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MG MGB Technical - Running - Not Running

OK, my first post... Here goes:

I have a '67 B, which ought to be a type 1. Bought it 2 years ago, and here's the story:

I bought it well over 150 miles away from where I live, and the road back home it worked just fine. No problems at all. Shortly after I bought it, I replaced the batteries (2 6V ones), as they looked more like calliflower than batteries.

Since about that moment (only week after I bought the car) I have had technical problems. It drives smooth and perfect and suddenly stops. Just like that. Nothing at all. Doesn't 'hump' or so, which I think means that it isn't a fuel thing (after trying to restart it you'll soon smell the fuel).

Sometimes I was able to drive 20, 30 or even 50 miles before it'd stop, and sometimes it'd run within 10 mins again.

I have burst about 4 coils so far, and after each coil replacement I have also replaced the contacts.

Everybody convinced me it is impossible that the batteries are faulty, but nevertheless I have tried that as well. I used a new 12V battery, which I cross connected and left the 2 6V's out. Started the car, ran smoothly for 5 mins, then when reversing out of the garage it'd stop within 20 yards.

Now, IF the B starts, it will not run for many yards before she stops, and can't start her again...

She's getting enough fuel. I never looked at the twin carbs, as I am afraid to scr** things up, and I presume that if it runs fine when she runs, the carbs must be good. Coil gets quite hot though but again, if she runs, the coil must be connected right I think.

Anybody any clue? She didn't have a decent drive since 1.5 years now, and she really deserves one (as well as her owner ;-)...

Thanks in advance!

Mark
mg at albadajelgersma putdothere org
Mark AJ

I had a similiar problem and it was a faulty fuel pump. It seemed ok (and it should have been as it was recently replaced) sometimes the car ran fine, others it started then stopped then restarted again. Sometimes just cut out when running. Eventually it wouldnt restart, called AA and they confirmed that it was the fuel pump. It had been sticking so the fuel got through most of the time but not all until eventually it jammed. (it was replaced under warranty and all is fine again). Hope this helps.

Alan
a g dunlop

Mark. I have a website at www.custompistols.com/ which may be of use to you. Go to the MG section, then the articles section. There is an article on the MG's factory fuel pump, the SU, on there by David DuBois. Dave has a lot of experience in rebuilding these pumps--something like 35 years. He also, when he agreed to write the article, Dave contacted Burlen Fuel Systems, the current manufacturer of SU pumps, to make sure the information was as accurate as possible. Thus, his article should allow you to identify and check out the fuel pump on your car.

Also on the website is an article I have authored which deals with the inspection and troubleshooting of the ignition system. That should allow you to go through your ignition system and see if there are any problems. The article on spark plug wires should also be read and checked out. These articles should allow you to focus in on what the problem is.

The other area you mention is the carbs. Paul Hunt has an excellent tech article on tuning the SU carbs on his website, The Pages of Bee and Vee. The website is http://www.mgb-stuff.org.uk/. Sorry, this did not come out as a "hot link".

As to my diagnosis of your problem. I suspect you are running a late model 6V coil in your car. You need a 12V coil. Information on how to test for which you have is in my article. The other possible cause is if the clamp holding the coil has been tightened too much. This will crush the body of the coil and cause it to short internally. But, what you describe seems to be coil related. Les
Les Bengtson

It is also possible that the PO did a polarity inversion conversion incompletely. (say that five times quickly!)... that that that that that (sorry)

Anyway, if the car was converted to 12V neg ground but only partially then many strange things can occur.

Mike!
mike!

Mark,
have you replaced the condenser aswell as the points?
A condenser with an intermittent fault could give the problems you describe. It could also be the famous ground wire for the points plate in the distributor.
Mike may be on to something here (although I remember that connecting a coil the wrong way round will only reduce the voltage of the spark). Are the batteries connected for positive or negative earth?
Where in the Netherlands are you?
Rufus
R Pool

Alan, thanks for your info. I will get a new fuelpump tomorrow and test this weekend... One of the few things I have not tried as of yet!

Les, I have a 12V coil, actually didn't know there also were 6V's for the MG B, as I thought ALL MGB's were 12V since start... The coil is pre-tightened, so that cannot be the problem really... I do have a new coil waiting for when I need one, so I could try hanging it loose with tie-wraps just for the check of course...

