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MG MGB Technical - Could my ignition coil be the problem?

My appologies to those that I've bored with this continuing problem before, but I'm still at wits ends.

My 72B (acquired May 06) had an occassional hiccup when driving. It started fine, drove fine, but for no reason and under no particular driving situation it would just jerk or hiccup. Not a repeated jerk or hiccup, just once but again shortly later and very random.

In '06 I found the 2 wires coming from coil - through tach were melted together under the steering wheel and below the glove box. I replaced the 2 from end to end and they are still fine. I was quite surprised car even ran.

Later I replaced points/cond with Ignitor system.

Eventually I installed an in-line fuel filter and cleaned out the fuel bowls, etc. And I've run various carb cleaners over the 2 years.

I did a valve job in '07.

All with the hiccup or jerk still showing up at random.

Someone on BB suggested ground in distributor, so I opend cap found wire and cleaned contacts by scraping and sanding and reattaching to frame of distributor.

Another symptom or feature of the B is that when it is quite hot out and I've been running car hot, if I shut it off it will not restart until it has cooled for 10 minutes or so with hood up. I've assumed this to be vapor lock of fuel system. (My fuel pump is still up on the firewall, I do plan to move back by battery box this year.) Just remember this for now!

But back to the jerk or hiccup. It is very harsh and abrupt. It is only 1 big jerk or hiccup that actually shakes the car, once. It is not a nice smooth, "oh I'm running out of gas" feel. It jerks the car. It can take the slack out the rearend, mine and the car's. Then its gone, only to happen again later.

I cannot prove but to me it is more likely to happen in OD while just crusing along.

I really have a hard time thinking its fuel.

Recently I started it up and drove off all fine, until 15 minutes later, all warmed up it does the hiccup and jerk.

Now I'm wondering - could this be the coil and could it also be the problem when it is hot out (90F) and car is running hot - that I'm attributing to vapor lock?

I'd spring for a new coil, but I'm confused as to which one. Mine is silver is mounted pointed down, there are the wires coming from the tach and the ones from the distributor. I do not know where to look to see if there is an external resistor that the parts books say to check for. If I would get a coil, how do I determine if its a resistor or non-resistor type?

Anyone think the jerking and not starting when hot are related and could be the coil?

Anyone have any other suggestions for what could cause such a harsh jerk, one at a time?

Thanks and my appologies for my usual long inquires.

R.W Anderson

When it does it's hop or jerk does the tachometer jerk with it? This might indicate what part of ignition system is causing the hiccup.
Sandy Sanders

RW. Your car uses a 12V coil designed to run without an external ballast resistor or resistor wire.

Normal failure mode for a coil is that it quits working. I have seen one which failed when it was hot. That one showed that the clamp holding the coil had been over tightened and the deformation of the walls of the coil was visible when examined.

None of them produced the form of "hiccup" that you describe.

You are not having an "occasional miss" problem which might be traced to a single high tension lead. The way you describe the problem, you are having a short of the coil lead or the low tension ignition system is shorting out for a short period of time, then functioning correctly again. Have you cleaned and examined the body of the distributor for signs of water/coolant? The heater valve is above it and drops of water have been known to leak onto the distributor and cause a problem similar to what you describe.

Hot starting problem is, most likely, either the location of the fuel pump or a starter which is dragging as it gets hot. If the engine turns over as quickly hot as it does when it is cold, assume the fuel pump would be the first thing to check. You can put a "T" fitting into the fuel line and attach a fuel pressure gauge. If, when the engine is hot, the fuel pressure reads lower than when the engine is cold, you have a fuel delivery problem.

Les Bengtson

I would give the coil a shot first. Surely someone you know has one laying around to sub in place of yours. Coils don't need to be grounded, so you can attach it any way you want just to try it out. I've held them temporarily in place with zip ties before. Heck, even duct tape will work. With the pertronix system you'll need two connections on the (+) terminal and one on the (-) terminal. You might have to make up some pigtails for a test coil, as US coils tend to have a stud and nut for terminals.

As far as a ballast or non-ballast coil goes: for testing it makes no difference. Ballast resistors or resistance wires were used to cut voltage to the coil while the key is in RUN. The coil got full battery voltage for starting. If you forget and leave a ballast coil in there forever, the coil life will be greatly reduced, but will work.

