Ignition problems for a newcomer please help!

Hi folk, I am hoping for some help and guidance.

I have just bought (from online auction) a 1975 chrome bumper MG Roadster, in need of some restoration. It is meant to have had an engine overhaul by MCOC in 2001 / 2002 but hasnt been used much since I got it. I was surprised to find the car was starting and appeared to be running well after just replacing the plugs. This however, wasnt the case.

On several occasions in the last week the car has broken down, the car becomes unresponsive to the throttle, and may even try to misfire / backfire, before stalling and dying completely, normally at an inconvenient point.

I appreciate that I have one of the last chrome bumper cars, I have tested voltage supplies to the coil, and I appear to have 6.5 volts supplying the coil which would suggest I have one of the resistor versions I have read about.

I have a new set of plugs in the car (yes they will be black as the fuel is running rich) and the distributor is said to be recent too. I have changed the HT leads. The only thing that remains "old" is the coil.

Can the coil cause this intermittant sort of problem? I drove 15 miles this morning, running well, then all of a sudden the car spluttered and ground to a halt. Waited ten minutes after riggling wires under the bonnet and checking the power supply to the fuel pump and it went, for another few miles, then the same happened again.

Yesterday this happened from cold. I started the car and drove her off the driveway and parked her up on the road. All of a sudden as I was locking up the garage the engine spluttered, and died. I couldnt get her going again at all for most of yesterday.

I have had the fuel line off, there is a supply (all over my driveway now!) and there is mains to the coil (6V)

Any ideas or suggestions please????

Dose it have a electronic ign.

If not could be a condenser, or a coil, or it could even be the ballast res..

when it dies dose it have the correct volt going to the coil? have you cked the ohms at the coil after it dies?

Anyway i am sure somebody else with more info. will chime in.

Dwight, you may also be encountering an issue with a wire that is frayed or corroded to the point of making intermittent connection. May I suggest you thoroughly inspect the harness for any signs of wire fatigue. Remove the wires from the terminals one at a time and give the connectot a tug away from the wire to see if it is in fact a solid connection. Pull the wire off of the connector using the connector itself and do not just yank the wire. While you are at it, clean those connections with a fine emery paper and replace the connectors as you do each one. A spot of dielectric grease after you are confident that the connection is good will reap many rewards in the future. The problems that were mentioned in the previous posts may also be present, but by checking the wiring harness first will eliminate a most likely cause of breakdown.

What distributor does it have - is it a replacement or a rebuilt original? Points? Opus? CEI? Start with the normal routine maintenance stuff like checking and gapping the points. Check the igniton timing, etc...

When it dies, is the fuel pump still in operation? Does it happen when you wiggle the igniton key? Is there power at the coil when it dies?

If you have an electronic igniton conversion, you need to rewire the coil to get 12V!

Its a 25D distrib, with points and condenser, not that I know much about these either. Ive had the cap off today for the first time, and didnt really know what to look for.

Its been hard to check the voltages to the coil as it has been intermittant, and when it has happened, ive normally been in awkward places. Other than when it happened on the drive.

Is it likely that a coil or condenser would fail in such circumstances, in an intermittant way?

I did have a lamp test screwdriver with me today when I broke down in the middle of newcastle and there were voltages showing on both sides of the coil (not the HT output but the two side terminals) so I guessed that voltage to the coil was present. I tried to see if i could see a spark, but in daylight, and the fact the HT leads have a good inch of insulation beyond the metal inner, prevented me from seeing a spark.

I can only describe it as a loss of power, very quick stall, or misfiring, normally resulting in me pulling over in an emergency manner.

I have basic electronic knowledge, and just wanted to know if there are any tell tale signs.

SORRY FOR CAPS, BUT WANT TO EXPRESS THIS POINT. I AM UNSURE HOW TO MEASURE THE RESISTANCE ACROSS THE COIL, CAN SOMEONE GUIDE ME STEP BY STEP? I HAVE A DIGITAL VOLTMETER, WHICH HAS A RESISTANCE MEASURING FACILITY. WHAT SHOULD I CONNECT IT TO, AND WHAT READINGS SHOULD I LOOK FOR, A STEP BY STEP CHECK WOULD BE GOOD PLEASE!

Although the terminals supplying the coil are corroded, i believe the rest of the wiring isnt too bad. I have noted that the cables are wrapped in electricians tape and Ive not seen any sign of the ballast resistor, yet... not too sure where that will be found..