Mike, somebody else gave me the same diagnosis as you just did... Seems unlikely to me, as the first weeks she ran fine. The lad that told me the same thing you did, also told me that IF this was the problem, I should decharge the batteries completely, than recharge them in order for it to do the polarity inversion conversion completely... I have tried it, no succes unfortunately.

Rufus, Yes, I have also changed the condensator every time I renewed the points... What's with the famous ground wire for the points plate in the distributor? I have not renewed the ground wire there as of yet, as the coil gets good voltage (which is after the distributor comes in). Batteries are connected in traditional UK way (positive earth)(as a true MG B should be ;-) it is also a RHD, and not a LHD *details*). I am based in Arnhem, car is based in Tilburg area (I have a lack of space living in Arnhem City Centre)...

If other people have other bright ideas, I love to hear about them, I am willing to try any solution now :)

Mark
Mark AJ

Mark,
I feel for you, having been through similar start-stop problems. The good news is that, in my experience, now it's really bad you have a much better chance of fixing it. My advice is to assume nothing and go through the full "car won't start" checks on Les or Paul Hunt's sites. You can change a lot of parts without getting anywhere but depressed.

The key in my case was to check for spark and fuel at the moment of the problem. The fault was a small piece of dirt blocking the fuel line to one carb. As soon as the engine stalled it would drop out of the way and then all checks would show normal. I used a gunson's Colourtune clear spark plug so that I could see what was happening in each cylinder. That way I could see the spark was OK but that there was no combustion in two cylinders as it stalled. A great cheap diagnostic tool. Good luck.
Steve Postins

Mark,

Before replacing the fuel pump there is a very easy test you can do. Recreate the problem (shouldn't be too hard from what you say!). So now you have a car that won't run. Disconnect the fuel pipe before the carbs and put it into a container of some sorts. Turn on the ignition. As there is now no back pressure the pump should be busy pumping out a lot of fuel into the container (if it is an original unit this will be accompanied by a ticking sound). If the pump is not operating then that is your problem. It is either faulty or having some sort of polarity issue secribed above. If it is working save your money and look for another fault!

Another simple check would be to test for a spark after re-creating the problem. Look at Paul's Hunt's site as recommended above. No point in just replacing parts when you could work out which is faulty first!

HTH
Iain

I D Cameron

Mark, there is plenty of good advice above and certainly going through the full "car won't start" procedure is a good idea.

All I will add is, if your coil is getting hot after driving a few yards then you have a serious problem! If the major fault turns out to be a lack of sparks, then you may find that swapping in a different coil gets you running again. If the new coil also gets hot, then the problem is still within the disi or associated wiring.

hth, good luck.
--
Olly
Oliver Stephenson

MMMMMMMMM (thinking)

I think I would be exploring all ignition options first. I doubt it is the carburettors as the car drove home OK. Fuel pump maybe but before fitting a new one check the flow and a check to see how much of the points are left. (See if the contacts are worn away)

As Olly says, you need to do a complete ignition check and a new condenser as R Pool from Holland says would be a really good starting point.

Most importantly, welcome to the board, this is a priceless resource if you are prepared to get your hands dirty!


cheers
Ian Buckley

If you have an electronic tacho, what does it do when the engine stops?

If it just drops straight to zero then it indicates a bad connection or broken wire on the 12V cicuit to the coil.

This could be a bad ignition switch or fusebox connection as well as the more obvious coil ones.
Chris at Octarine Services

Don't forget the short flexible earth wire inside the distributor which links the base plate to the body. It is sometimes difficult to spot this as the very thin wire is cotton covered. This a source of much confusion as it sometimes makes and breaks as the baseplate rotates to advance / retard the engine and the cutting out does not make a lot of sense.

I feel that you may have a 6v coil fitted, better check on this. The polarity doesn't really matter too much because if it is wrongly connected it is only the efficiency that is reduced by about 10/15%. Pump seems to be duff though as these things can often pump continuously into a bucket but more usually will not perform reliably intermittently when asked to and this condition can be likened to light throttle useage.
Iain MacKintosh

A few details will help the pundits here to help you.

In my experience, (in general) in a fuel shortage the motor will sputter to a halt giving a few last tries, while an electrical fault cuts the motor off instantly. Which does your motor do?

