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MG MGB Technical - She just won't start !

Hi, Please help..

Thought I'd start the MGB today (she's a 1979)after clearing the driveway with the snow blower.
She started first time, so I thought that I'd let her run for 20 or 30 minutes.

After about 20 minutes she started to run rough and she finally quit. She cranks over but won't start. I checked if a spark was going to the plugs. No spark but found that the plugs were black. Then I checked the points, still no spark, then the HT wire from the coil to the distributor, no spark.
The distributor was replaced two years ago, so I think it should be O.K. Any ideas please.
Thanks, Harry.
Harry Scanlan

Harry. From your comments, I am assuming that "no spark at the points" equals a points type distributor. If so, it sounds like you have a problem in the low tension ignition circuit. Go to my website, www.custompistols.com and click on the MG section, then, on the tech articles. There is one on how to trouble shoot the LT circuit to find out where your problem is. I would also replace your spark plugs while you are at it as fouled plugs (oil or soot from running rich) are harder to fire than clean plugs. Les
Les Bengtson

Thanks Les. I'll check out your page.
Harry.
Harry Scanlan

<<<snip>>>
After about 20 minutes she started to run rough and she finally quit.
<<<Snip>>>

Out of gas? Check to make sure there's fuel in the carb.
Blake

Harry

rough running preceding stopping suggests fuel rather than ignition even though there is apparently no spark

gradual enrichening of mixture , particularly if the motor is left idling, suggests leakage past the needle valves, so that the float level gradually builds up and chokes the motor with a rich mixture cut

try spinning the motor over with full throttle and no choke

also check that this was not the distributor clamp loose allowing rotation and losing the timing!

Chris
chris

MY 80 did the same thing...except I was on the road. Solution: bad distributor rotor. Was cracked, damp day, shorted out.
R. L Carleen

Harry. Also be aware that, if your car still has the stock coil and wiring system, you have two different current imputs to the coil. One from the starter, at full voltage, and one through the ignition switch in the "run" position. It is possible to have no spark when the key activates the starter, but to have good current with the key in the run position. If you have no current input in either position, begin checking for input to the key switch. All of the other comments are valid, but, without more input from you, it is hard to focus on an exact problem. For instance, the ignition switch, when in the run position, powers both the coil and the fuel pump. Hence, a problem in the white wire system will give indications of both an electrical and fuel related problem. However, in this case, the classic test is that the car will start when the key is turned to start and die when the key is returned to run. Let us know what your tests reveal and we can focus in on the problem. Les
Les Bengtson

Blake, I removed the fuel filter and found gas to be present. I blew it out in both directions and it seems o.k.

Chris, I'll try tonight with full throttle.

R.L. Carleen, I'll check the rotor.

Les, The wiring is stock. However, I replaced the coil 2 years ago. The pump "clicks" when the key is turned on as normal.
I have checked your page. Excellent info, thankyou. I'll take a meter home tonight and do a few tests as you suggest.

Thank you all for your response...

Harry.

Harry Scanlan

My mga ran great for 2 years. Then I bought a brand spanking new "sport coil". Within a month, the car stopped in the middle of an outing. I thought I was out of gas. Long story short - it turned out to be the "new" coil.

I put back the old, used, dirty coil and am still running on it. I've noticed more threads on instant breakdowns due to coils than any other single problem.

fritz
Fred Horstmeyer

I would strongly suggest that it is the coil. Mine packed up a few weeks back with similar symptoms. The easy way to check is to remove the coil lead from the distibutor a either place the end a quarter of an inch away from a good clean earth point (head bolts or similar, or fit a spark plug to the lead and ensure that the plug is earthed. The take the wire that goes from the coil to the distributor and flick it across an earth (make sure that you have the right lead) and make sure that there is no fuel around or smells of fuel first.

If the coil is OK and you have 12v to the feed side of the coil then you will get a reasonable spark, no spark either means that your coil is duff or the 12v side is not getting any current.

Steve Birchall
S C Birchall

Just a point about the rubber-bumper voltage feeds to the coil. It does have two - one from the switch and one from the solenoid, but if the one from the switch is missing the engine will still start using the one from the solenoid but cut out as soon as the key is returned to the run position.