I did get a wiring diagram of the ignition system, but being new to the MG and world of classic cars, I was a touch lost off especially with the way the tachometer works, any further guidance would be appreciated!

Ray Burke

The problem doesnt seem to be related to the barrel, or the key, as the red light comes on, and stays on until i turn the ignition off, so I am guessing its not a key problem. There seems to have always been a 6V supply to one terminal of the coil. But that is as far as my diagnostics go so far.. apart from knowing the distributor looks new and in reasonable condition, and the plugs are all new, and the HT leads have been replaced.

Sometimes it has occured from cold. I have tanked up with fuel today, its not fuel starvation.

Distributor, is the 25D - having taken the cap off. Points and condenser, not the electronic type.

I would have started with the gap and points stuff, but surely if that was the problem, it would be permanent and not intermittent - as my fault seems to rectify itself by doing very little, and it comes back sometimes an hour later, sometimes 30 seconds later!

Its random, frustrating, and GRRRRR!

If anyone can detail what sort of problems a faulty coil and condenser can cause (symtoms) I would be grateful.

Is there a tester for HT spark? To measure spark strength etc?

Hello,

Welcome to the world of MG's. When I got started a few years ago, I had the car tuned by a noted professional so that as I learned more about the car, I was at least starting with a clean slate. A real wizz (not me!) can often sort these things out in a few hours work. As I have learned more, I have dug deeper into the car. The internet is great in that you can find that one person in your area who has decades of knowledge in tuning the car and is highly recomended. DPO's (Dreaded Previous Owners) find all sorts of sneaky ways to mess up the carb and dizzy settings.

In case all else fails...

Clean every terminal. No corrosion is acceptible of you want this problem to continue! You should probably install new points and condenser - as a corroded set of points or a flaky condenser can cause these problems. Corrosion on the points contacts and connecters can also cause these problems! Also remove the points plate and clean the areas where the screws hold it down - top and bottom - so you have a good ground! make sure the terminals inside the cap and at each spark plug wire end are clean, including the coil wire on both ends!

To measure OHm on your coil, disconnect the distributor wire from the coil. Red wire from the tester goes to +, black wire goes to -. Have it set on the Omega (Ohms) setting at the lowest number. You should have 3 Ohms. The later style are 1.2 Ohms, and that would be fine with points too. You probably don't want to invest in a tester to check power output - they're not cheap! You can also test the Ohms from the spark plug end of each wire baack to the inside of the distributor cap to make sure you don't have a short. Readings can vary from 0-10,000 Ohms depending on what wires you have. If one wire is 30% longer than another, expect a 30% higher reading on the meter.

Another way to test the spark to to take a WELL INSULATED screwdriver and short each plug to ground while the engine is running. Everytime you short a good plug/lead combination to ground the engine will stumble and run rough. With a bad one you will nopt see any difference. BTW DO NOT use a screwdriver with a shaft that goes all the way thru. Made that mistake in the dark on night.

B-Racer two things on your suggestion - firstly a lot of meters have the leads reversed when in Resistance ranges, that is the black lead is actually positive. Secondly as you are only dong a resistance measurement, hence no power applied, it really should not matter which lead goes to which terminal on the coil - coils are not really polarised.

FWIW----A '75 chrome bumper???? last of the chrome bumpers???? lemmee ck my calendar.
I certainly appreciate all the electrical jargon going on but seems to me from reading initial problems, could also be bad gas/fuel supply mixing into the other problem. When it happens again, shut it off immediately and check fuel filter to see if its full. Does the fuel smell like varnish? Turn on the ign. and let it pump 2 qts into a milk jug or suitable container (I like those litre bottles w/little feet on the bottom) Is the flow continuous and strong? Let it settle and check for water/debris at bottom.

If it always dies after the same amount of time then it is probably heat related.. When the coil, resistor, condenser, etc gets up to operating temp one of them is breaking down.. (breaking down, losing continuity). I assume the fact that you have a 1975 chrome bumper is due to home model configurations..
Gary

It is very hard to properly test an ignition coil. More precisely, quite a number of simple tests can assure you that the coil is bad, but there are no easy tests that will guarantee it is good.

Coils have thousands of windings of very fine wires coated with very thin insulation. Sometimes the wires develop cracks that only separate during heat and/or vibration. In other cases, the insulation begins to break down and will suddenly arc out. Coils are cheap enough that the easiest test is to substitute a new one - if that wasn't the problem, you haven't lost much.

