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MG MGB Technical - Help! How do I fix this?

Hello,

My car is a 1973 MGB. The engine, etc, was professionally rebuilt about three years ago. The car has always run perfect since then and I have never had a moments trouble and it has always started on the first crank. Until today....

This morning I went out and my car started perfect. I drove it about 25 miles on the expressway and it ran fine. However, after I got off the expressway, when I would stop at a light or stop sign and then start off, the engine would hesitate till it got to about 2000 rpm. Once going, the car ran fine. When I got to where I was going I parked the car for about an hour. When I started back home, the hesitation was gone.

On my way home, the car was running fine. However, on the way home, in traffic, I stopped at light when suddenly the engine just died. It was idling fine when it just stopped and would not restart. I had to push the car to the side of the road. I tried starting it several times and the engine would turn over but there was no indication it was trying to start. Then after letting it sit for another few minutes, I tried again and it started fine and ran fine. Therefore, I continued on home. However, about four miles later, at another light, the car did the same thing, but it would not restart this time. The road was a hill, so I let the car roll and popped the clutch and it started. It was hesitating , but once it got going it ran fine.

Then about a mile later, I was turning right at a light (I was keeping the engine revved up and the car moving) and it just died again. The engine turned over fine, but would not start or even give any indication of starting. Then after letting it sit for about three minutes, it just started right up. Luckily, I was near home and made it back.

When I first got home, I shut the car off and then tried to restart it. It acted like it was not going to start then it suddenly started. Now it has been about thirty minutes and I just went out and it started fine with no problem.

So, what is this problem? Where do I start looking? What is causing this? The car has about 3/4 of a tank of gas. The temperature is in the mid-90's. It is not raining. The gas cap is fairly new, and I took it off and checked it to be sure there was not a vapor lock.

I am not sure where to even start looking since it will now start. Suggestions?????

Thanks,

Robert
Robert Browning

My first check and adjust would be the contact breakers Robert. Has the car been serviced lately? Also listen carefully at switch-on for the petrol pump ticking as it should. First guess at a failing item with these symptoms would be the coil. Rich.

Rich

Rich,
Yes, I get the fuel pump ticking noise.

As for recent service, it only has been about 3,000 miles since the last service.

I am not sure what you are referring to when you say contact breakers? Do you mean the points?

As for the coil, I was wondering about this, but I was always under the impression that coils either worked or they didn't work - am I wrong? (Probably!!)

Thanks,

Robert
Robert Browning

Robert,
It's the fuel spark issue. A quick check of the ignition, when the car died did the tach just drop or slowly fall off. If it dropped quickley then you have low tension issue. Fuel pump clicking normally is an other indicator. No clicking no gas. A coil would cause this as well and easy to substitute. Rotor cap points etc. Basically verify the fuel and spark. I always like to recheck anything recently repaired or replaced.

HTH

Ron
Ron Smith

Ron,
As for the tach falling off, the only thing I can remember is that it fell off just like if I had turned the key off.

What do you mean by a "low tension" issue?

Also, as I asked above, will a coil work and then not work and then work again, or do they just quit all at once?

I will check the rotor and cap and points and see what they look like.

Thanks,

Robert
Robert Browning

Look for a broken wire on the 12 volt (low tension) circuit to the coil and from the coil to the dizzy - also check the thin flexible earth wire inside the dizzy - between the moving plate and the case.

If all is OK then check the ignition switch itself.

The fact the tacho drops like you had turned it off suggests that there is a break in the 12V circuit - if it had just fallen slowly with the revs of the engine as it died then it would point to a high tension (spark plug voltage) problem like a faulty rotor arm, cap centre electrode or king lead failure.
Chris at Octarine Services

Robert. As Ron notes, if the tach just dies it is a low tension ignition circuit problem. You need a good quality copy of your wiring diagram, preferably enlarged about 300% so you can trace the circuits more easily.

The low tension circuit is the distributor circuit which includes the power input to the coil, the coil, the line from the coil to the distributor and the distributor itself. Depending on year, you may have an ignition relay involved before the coil. The tach is driven off the low tension circuit and, when it fails by dropping immediately when the engine quits, it is a sign that electric current within the low tension circuit is interupted.