The 10 minute cool-off leads me right to the coil. When you turn the engine off the temp under the hood rises. If the coil has a hot open, the windings will electrically open until it cools off.

When you buy a replacement just tell the parts guy you want a coil for a non-ballast system.

I will add that when I opened the distributor cap, it looked like it rained inside the cap, not just a build up of condensation.

I dried the cap and at the same time noticed the heater valve was dripping and the route for drips lined right up with the tap/insert for the cap; providing a potential straight entry point for moisture.

I've since acquired a rubber boot from another car that goes over the cap. This will keep heater valve or other moisture from entering cap.

ALSO, I haven't noticed that tach drops when it hiccups or jerks. If it does it is for a split second just like the hiccup/jerk. I watch more closely. But I think I watched for this last year with limited sucess in seeing anything.
R.W Anderson

A single jerk like that doesn't sound like coil to me, when I had that the ignition would die for a few seconds, but the length was random. The tach would also suddenly drop to nothing until it picked up again, even though the motion of the car was still spinning the engine. A faulty ground wire in the distributor, or faulty ignitor/points, or any connection in the ignition LT circuit would affect the tach in the same way. If this doesn't happen to your tach then it isn't ignition LT, but could be ignition HT or fuel. If it were ignition HT then I'd expect a backfire in the exhaust when it picked up again, as it ignited the unburnt fuel in the exhaust from prior non-firing cycles, sometimes at least. I also wouldn't expect condensation in the cap to cause just one sudden but brief occasional loss of HT, but the cause of any condensation does need to be removed. If it's not that, then it only leaves fuel. And again that is not my experience, loss of ignition and fuel has only resulted in loss of power, no jerk big enough to shake the car. Is perhaps a rear brake grabbing? I've had that, albeit when reversing, and that *does* give a heavy shock.

Having the pump in the engine compartment rather than by the tank could cause problems with vapourisation between tank and pump. As this seems to be easily reproducible I'd tackle that first. When it won't start clip a timing light onto the coil lead and each plug lead in turn, and watch the flashes while cranking.

No flashing on the coil lead means no HT, which means bad coil (these can fail with the higher temperatures of heat-soak but work OK when running) or ignition LT. In that case connect a voltmeter or dwell meter to the coil -ve (points/ignitor connection) and you should see 12v switching on and off as you crank.

If the coil lead flashes but one or more plug leads don't then the cap (some leads) or rotor (all leads) is breaking down. If you have regular and consistent flashing on all leads - bearing in mind the coil lead will flash four-times faster than any plug lead, then when on No.1 lead point the timing light at the crank pulley and check the timing is about right. If that's all OK it must be fuel. Remove a fuel delivery pipe from a carb and direct it into a container and switch on the ignition. Be careful, fuel flying about and a hot engine don't make happy bed-fellows. If you have fuel delivery, then if you have HS carbs it could be a bad heat-shield allowing vapourisation in the jet pipes. HIFs should suffer from this.
Paul Hunt2

Another faint possibility to explain the jerk is the overdrive dropping out and back in. This gives a similar symptom to the "hiccup" described. I've had this occasionally - tracing it is on my "to-do" list.
David Overington

Just today on my ride home I experienced the hiccup/ jerk R.W. is talking about. It lasted for approximately a mile, was intermittent and seemed to be worse while I was in fourth gear and not the lower gears,( i started to pull over but saw someone too close behind). I did notice that my tach did not drop and it did not feel like fuel. I was very near a hard right hand turn at the time, took the turn and did not experience any more problems all day. That got me thinking ground wire some place and I will pop the hood and investigate throughly on the morrow.
Peter Murray

A sudden jerk that shakes the whole car sounds more like an ignition switch that's cutting out to me. Definitely not the coil. Then again, I'd suggest fixing the heater valve! If your distributor has been filling up with coolant, I guarantee the advance mechanism is sticking and not functioning propely, so your distributor's mechanical advance could be the root of your entire problem. Drive that car over to my shop and we can fix that!!! Its supposed to be nice out Sunday!
Jeff Schlemmer

I'll look into driving the car over in the near future, once I get my brake and clutch master cylinders back and into my car later this week. I keep thinking there is something electrical that is causing this problem, but cannot find it. I agree that if things are gummed up in the advance that this could be the problem.