If the car dies instantly, it is probably ignition. If it sputters a bit while dying, it is more likely to be fuel. In particular the fuel pump may be intermittent.

Tell us about your B. Does it say 75 on the tag on the door jamb? If it does it can only be 2 things. Its an original chrome bumper car with Vin changes or its a Rubber bumper car that was converted to chrome. "Inquiring minds want to know". I know the answer to this question won't help your ignition problem. Sorry.

<<<<< '75 MGB nearly original through and through, right down to no vacuum line on the distributor.

Years ago (1975) I had a similar problem with a 74B - it would run fine for a while, and then die on the side of the road. Sometimes it would crank again, sometimes wouldn't. I didn't know much about cars, so I didn't have any idea what the problem was. It turned out that the dealership that was servicing it, that also sold GM cars, used GM points in the distributor instead of Lucas. When I took it to a British car mechanic, he used Lucas points, and everything was fine from then on.

All this to say that since you don't know what's been done in the past, why not renew all the renewable parts of the distributor, including the plug wires, before you go too far? It's not expensive, and at least you'll know that its NOT that stuff.

Start at the points. Use a matchbook cover to gap them, change out the condensrer/capacitor in there. Make sure the little wire from the points is not getting grounded in any way. That wire chafes over the years and can touch the metal distributor housing. Re-cover it with Shrink Sleeve if necessary. The points and condensers of MGs have been the most single point failure item in the past 30 years of production. When you stop to consider the engine idling at 1,000 rpms, those points are opening and closing 4,000 times per minute. There is a constant surge (avalanche) of electrical power cycling through those points and they have a tendancy to build tiny cones of conductive material between them. In the old days, most MGers carried around a matchbook cover and a small flat Swiss file for such contingencies. 99% of the time when a coil quits, that's it. It won't come back to life, not even cleaning it up and painting it will do. Invest in an electronic ignition system and you will never have this type of problem again. BTW, usually the ballast resister is located inside the base of the coil can itself, usually, on some coils. Change out your fuel filter as well so that you give yourself a good baseline to troubleshoot your problem.

No one else has asked yet, but if it is a '75 I would want to ask, do you have a single Zenith Stromberg Carburetor, Dual SU's, or a Weber conversion...?

These symptoms sound a lot like a Zenith Stromberg carb. with a torn diaphragm.

If you have the Zenith, simply pull the 4 screws out of the top of the carb, and lift the top STRAIGHT off, inside you will find a long spring (don't worry it's not a very strong one) and a big black rubber diaphragm. IF that diaphragm is torn or has holes in it, replace it that will probably take care of your problem. There is a tab on the bottom side of the diaphragm that needs to fit in an indentation in the carb body, make sure that lines up when you put it back together, and be sure that everything you take out is lifted STRAIGHT up. There is a big needle in there you don't want to bend.

I am beginning to question my sanity!

I have replaced all the HT leads. I have replaced the coil, and the connectors to it. I have replaced the condenser & rotor arm, and still have the same problem, only this time its happening more, and its getting more frequent (ill consider that a benefit for the moment).

I think, i have a spark (but no easy way to check is what I am being told). The plugs were black, in terms of mixture. That I already knew.

The car sometimes starts, sometimes fine, sometimes rough. Fuel remains in the fuel filter, although Ive not taken the fuel line to bits again, ill assume there is fuel.

I bought some points but they simply dont fit onto the pivot in the 25d Distributor. So I have reverted back to the old points for now!

I am suspecting it may not be electrical afterall, I have twin SU carbs fitted, currently running without the air filters.

Any further advice as to what to look for? There is voltage to the coil all the time it would appear, 6volts, seems constant. I am struggling for what to look for.

Change the dizzy cap and rotor. Dizzy cap is cracked or corroded

May have found my problem (as mentioned in a new posting i have had to create!)

Fuel line was dribbling when the car just stalled on my driveway!

So I went to the fuel pump, and tested the supply (+) and its fine. Pump failure i thought... Intermittent pump failure!

No, that wasnt it quite either, it appears the earthing of my fuel pump is very very poor! I dont have a clue how the fuel pump earths to ground, but I am about to find out as that is what is causing my problem! So ill put the bonnet down, and open the boot!

A big thank you for all the input so far, my car has had an ignition overhaul almost, and will soon be running fine, if anyone can help me to put the fuel pump right please drop a message, how they earth, and how to fix. I doubt the pump is faulty, giving it a good shake seemed to make it click and make contact again!

Many, many thanks!