When the engine quits and the tach keeps indicating as the engine winds down, it is an indication of an ignition system high tension circuit problem (coil, king lead, distributor cap, rotor, spark plug leads and plugs) or a fuel system problem.

Your indications are of a low tension (LT) electrical fault affecting the distributor. You may wish to visit my website, www.custompistols.com/ where there is a tech article on ignition system troubleshooting which may help you. It is on the MG side under articles.

Les
Les Bengtson

Robert,
Sorry I should have explained in more detail. I think that Chris and Les have pretty well summed up the igition circuit. I have seen both the small wire inside the distributer fail as well as the key switch go bad. Both will cause an intermittant failure. The wire inside the dist runs from the side to the point plate and moves with the point plate as RPM change. What you will see is the outside cover, not the fine wire inside the outer cover. Best to use a multimeter or with the ignition on a circuit tester (looks like a awl with a light insid, clip one end to ground put the point on the circuit you want to test. If 12v is present the light will glow. Wiggle the wire carefully and see if the light glows steady. Have a helper wiggle the key in the switch as well, same type test. The main 12v input is the wire going to the side of the distributer.

Ron
RJS Smith

Ron, Les, and Chris,

Thank you very much for your suggestions. I will look at these tomorrow or as soon as I can.

As for the ignition, the key switch is fairly new - less then two years, so I will
concentrate on the wires at the points and the coil.

Again, can anyone answer the question for me about the coil? Do coils stop working all at once, or will/can they start and stop working like I have described?

As for the tach falling off, I admit that I am not positive about it. When it happned I was more interested in not getting run over then watching the tach. However, from I remember it seems that as the engine died the tach just dropped, so maybe everyones suggestions are correct.

I will let everyone know.

I would still other peoples thoughts on this issue - so everyones suggestions are appreciated.

Why do I get all the hard problems????

Thanks,

Robert
Robert Browning

A coil can be intermittant, usually heat related as it gets hot either a wire will expand and seperate or will expand and shortout. After cooling it will work again until it gets hot. I've only seen this once, but then again a coil is relatively cheap and easy to replace. If yours is the original I'd say its done its service.

Ron
Ron Smith

Ron and everyone else,

I have been thinking about this and I have a question for everyone.

If this is an electrical problem such as a bad wire or a short in a wire, why does the problem only happen when the car is at idle or the engine rev's are low? Once the car was running and above say 2000 rpm, it ran fine. i could cruise all day at 50 moh with no problem.

If this was a bad wire or bad high/low tension circuit, wouldn't it be causing the problem all the time and not just at idle?

Just wondering?

I think replacing the coil may be a good starting point. Agree??? If as Ron says it could be heat related, then today would have been the day it would have happen - it was boiling hot!

Thanks,
Robert
Robert Browning

Robert. It is easier to theorize about why something happens at a certain time, but not at another, when you know what the problem is. Since we do not know, nor do we know the exact conditions under which the power dies, it is difficult to give a cause with any real probability that it might be correct. One thing which might be a factor is that, depending on throttle opening, the vacuum advance unit will be operating differently and it connects directly to the points base plate. Thus, if the base plate were loose on its mounting, if if the ground wire is breaking up, the movement of the base plate could be a factor. If it is a coil problem, higher rpms result in less saturation time than lower rpms and that might be a factor. But, all this speculation is of no use without some facts on which to base such speculation. Provide us with some test results and we can speculate effectively. This is not the case at the current time. Les
Les Bengtson

Les,

Makes perfect sense. I will investigate and let eveyone speculate more then...

Thanks,

Robert
Robert Browning

I've had a condenser go bad decades ago that sounded kind of like that.
JimS

You might want to look for a loose wire or one at a connector about to become loose (hanging on by a strand!). I have to wonder about the sequence of events leading up to the stalling of your engine. It could be heat related, but it could also be a wire that moves as you accelerate/decelerate???

Another question would center around the carbs you are using. If HIFs, is there a possibility that they might flood somehow? I am not very familiar with the details, so I may be mistaken in my assumptions about how they could flood.

If simply cranking does not start the engine right off, but it will catch after some minutes and then cranking, it sounds a bit like the fuel/air mixture is too rich when it dies and then thins out to "normal" when the excess gas has had a chance to evaporate.