When the motor cuts out, what other electrical things stop? Does the red charging light come on? As Chris asks, if the tach is electric, what does it do? Do the radio/wipers/blower cut out as well? headlights?

"I have burst about 4 coils so far" is very worrisome, and the reason Les asked to verify the coil is a 12 volt model. The later cars had 6 volt coils. For better spark, 12 volts are supplied to the coil while starting. Then as the key released to the run position, a circuit with resistor reduces the supply to 6 volts. If you run a 6 volt coil at 12 volts for normal running it will overheat and fail.

If your coil is definitely a 12 volt coil, could it be that your alternator is pumping out too much voltage? I believe you should have no more than 14v at the battery posts when the car is running. Hopefully another will chime in to confirm. Of course, this will be affected by the resistance in the connections and cables between your alternator and battery. Most American auto parts stores will test an alternator for free. I don't know the custom in your neighborhood.
Matt Kulka

I believe Mark's car has a generator/dynamo assuming it is original. It also has an external regulator. Mark, have you observed the generator/battery output voltage with the engine running at say 2,000 rpms? More than around 14.5 volts would indicate you have a faulty generator/regulator. A high input voltage at the coil could cause the coil to burn out. Good luck, Clifton
Clifton Gordon

Wow, what a reactions.... Let me try answering all relevant questions...

Today, when I called my supplier, he also advised me not to just renew the fuel pump. I have not bought the pump yet. If it ticks, it must be good. And it does make the ticking sound... So this weekend, I will disconnect the fuel pipe before the carbs and put it into a container... I will keep it running for about 20 litres with a sock in between to look for dirt of any kind, just like Ian describes.

Ian Buckley, contacts have not worn out, they look good, also after car stops.... Have been sanding them a few times before I renewed them...

Chris, I have tha tacho, and it has been about a year since I checked it (and will do again this weekend to make sure), if I remember well, it goes up to appr. 14.7 (!)...

I have called my parts-supplier, and he confirmed it is a 12V coil... I have the newest coil here, and it has nothing printed on it (except for DLB-101)...

Matt, I'd say a fuel shortage would sputter as well... Although, I have had a problem on a pre-war car when I drove it to Prescott (Gloucester) which was fuel... And it'd stop instantly... This car had 2 fuel tanks (one tank, one reservoir)... When driving highway, the underpressure did not 'suck' enough fuel into the reservoir (has no fuel pump) causing the reservoir, which feeds the carbs, to become empty thus stopping with no notice... So I will check the flow just to be sure...

No other electrical things stop, the red ignition light comes up, lights do work straight away, horn works fine too...

Clifton, as said just earlier... I think it does 14.7 when car hasn't even started yet... IF it starts this weekend, and if she's willing to let me go to 2.000rpm, I'll check again and post my findings here...

It's a shame I do not have the car here, so I would be able to check all your replies right away.... I will try most of them though!
Mark AJ

Mark, I don't know about the Netherlands but in the USA most of our parts people wouldn't know a 12 volt coil from a 6 volt coil unless that information is printed or stamped on the coil. Since you have the latest coil in hand, do you have an ohm meter you can use to measure the resistance of the primary winding? As you may know a 12 volt coil will measure around 3 ohms resistance and the six volt coil will be around 1.5 ohms. These values may vary .1 or .2 volts. Here most cars use a 6 volt coil so a 12 volt coil is hard to find. I'm not suggesting you not listen to your parts supplier but having three coils go bad is reason to suspect that he may not actually know what he is selling. Good luck, Clifton
Clifton Gordon

Mark. A couple of things.

First, all coils are "12 Volt Coils" because the system voltage on all modern cars is 12 Volts. What we are referring to as a "6 volt coil" is the DBL 102 with a resistance of about 1.65 ohms across the two primary terminals. This type of coil is designed to have either a resistor wire in the circuit, or a ballast resistor in the circuit, which drops the input voltage down to about 6 volts. Your car needs a coil designed to run at a full 12V input, all the time. As I remember it, the coils with the marking DBL 101 on the clamp were the correct ones for your car. (I run Sports Coils, so cannot find an original coil in my spare parts collection.) So, it would seem that you have the correct coil and, if there are problems in the future, you can use an ohm meter to verify which you have.

