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MG MGB Technical - Help please! Won't start-will start-dies

I went to pick up my car from its MoT and could only get my '74 GT to start one time in ten. When it does start it runs fine, for about 20 seconds, then dies like you switched it off, with the tacho dropping dead to its stop. Only other observation is that it seems to try to fire as the key is turned from ign to off as you take it out. There is fuel in the float bowls and the fuel pump still ticks OK. It has Lumenition optical ignition, and has just come back from heaps of bodywork (sills, wings etc) but ran fine to the garage and on my driveway over the weekend. Any help would be very much appreciated.

Thanks,

Steve
Steve Postins

Meant to say, if it doesn't start the engine doesn't fire at all on cranking. One the few occasions it does start it's on the first turn of the crank.
Steve Postins

Steve. First step is to get the car somewhere where you can work on it and have some test equipment on hand. Second step is to recruit an assistant.

Your problem is either fuel or spark. My feeling is that it is spark related.

First, pull the fuel line to the carbs, insert it into a catchment bottle, then have assistant turn the ignition switch to run. You should have fuel. Proper volume and pressure is important to proper running, but will not affect the ability to start. David DuBois has an article on the SU fuel pump at www.custompistols.com/ if you need to investigate the fuel pump.

If the fuel pump is working, pull the coil lead and have the assistant crank the engine over while you hold the lead about 1/2" from the block. You should see a strong, blue spark. If not, check out the low tension ignition system. I have a tech article on this on the same site as Dave's article.

If you have good spark at the coil, check the spark at each of the spark plug leads. Good spark at the coil and bad spark at the spark plug leads indicates a bad rotor, dizzy cap or leads. If you have good spark at the spark plug leads, pull the plugs and sniff them for the smell of petrol. You should be able to smell it on the plugs. If not, you have some form of weak mixture. Look for a carb problem or air leak between the carbs and cylinder head. Also a good time to check for adequate spark on each of the plugs. Hook the plug up to its lead, ground the plug and have the assistant crank over the engine. Holding the plug to the block with some form of insulated tool is an excellent idea.

If these ideas do not put you on the track of the problem, post what you have done, the results you have obtained and we should be able to provide a more directed analysis. Les
Les Bengtson

Many thanks Les,

I spent an hour getting nowhere with one small screwdriver in front of the chuckling mechanics at the MoT centre and when I returned this evening with a full set of tools it started first time and has ran fine the 3 miles home. Why didn't I have my tools with me this afternoon? Because I got enthusiastic about the exercise and ran to the garage to collect it. Having exhausted myself getting there I then had to run back. I just know it will wait until it's dark and wet before it stops again. I'll have a close look at the wiring tomorrow to locate its sense of humour circuit.
Steve Postins

Steve,
I experienced something quite similar recently and I pestered many on this board and another until it was solved.

It would not start in the morning. Come home in the evening and it would crank right up and run like a top. Drive it around several hours. Next morning wouldn't start. I eliminated all the usual suspects ignition wire OK, charge in and out of dizzy, timing, fuel, etc. The odd thing was that when I took dizzy out and put it back it seemed to start...must have been a red herring, had just had it rebuilt by one of the most reputable shops on this side of the Atlantic......finally replaced the coil....and that was it...but only after testing everything else. If you can get your hands on one that you know works and you don't have to purchase...try it out. You never know and it only takes a few minutes. As my dizzy rebuilder told me ...."coils never fail" corollary #43..."sometimes they do". Low liklihood but possible and symptoms sound similar but I am not one to jump to conclusion that it has to be that. Hear it sometimes on the various BBS where people are quick to suggest that their exerience is yours. As Les suggests .....eliminate possibilities till what is left is what it must be...I definitely went there over a couple months because I didn't have a coil to throw in and check.
J.T. Bamford

Steve. The "sense of humour circuit" must be located in the driver, not the car. If not, what is an adventure turns into a nightmare. Everytime I think about the purchase of a Corvette, my wife and daughters remind me of how all of my Corvette stories start out, "Now, I remember the time my Corvette broke down in ....". You need a much stronger sense of humor circuit with a Corvette than with an MG. Les
Les Bengtson

I had a odd problem once with a Lumenition. The red wire had an internal break that was not visible on the exterior. The car ( actually a Land Rover SII) would run great but every so often act as you describe - like someone was turning the key on and off. I found only by fiddling with the wire on a hunch.
Andy Blackley

Steve,

The problem is alost certainly in the low tension side of the ignition circuit - start by checking the ignition switch itself - they sometimes get a bad contact which gives your symptoms.

PS Parts on their way yesterday.
Chris at Octarine Services

Thanks all,

I do have an old coil so I'll keep it ready and will check over the rest of the LT circuit in the meantime.

