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MG MGB Technical - Electrical or Fuel Problem

78 MGB. Been running great for last six months. Took it to work last week. Time to go home..the car stalled less than 100 yards away. Turned the key and started up again, got less than one block and died again. Left the car overnight and picked it the next day, hoping to nurse it to the house. Ran great, like nothing happened the day before. The next day, stalled again twice. Changed the in-line fuel filter as well as the one on the Weber Carb. No change Plenty of fuel going to the carb and filter. fuel pump working fine because I can hear it ticking with the ignition in the ON position. A backyard mechanic friend seems to think this might be related to the ignition system, possibly the coil. Replaced plugs and coil wires four months ago with new plug wires.

I'm leaning towards electrical problem for mere fact that I'm getting fuel to the carb.

Anyone have the same experiences?


Sounds electrical to me, probably the electronic ignition (if you still have it). Same thing happened to my 1978 MGB. Gets hot and shuts down. Cools off and runs perfectly. All unpredictable.

I now have a points ignition.


rick ingram

hello ive had that problem twice first prob bad earth seondly one of the switch units next to the fuses had burnt out. i have the prob again remember therebritish leyland cheers daren

Anytime I've had fuel problems (fuel pump quit) the car always stuttered a bit before dying. Once I had an electrical problem (dirty points) and the car cut out right away. Your problem sounds more electrical.

Simon Jansen

Shook. If it is electrical, you will probably see the tach drop straight to zero when it happens. If the tach winds down, it is probably fuel related. Sounds to me like it might be a white wire circuit problem, perhaps off the ignition relay. Ignition relay is the one forwards of the fuse box and receives its power from the ignition switch. From there, the relay powers the rest of the circuits that control things like distributor and fuel pump. A simple test light will show if you are getting power to the white wire circuit (front of the fuse box, second fuse from the bottom) when the engine dies. Les
Les Bengtson


You described exactly what the car is doing. Tach loses power 1/2 second before it shuts down. Should the white wire be getting power all the time or just when the ignition is on. If I remember correctly, only the bottom fuse has power all the time.

I've seen the ignition relay before and not paid much attention to it. If nothing else, I'll make sure there's a good connection at both ends.



What ignition does it have? An LHD 78 would have had a factory installed electronic system, either the very fault prone 45DE4 with the attached electronics module or the much better 45DM4 with the remote module. From the factory the white from the key operates the ignition relay when the ignition is on, then a white/brown from the relay to the fusebox and other circuits powers the ignition via the loom ballast. When the tach shuts down do the temp and fuel gauges also start to die? Turn signals stop working? If so it could well be the ignition relay or connections around it. If they continue to work then the problem will be closer to the coil etc. And yes, only the bottom fuse (brown to purple) should have power all the time.
Paul Hunt


Ownership of my B goes back to last year and so does my parts recognition. I know I don't have Electronic Ignition. I also have to assume that the original distributor is still installed. When the car first stalled, I noticed nothing as I was in traffic. The second and third times, I noticed that the Tach crapped out, but didn't notice the temp or fuel gauges. Makes sense that they would also lose power if the problem is in the ignition area. I'll check all the connections this evening and take it out tomorrow on a beer run. I'll let you know what happens.




re: I know I don't have Electronic Ignition. I also have to assume that the original distributor is still installed.


You *may* still have an electronic ignition...all '78's came from the factory with one, as Paul advises.

Obviously, you can look for points beneath the distributor cap to see if your MGB has been converted back to a points style distributor.

I've also had an ignition relay go bad in the past. The car would stutter and gradually (sometimes rapidly)lose rpm's...then act as though it wanted to fire but would not catch when I attempted to re-start. The first time this happened I had absolutely no clue what was going on...the second time took me about a minute to open the bonnet, pull the old relay, and install the new one. I now carry a spare ignition relay in the glove box as well as a small jumper wire to TEMPORARILY get me from point A to point B in an emergency.

Good luck.

rick ingram

Since your tach is involved, it's probably not the fuel. But I'll mention this anyway.

I had a similar experience. The fuel pump on my 77B is grounded inside the trunk. Turns out that the bolt holding the ground wire was loose. Sometimes the car would start, and sometime it wouldn't. If it did, it would eventually die of starvation - leaving me in the middle of traffic.