If Harry doesn't have turn signals, gauges, heater fan etc either then the problem is more likely to be the ignition relay, but a failure there with the engine running is more likely to cause a sudden cut-out than a fade and die, but the latter is not impossible with an electrical fault.
Paul Hunt

If it was idling without the choke being adjusted for 20 minutes(?), then it is surely bound to run richer and richer until possible damage could be done to the rings and the oil because of unburnt fuel accumulating in the cylinders and running down into the sump? Engines don't like to be left idling for long periods, esp. diesels.

Given the circumstances, I realise this would be difficult, but I thought modern recommendations were to warm up the car by starting and driving off as soon as possible and until the engine warms up properly drive it as if it was being run in - no over revving or loading in the wrong gear?
Martin

Fritz, all of the above ideas are excellent. However, since I'm no mechanic, I'm going to try another new coil just for the heck of it.

Steve, I've tried checking the coil with this method and still no spark.

Paul, heater and gauges are o.k.

Martin, I'm sure it wasn't such a good idea letting her idle. However, when you have 2 feet of snow in the driveway, it so tempting to start up the "B" and stare under the bonnet for a while. It's a 79 with a Zenith carb / automatic choke.

Gee, this is so much different from when I was a teenager in Glasgow, Scotland. In 1967, my dad bought me a 62 MG midget, twin SU's etc. soon followed by a Spitfire and finally a TR4 IRS. (Then I was daft enough to get married) I'd never heard of pollution air pumps, carbon cannisters and automatic chokes. After being in Canada for 25 years. I thought that I treat myself to an MGB (middle age crisis etc).

I wish my MG would start !!

Thanks for all the info..


Harry Scanlan

Harry. New coil or old coil, if you do not have power into the coil, when cranking and when the ignition switch is in the run position, it will not matter which you use. Trouble shoot this in a logical manner using a test light or volt meter to follow the current through the system. If you have power into the coil, do you have power out of the coil? Do you have power to the points? Across the points? What is the voltage drop across the points? Should be less than one volt or the points are bad. Is the ground wire for the base plate present and intact? "No spark at points" indicates that the LT circuit is open somewhere. Begin with that, then progress onwards. Les
Les Bengtson

I'm with Les - diagnosis before replacement. As well as the cost of replacing bits willy-nilly when they aren't faulty, and the frustration of replacing things and it still doesn't go, if the problem is a bad connection you could disturb it while changing a component and think you have fixed the fauilt, only for it to reappear again some dark wet night in a dodgy area of town whereupon you will probably start blaming Joe Lucas, Abingdon, some hapless PO, etc.

Diagnosis is very easy. You should have 6v at the coil +ve and near 0v at the -ve with the points closed. With the points open you should have 12v both sides of the coil, simple as that. That is with a rubber bumper, on a chrome bumper you should have 12v on the coil +ve both times, otherwise the same.
Paul Hunt

No points: its a 79.

Perhaps the inductive pickup has packed it in. They can quickly fail without warning. As several have mentioned make sure you have power to the coil. Then create an imaginary set of points. Turn the engine to TDC on #1. Pull #1 plug wire and attach it to a spark plug and ground the plug. Use a short piece of wire between the minus terminal of the coil and ground. Each time you pull the wire from ground, the plug should fire. A light scratching of the jumper wire across a good ground should creat multiple fires. If this test works, the inductive trigger in the distributor or the wiring to the trigger in distributor is bad.
G'day

Blake

Blake. The original posting states, "no spark at the points". I have, twice, tried to get Harry to confirm he is running a points type dizzy, but he seems to keep missing this. A points type ignition system is very easy to trouble shoot when there are problems and it is fully possible that Harry's car has been retrofitted with one. My 79 has. Les
Les Bengtson

Les, I plan to try some of your suggestions on the weekend. My car definately has points, I replaced them 2 years ago along with a new cap and rotor.

Paul, I will definately check the voltage.

Blake, I will check power to the coil.

Thank you all.

Harry

Harry Scanlan

This thread was discussed between 06/01/2003 and 09/01/2003

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