The next time the car dies on you like this, try putting your gas pedal to the floor and holding it before and while you crank it. If it starts right up or much more quickly doing this procedure, the extra air allowed in by the wide open throttle may be enough to thin out the mixture and this conjecture of mine might lead somewhere. However, if there is no difference in how quickly the engine starts again, then I guess it is back to checking for heat/wiring problems. Good luck.
Bob Muenchausen

My experence has been if the motor dies suddenly without coughing or spiting, it usually means the problem is ignition and not fuel related. I've had a weak coil that would show a spark when the plug was touching against the block but not fire in a cylinder. When it was cold, the coil worked great. I have also found that coils get weak with age. One other bit: after fouling the plugs when the owner set the gap at 0.050, no amount of cleaning and scraping would get the plug to work, only replacement solved the problem.

Safety Fast!
Jeff
Jeff Pintler

Just to offer another avenue: Is the piston in one of the carbs sticking open? If one of these was stuck open,the car would lean out and not run at an idle. The piston eventually drops and all is well for a while. Easy to check and if they have not had any attention in a while, worth cleaning anyways.

Pete
Pete

I had this problem, it was the 3-way bullet connector where the white wires join under the steering column. The connections were loose. Worth a look?
Samuel Sullivan

Hello,

This afternoon I checked the tension wire going to the points in the distributor and it was fine. All the other connections also checked out fine.

Therefore, I am getting a new coil and condenser tomorrow.

I will now go check the pistons the carb and the bullet connection just to eliminate thses items.

I will let everyon know.

Thanks,

Robert
Robert Browning

One thing you said struck me. The gas cap is fairly new. Is it vented? Is the vent plugged? If you take the gas cap off, is there still a problem? Easy thing to check before you spend money on parts.
Tom

Hello,

I am still working on the problem with my car. We now think it is a combination of the air/fuel mixture, and the heat, and possibly an ignition problem - and/or combination of all the above.

I have ordered a pertronix Ignitor system and hopefully it will be here tomorrow, so I can put it in this weekend. Once I get this installed, I have someone that knows what they are doing that will adjust my air/fuel and timing.

Now the new problem and question I have for everyone is which is the correct coil for my car - a stock 1973 MGB. I bought a new Lucas coil and the number stamped on the bottom is 11P12. The number on the box is DLB101. The number stamped on the bottom of my old coil is HA12. Do these numbers mean anything?

The Moss catalog has their part number for a coil for a 1973, as 143-220.

The problem I am having is that the diameter of the top of the coil, where your coil wire connects is much smaller on the new coil then on my old coil - about 1/4" smaller in diameter. The rubber boot on the coil wire is so loose I am afraid the wire will fall out.

Does anyone know the part number for the correct coil for my car - both the Moss number and the number on the Lucas box?

Thank you.

Robert
Robert Browning

In addition, on the top of the box for the new coil, it list about ten cars that I guess this coil is suppse to fit. MG is not listed.
Robert Browning

Robert,
",,,new Lucas coil and the number stamped on the bottom is 11P12. The number on the box is DLB101. The number stamped on the bottom of my old coil is HA12. Do these numbers mean anything?"
"11P12" and "HA12" are the model numbers. HA12 is a semi-sports coil as fitted OEM to most "high performance" or sports models. Higher voltage output than Morris or Austin etc., lower than a "Sports Coil". 11P12 is something new - a supersession no doubt. The critical point is that the "12" means it is for 12V (non-ballasted) operation; coils for ballasted cars have models ending in 6 or 7, since that is their design voltage. DLB101 was a Lucas attempt at "number rationalisation" - a generic coil that should work. They did this c1980 with tune-up parts, filters and the like. I believe it is correct but can't find my cross references to that number system.

"The Moss catalog has their part number for a coil for a 1973, as 143-220."
Moss make up their own numbers; you assume they are right, and have to talk to them to try for what Lucas numbers they might match.

"The problem I am having is that the diameter of the top of the coil, where your coil wire connects is much smaller on the new coil then on my old coil - about 1/4" smaller in diameter. The rubber boot on the coil wire is so loose I am afraid the wire will fall out."
The rubber boot is only to keep moisture out, it IS NOT to hold the wire in. The wire should snap quite tightly into the tower. A loose wire here is one of the very few things that will kill a coil, as it causes an arc inside the tower, burning the insulation. The wire MUST go all the way in tightly.