If you suspect a faulty pump - or ground - listen for the clicking noise. If it doesn't click when you switch on the ignition, you're pump is bad, or not grounded (as was my case).

After a week of stalling, it only took a quick tightening to solve my problem. I hope yours is as easy.
Tony Calleja

Hi Shook,
I have a 79 B, It was having a similar problem of cutting out while crusing along about a year ago. Mine does not have an electronic ingnition. Everyone has given you good advice. My problem turned out to be the fuel pump. When it was working it worked, I even measured the rate of flow and pressure at the carb. But the pump *points* had gotten bad and sometimes it would just quit pumping. One time it really started acting up and would quit about every 5 miles, I was out on the plains about 50 miles from home, and I had to get under the car each time it quit and rap the pump with a hammer to get it ticking again. Finally got home, then installed a new fuel pump, never had the problem again. What part of Colorado do you live in. If you are anywhere near the Denver Metro area give me a call I wouldn't mind stoping by and give you some help. I have owned my B for 11 years now. E-mail me and we might be able to get together.
If you don't know about it there is an annual Engish car meet in Arvada once a year in the fall, 2 day event with well over 500 cars in attendance. Lots of fun let me know if you would like the details.
John F

Shook. As the others have noted, your car came standard with a Lucas 45DE4 distributor. This one has a round body with a square box extruded from the side. The one on my 79 went out in three months from new. My understanding is that, when these were replaced under factory warranty, the Lucas 45DM4, an upgraded distributor was used. Do not know at which point they started doing such replacement.

So, the first thing to do is to look at your distributor and see what it looks like. If it has the round body with the extruded square we need to factor that it. If it has a round distributor body and a black box bolted beneith the coil, it is the later style, Lucas 45DM4. If it has a squat round body and no box under the coil, it is probably a Lucas 45D4 points type. If it has a round, less squat body, it is probably a Lucas 25D4. Finally, there are the aftermarket distributors and the points replacement systems used with any of these models. Hence, you will want to know what you have in your car so you will know what problems it might, or might not, have.

While under there, check for loose connections at the ignition relay, fuse box, coil and distributor. Loose connections and wires about to break at the connection are a continuing problem with older cars.

You can pick up a small, simple test light, having a probe with a light inside and a ground wire with a clip on the end, for about $2 at most discount parts stores. I keep one of these in each car's tool box for quick checking of problems. If you have the engine quit again, you can hop out, hook up the light and start tracing where you have power and where it stops.

A copy of the factory wiring diagram, expanded 300% and laminated is an excellent idea. Allows you to trace circuits on the diagram and see where the power comes from and goes. This has helped me greatly over the years. With the diagram laminated, you can use a water soluble marker to trace out the circuits. Makes it much easier to understand and figure out the problems. Les
Les Bengtson

Three beers later and optimistic. Checked, cleaned, and reconnected wiring to ballast, coil, and fuse box. What is that little blue, nickel size cylinder object on the fender wall with the small wire going to one of the coil posts? None of my three MG books shows a picture.
Checked coil leads and noticed that the distributor was quite loose. Was able to rotate by hand (quite easily) 1/2 inch in both directions. Not sure exactly which way to go, I adjusted the timing by ear until it sounded right and tightened it up. Let the car idle 15 minutes and not a problem. My next step is to take it out on one of our country roads and put it under pressure. Won't know if the loose dizzy was the culprit until it happens (or doesn't happen)in the future. From the various descriptions, it appears that my B is equipped with the 45DE4 Distributor 'cuz it doesn't have points under the cap. Is the 45DM4 better than the DE4 or is it not as bad?

The little blue thing is almost certainly an interference suppressor. I don't know whether they were fitted by the US importer but UK cars don't seem to have them as a rule, whereas I have heard of many North American cars with them. A loose distributor *could* cause the tach to drop as well as the engine to cut out as the ignition path is through the distributor body to the engine ground. You really must check the timing now and get it right. The 45DM4 is a good unit, not just better than the 45DE4. The electronics module was widely used by other manufacturers and is still available. If you *do* still have a working (well, some of the time at least) 45DE4 then it deserves a place in a museum, it must be one of a very small number.
Paul Hunt


Do you have a model number of a more-reliable Distributor available in the states? I'm more concerned with reliability than originality. I'm guessing that the interference suppressor is there for the radio and nothing else?