I'm not down on the car just yet especially when I recall it took a long time to track down a similar problem on my wife's car. Being a modern vehicle and full of electronics, these were immediately blamed, and with diagnostic laptop wizardry at their disposal Audi went to work. Two months and £2000 of parts including new dash and engine management system didn't help (under warranty I'm pleased to say!). In the end the cause was simply a broken wire from the starter, just as might happen on an MG. Only on an MG, that's what you'd suspect to begin with!
Steve Postins

Yep, Chris has hit the nail on the head here. It just might be the coil but I too think it is the LT circuit and your comment that it seems to try to fire as you turn it off points me to this.

Hot wire the coil by connecting a wire from a live connection on the fuse box directly to the coil +ve then turn the key. If this works you will have to check the LT circuit. What year is your car ? because if it has a ballast resistor fitted that may be the problem.
Iain MacKintosh

No ballast resitor ('74 chrome) so I'll give the coil hotwire a try Iain. If it is LT, I guess this would get me out of trouble if I get stuck again?
Steve Postins

If the tach drops instantly even though the engine is still spinning as it winds down then it is ignition LT. If at the same time the ignition light glows then flickers and dies as the engine stops spinning then it is the 12v supply from the ignition switch that has failed. When it won't start check for 12v on the coil +ve. If that's there measure the voltage on the coil -ve wrt to earth with an analogue instrument while cranking. You should see the needle wavering up and down as the points open and close, averaging around 6v. If it stays at 12v the connection through the points and distributor ground is open circuit. If it stays at 0v then either the coil is open circuit of the points are shorted out for some reason. You can tell which by cranking with the meter connected between the coil -ve and the purple at the fusebox ... always assuming the interior lights still work when this problem occurs. If the meter stays at 12v during cranking then the points are shorted, if it wavers again the coil is open circuit.
Paul Hunt

I remember the tach dropping dead but didn't note the ignition light. The car has behaved perfectly all day now. Even so, I have just been through the wiring and found a couple of suspects but no eurekas. The first was the ground from the lumenition optical pack in the dizzy that had a wobbly contact going into a connector block. The second more obvious problem was that the earth terminal clamp on the battery was undone and rattly, presumably an oversight post the welding work. I can't see that this would give the symptoms but it certainly needed putting right. The coil values were good (and interestingly identical down to the last decimal place on my fitted expensive Lucas Sports coil and spare old Unipart item) so fingers crossed.

Many thanks to all once again.
Steve Postins

Ah yes, forgot the Lumenition. First rule of diagnosis with those (or any electronic ignition) is retro-fit points and condenser and then see what happens :o)
Paul Hunt

...and that will be what I'm going to do now. It got 5 yards in the direction of the MoT retest appointment before doing it again. The coil had 12v on the +ve, but my timing light showed no spark on the coil to dizzy HT lead. Measuring -ve vs earth on cranking showed 1.5v constant, but it did this on the times it started too so I guess this may be a function of the Lumenition or my meter. Now where did I put that old baseplate?
Steve Postins

Okay, I'm back to points and bypassed the black/white wire in the process and as far as I can see (the problem comes and goes so quickly) the coil -ve shows 10v (better meter) during cranking when the problem is there but 6v when the car starts so I seem to be looking at the points and distributor ground going open circuit. I understand the theory but could you help me with where to look in practice? As the problem has repeated with Lumenition and points I can't think where to thump my hammer next.

Thanks

PS Parts arribved safely thanks Chris
Steve Postins

Steve,

The dizzy moving plate earths through the engine block via a thin flexible wire inside the dizzy.

I assume you have fitted this wire and it is intact? One end goes on the capacitor screw ( if memory serves me right) and the other goes under the head of one of the screws holding the fixed plate to the dizzy body.

If it is missing, broken or has a bad contact the earth circuit will go intermitantly open depending on how the moving plate rests on the fixed one, and a possible route to earth thrugh the advance unit spring.
Chris at Octarine Services

10v when cranking is full battery voltage so implies the points circuit is open-circuit. As well as the distributor ground wire, which is a very flexible braided tinsel wire to cope with the flexing caused by the movement of the points plate under vacuum, the points wire from the coil can go open-cicuit inside the insulation, and this is *not* as flexible a wire as the ground wire even though it gets the same amount of flexing :o)

Unless you connect a test wire to the points mounting screw (under the top insulating washer) and run it outside the distributor it is going to be difficult to diagnose any further. If this wire shows 10v when it won't start then the problem is the points or the ground wire. If this wire drops to 0v when it won't start it is the coil to points wire. But that is the white/black that you have already bypassed?
Paul Hunt

Sorry, I think my measurement of things going wrong after changing back to points was mistaken, and the 10v must have been recorded with the Lumenition. I changed to and fro several times whilst getting an old base plate sorted, and suspect I got mixed up with what was in there the last time it wouldn't fire. The car has certainly started fine all weekend with points. Thanks again.
Steve Postins

This thread was discussed between 12/05/2004 and 16/05/2004

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