Secondly, you mention a "tacho that goes up to about 14.7". Sorry. The "tacho" is the dash mounted tachometer or rev counter which indicates the engine revolutions per minute. On your model, it is electric and, when there is a fault in the low tension ignition circuit, the engine faulters or dies and the tach needle drops for the current engine rpm reading to zero. Immediately as if a light had been turned out. If, when you have your problem, the tach/tacho/rev counter drops--immediately, to zero, it is electrical. If the engine winds down or sputters and the tach does not drop to zero, it is either fuel or high tension ignition circuit.

I am not sure what you are referring to when you say "I think it does 14.7 when the car has not started yet". The only 14.7 I can think of is the system voltage when the car is running. You should have either one or two batteries in your car. It originally came with two 6V batteries wired in series to produce 12V to the system. The voltage of the battery/batteries, measured at the terminal going to ground and the terminal having the cable going forwards to the starter solenoid, should be between 12.0-12.5 volts when the engine is not running. With the engine running, use your volt meter on the clamps (not the terminals themselves) and the system voltage should read between 13.5 and 14.5 volts (although 14.7 volts is not a problem).

On the fuel pump. Just because it ticks, it is not always good. Again, go to my website and check out the fuel pump article. It will tell you which pump you should have and the volume and pressure that pump should be producing. You can actually measure these two items and, if you have any questions about the pump, there is a hot link to David DuBois where he will answer your questions.

Nothing you have said indicates you have a problem which cannot be solved. But, we need to be talking about the same things and understanding the same terms if we are to help you effectively. Les
Les Bengtson

OK, just got back from the B...

First, I checked the fuelpump... I disconnected the fuel pipe before the carbs and put it into a container, kept it running for about 10 litres, ran smooth as could be. Conclusion: Fuel is all right.

Second, I disconnected the batteries, and used a booster instead, as I wanted to make sure the batteries were not the troublemakers.

Third, time to start the car... Would not start at all..... After 4 minutes of starting I disconnected the coil and installed the new one... Started within a second. I kept it running for about 20 minutes total, in which I have tried to find the problem...

I measured the voltage (low rpm and high rpm) which seemed all to be in order. No extreme peaks or whatsoever.

The problem seems to be in one of the 3 wires connected to the coil... When holding the coil in my hand (I didn't screw it to the sideplate) and turning it a little, she'd try to stop, but turning it back would 'restart' the engine. Turning it again would try to stop the engine, turning it back would undo the stopping of the engine again.

So I took a plier, and tried gently to pull all cables connected to the coil... The earth wire didn't make a change, neither did the mass. The 3rd one (thick one in the middle, sorry, forgot i't's name) did cause it wanting to stop the car... Conclusion: Wire will need replacement. This cable might cause all trouble.

New conclusion: Wire needs to be renewed, but this does not answer the question why my coils keep dying... Then I wondered... what if I had an operating microwave oven, unplug it's power plug, and putting it back RIGHT AWAY... Wouldn't it harm the microwave oven like, a lot? Yes.... I think it would.

But I have turned the coil, causing it to stop nearly, and by turning it back again she would just not stop completely... So I have reconstructed the problem, which did not kill the coil... Has anyone any ideas how this could be?

Tomorrow I'll go back again, and renew the wire (due to the lack of a new one, I will use one of another classic car) to see how it keeps then...

To be continued............
Mark AJ

"ran smooth as could be" yes, but did you check its rate? Should be at least 1 pint per minute.

Cutting-out while you twist the coil back and fore definitely sounds like an intermittent electrical connection around the coil, but if by "thick one in the middle" you mean the HT lead, in a good ignition system the spark should be able to jump between 1/4" and 1/2" altogether, which when taking into account the rotor/cap and plug gaps still leaves quite a bit. However I did have a problem some years ago with a silicone-cored HT (the only problem I have ever had with them in over 30 years of use) in that the brass connector at the coil end developed a very hard bright blue coating that resisted all attemps to scratch through it and was impervious to HT voltages. I would check just how far you can remove the HT lead from the coil and keep the engine running, if not at all I would suspect a non-functioning condenser and hence a very weak spark. In a good ignition system if the lead to the distributor was removed but the engine was still spinning the resultant HT could well find an alternative route to ground, and with you holding the case of the coil it would be via you!
Paul Hunt

This thread was discussed between 05/07/2004 and 11/07/2004

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