Shook. Even if you have an original Lucas 45DE4 distributor body, the internals may have been replaced. My daughter's car came with a Lucas 45DE4 distributor having Lumenition internals, making it much more reliable.

Many of us have used the "Euro-Spec" Lucas 45D4 distributor offered by Brit-Tek to replace older, malfunctioning distributors. If you wish, the 45D4, a points type distributor, can have the Petronix points replacement conversion done to it. I am using the Petronix system on my IH Scout II, so far with good results. My MGs however, are still running points.

You need to find out exactly what you have installed, then move onwards from there. Les
Les Bengtson


I've checked and cleaned all wiring to and from Distributor. Re-newed a couple connectors and splices. Replaced Lucas Coil with new PS40 left in car from PO. Need to have timing adjusted. Ran car last night in garage for 15 minutes with no hiccups. Weather depending, I'll jaunt across the country side this weekend, hoping to duplicate conditions when I first broke down. Thanks to all who have contributed to my dilema. Electrical snafu's are not the easiest to diagnose. It appears from all the threads that it's best to ELIMINATE obvious faults before jumping in making rash changes or replacements. Thanks again

re: It appears from all the threads that it's best to ELIMINATE obvious faults before jumping in making rash changes or replacements.


Ah, you learn rapidly, Grasshopper!

Welcome to MG-dom!

1978 MGB
1969 MGC
1974.5 MGB/GT V8 conversion
rick ingram


Flat tires, spongey brakes, leaky hoses are obvious and generally isolated to one visual area. Electrical, on the other hand can travel throughout the entire frame. EVERTYTHING on the car is a potential ground problem. There is no such thing as common sense electrical. My co-worker referred to Lucas as the Devil. When I bought the car last year, I graded my mechanical sucesses on how many Blue Moons (Belgian White)it took to complete the job. It's cost me over a six pack in dealing with this latest malady. I don't have a tee shirt or hat just yet, but I think I'll be earning them in the future.


I am probably wrong about this, but if there is a thin little black insulated wire inside your dizzy which connects the rotating plate your electronic pickup is mounted on to the outer body of the dizzy, it could be broken internally and your electronics has a faulty ground.

Unfortunately, I am not as familiar with the OE electronic ignitions as the others above are, but on points type dizzies, this can be a culprit for similar fun as you have been having. If it were a bad "breaker plate" grounding wire, I believe Moss sells spares. I would use a genuine spare as they are made to be very flexible to accomodate the rotation of the plate by the vacuum advance. FWIW
Bob Muenchausen


I do remember seeing a three-some harness of small gauge wires (20 gauge)going from the dizzy to possibly another harness. All three wires are contained within a gray insulating sleeve. Its' origin is at the Dizzy, but the destimation is unknown until I can cut the wire wraps from the electrical bundle courtesy of the PO.

I'm taking the B to a car care mechanic tomorrow for an electrical diagnosis and re-timing. I'm ignorant of elect diagnosis methodology or I would do it myself. I've learned quite a lot from this and other threads, but I'm still not confident in my ability. I will be getting a timing light shortly so I can check the timing when this type of thing happens again.

Would your comments re: black wire be amplified during warm engine conditions?

I'm still leaning to a loose dizzy being "automatically" adjusted during shifting as the main culprit. I don't know what kind of torque the 1.8L puts on a loose fitting distributor during routine driving.

I'll no more after a qualified mechanic looks at it tomorrow.




Mechanic checked everything out and found no faults. Although he is not a LBC mechanic, he is quite competent with other autos. He adjusted the timing and air/fuel mixture. As advised earlier, I'm getting a spare ignition relay..just for grins

Thanks to all who participated..

Proves that MGB'ers care about the car and owners.


The distributor internal ground wire is used for points systems and fully 'in cap' electronic systems like the pertronix, but there isn't one for the factory electronic systems where only the pick-up is in the cap, and after-market systems with only the pick-up in the cap don't need it either. The problem comes when fitting a Pertronix or similar to a 45DE4 or 45DM4 in place of the factory pick-up, you have to *provide* the ground strap or it will not work properly.
Paul Hunt

This thread was discussed between 08/03/2005 and 15/03/2005

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