"Does anyone know the part number for the correct coil for my car - both the Moss number and the number on the Lucas box?"
The original part number (probably 45212) is stamped on the bottom of the coil along with the model (HA12) and date (week, year)of manufacture.

You have a 99.9% chance that nothing is wrong with the HA12, if there is no visible damage and no burning inside the wire tower. Keep it or send it to me!

It is possible that you are having boiling fuel problems, although this is usually seen in early spring, when cars are still using winter gas and a hot day comes along. The fuel boils in the lines, and since it can't go backwards past the pump check valves, it pressurizes the carbs, causing both rich running (vapor) and fuel starvation (liquid), and sometimes flooding. I have put a tee in the line at the carbs and run a line to a fuel pressure gauge in the car. Results showed pressures above 20psi under exactly the conditions where you are having your stalling. It usually does not flood, as the pressure is caused by vapor, not liquid. As soon as you start off, cool fuel flows to the carbs and the problem ceases. The only real fix for this is to run a return line to the tank, with a small orifice at the tee where you check the pressure. The orifice should be about .030 and pointed upward from the high point of the line. This will let the pump run all the time as it bypasses fuel and vapor to the tank, but will maintain the required 2-4psi at the carbs. Otherwise, wait til it cools off, or keep moving!
FRM
FR Millmore

One other thing- I once heard an investigative report on CBC radio regarding gasoline and diesel/heating oil. Turns out these are major components in illegal hazardous waste disposal. All manner of solvents are mixed with fuels to get rid of them; the people doing this get paid to dispose of the waste, and they get to sell more "fuel"! At today's prices, it must be happening a bunch - bet it screws up the vapour prssures of the resultant stuff.
FRM
FR Millmore

FRM,

I was doing some research on fuel pressure gauges and I keep finding the following information:
"DO NOT ROUTE A FUEL LINE INTO THE COCKPIT! TO PLACE A FUEL GAUGE IN THE COCKPIT YOU MUST USE AN ISOLATOR OR AN ELECTRICAL GAUGE WITH A SEPARATE PRESSURE SENDER.
If you are installing an electric pressure gauge you can install the pressure sender in the same place in the same way and then route the output wire through the firewall to the gauge."

How did you install your gauge inside the car? What gauge did you use?

Thanks,

Robert
Robert Browning

Robert,
Yes, that is standard advice re fuel lines in the cockpit, obviously not a great idea for fire reasons. Coming from racing and liability concerns, but then how many cars have you seen that really have a "totally sealed firewall", also good practice? Lines can break and gauges can rupture, but it is no doubt less hazardous than the same lines under the hood in normal driving, especially with low pressure electric pumps.
But, I am talking about a temporary test setup. I just run a piece of fuel hose through some convenient hole in the firewall, then to a standard fuel pressure/vacuum gauge. Good idea to at least use real fuel hose (instead of the push-on unreinforced rubber supplied with the gauge) and put hose clamps on it.
BTW, check to see if someone has replaced the pump to engine line and put it close to the exhaust.
FRM
FR Millmore

Correction: I started out with the standard test gauge, but it topped out at 10 psi. That's enough to diagnose the problem, but not to tell what is really happening, and it might screw up the gauge at higher pressure. So I went to a 0-30psi gauge to find out the max pressure; that had a threaded 1/8 pipe fitting, so needed a hose barb adaptor. I had 4 cars (2Bs, TR6, Spridget) all behaving this way on a 90+ day in March, and measured all of them. Highest was 24psi I think, but all got to around 20; only the TR6 flooded.
FRM
FR Millmore

FRM,

I think you may have hit on the problem with my car.

Today, I did however replace the coil and while I do not think it is my imagination, the car just seems to run better. It started easier (though it has always started fairly easy) and seems to idle smoother. However, it is much cooler this Saturday then last Saturday.

I mis-understood about the gauge and thought it was permanent hookup. It sounded good to me!! Anyway, I think I will try your suggestions to just see what happens and get some measurements.

I have not adjusted the air/fuel or the timing on my car yet, as I have ordered a Pertronix ignition Ignitor and just decided to wait and do everything at once.

I will keep you informed.

Cheers and thanks for everyone's help.

Robert

Robert Browning

This thread was discussed between 20/08/2005 and 27/08/2005

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