connectors. The covers on those are black of course, and the innards 'silver', but whether its tinned brass or copper I've not investigated. Definitely wouldn't have been aluminium has that has no springiness when that thin.
Paul Hunt

Paul - there are times I wish I could pop for a digital camera. Picture - thousand words.

Any word from the patient?
John Z

Going back to the Britpart electronic ignition for 45d ignition. The sit ethat seels them has new 45d distributors for 25!. Are the 25d bob weights interchangable with 45D. In other words can I buy a new 45d distributor and electronic ignition for about 35 and fit it with the advance curve from a 25d distributor?

No, the internals are quite different. However you could play with the springs to get the 45D close to the 25D - depending on what the 45D is to start with. What reference number is it, or don't they tell you? It's largely irrelevent anyway, the engines need quite different curves today than they did originally because the fuel is so different, even if you use 98 or 99 octane. Short of spending a lot of time and money on a rolling road to identify the ideal curve for your engine and your fuel, and then tweaking springs and weights to suit, the only thing you can do is set the timing just short of pinking at any combination of throttle, revs and load. Even then if you live in Lincolnshire and take it to Scotland or Wales you are likely to find it starts pinking again on hills. Bear in mind that some of these eBay distributors are crap in that they have the wrong phasing and the spark occurs when the rotor is between cap contacts.

If you want a 45D get a 41610, which was the one used on UK rubber bumper cars, the engines on those are not much different to earlier. The 45D is better as it is smaller making it easier to remove and refit together with its mounting plate, and has +-5 degrees tolerance which means it stays in spec longer than the 25D with its +-3 degrees. I ran one set of points in a 45D for about 18k and they stayed in spec the whole time.
Paul Hunt

Britpart have run out of the replacement ignitions - next batch in mid April. HOWEVER do a seach for Britpart ignitions - a lot of the blue ones appear to be defective, the red ones are alledgidly better but.....
d brenchley

This thread was discussed between 16/02/2009 and 26/03/2009

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