I think I described this problem last year on web, and it is slowly coming back to me that there was some connection to deaccelerating. If I'm cruising along under steady speed in 4th and in OD the jerk sensation was more likely after deaccelerating. I'd shift out of OD and jerk incidence would decrease.

Hope to see you soon, an extra set of eyes is always good.

R.W. Anderson

FWIW any fault that stops ignition HT will produce exactly the same symptoms, if the ignition switch were cutting out that will stop the sparks, just as a failed coil, cap, rotor etc. will. But when it's the ignition switch the rev counter will instantly drop to zero, and the ignition warning light will come on while the momentum of the car is still spinning the engine. Neither is it a ground, the only one is in the distributor, and again it will cause the tach to instantly drop, but this time the ignition warning light will remain out.

If it's the OD then you will see the tach jump up a bit if it only happens in OD, or down a bit if it only happens out of OD. If it happens both in and out of OD then I'd say it was unlikely to be the OD wiring, but could be a major internal mechanical/hydraulic problem.
Paul Hunt2

An intermittant condensor on my V8 caused a similar fault. As I say, similar, my 'hicup' had a greater frequency - but it was like the engine just shutting down completely. Maybe your condensor isn't failing and just suffers from a bad earth etc.
C E Muir

R W,

The first thing you have to do is replace that heater control valve. This problem is legendary in MGBs. What kind of a morbid engineer would would not only design such a failure-prone valve, but then diabolically place it directly above the distributor. It's gotta be the same guy who placed the leaky ZS carburetor directly above the red-hot catalytic convertor on the late model cars.

Even with the boot you found to cover your distributor cap, dripping coolant can raise havoc with the spark wires. Replacing the $25 valve is a 20-minute job. Just get it done and permanently rid yourself of either THE cause, or a contributing one that cannot be evaluated.

Having said all this, there is a soft black ground wire inside the distributor that, for lack of a better term, is "riveted" to the breaker plate. The only time I've had a problem like you describe, it was this wire that had broken free from the breaker plate. The plate is, to some extent, otherwise grounded so the engine will run, but it is a very intermittent ground, causing the symptoms you describe as well as a good deal of backfiring.

Allen Bachelder

Once I get the clutch and brake master cylinders back in the car and I'm road worthy again, I'll be taking advantage of one of the above offers to bring my 72B by for a check for a gummed up advance system inside distributor.

I've checked that black wire inside the distributor, and cleaned all the contact areas where it is attached by a screw to the outer surface of distributor. This did not make a difference.

Of note is that I do not have backfires when hiccup/jerk happens or at other times. I've not been able to watch for ignition light or tach drop at every occurance, but knowing I should watch has me ready to notice it if it happens, but I haven't seen tach drop off or ignition light come on yet. The hiccup/jerk is only for a second. But it may only be a minute before it repeats.

And yes, I'll change the heater control valve to elminate the drip on the distributor.

I need to first win the lottery so I can retire, and have the time and money to do all the repairs. I still don't have horns or wipers either. Although if I won the lottery, I'd probably take a vacation while someone else does all the repairs. I'd try to keep my loss of personal satisfaction at doing all the repairs myself under control while sailing in the BVI.

thanks for all the guidance so far.
R.W Anderson

RW. I've sailed the BVI. Good plan. But until you get there I would focus on whether you are in OD or not when it hicups. Also accellerating or decellerating. If it is an OD issue, you will not see any of the other symptoms mentioned. Could be an intermittant wire or marginal OD switch. I would guess that with most of the other possibilities there would be more than one hicup at a time.

Just my guess,
Mark R.
Mark R

Hi RW, I don't think anyone has suggested changing the rotor. I had basically the same problem last year and it turned out to be a rotor that was grounding out every now and then. My "B' would stall and then start up after sitting for a few minutes. Moss now sells a heavy duty rotor and I have one on backorder.
Good Luck, Doug
Doug Madson

"Moss now sells a heavy duty rotor"

You mean a *proper* rotor, the other ones they are selling being crap.
Paul Hunt

This thread was discussed between 02/05/2008 and 23/05/2008

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