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MG Midget and Sprite Technical - Coil Woes

I was troubleshooting a missfire so I started pulling spark plug wired one at a time to see which cylinder was not firing and I found out cylinder #2 was not firing.

When I pulled the leed from the plug I noticed I was getting spark and when I held the wire about 75mm from the top of the plug I started getting a strong spark from the tip of the leed to the plug and the motor started running smoothly.

When I put the leed back in contact with hte plug the motor started running rough.

The only thing I can figure out is I am increasing the resistance between the leed and the plug until the coil builds up enough voltage to jump the gap between the plug and the wire.

Do I have a bad coil?

The coil I have is from the local autoparts store and "does not require external resistance" so took out the external high watt resistor I had with the previous coil. I am running a Crane electronic ignition with a modified Lucas distributor. I put the coil on my other midget and had the same fault and that midget is running a Mallory optical distributor.

Should I get a new coil or do I have something set up wrong?

What would you look at?
Stefan

I had a similar problem but it turned out to be a bad lead. Have you tried swapping leads around to see if it has any effect?
G Lazarus

I just got a new set of leeds in from Rover UK and installed them. Same problem.
Stefan

Must be the condenser ;-)
Bad plug? Have you rotated them? Obviously you haven't checked how the engine runs when all four leads are held away from the plug (unless you are part octopus).

Did you really mean 75mm??? If your spark can really jump nearly 3 inches, you DO have a coil problem! Did you mean 7.5mm?
David "Someone had to say condenser" Lieb
David Lieb

David, yes 7.5mm. I had the same problem with the mallory coil as well as the lucas coil with the Crane ignition installed.

The condenser is inside the Lucas points type distributor right?

Tonight I am going to try to move the plugs around and see if that helps any. I might end up changing out the coil for one that requires the ballast resistor. I think I might have the installed a bad coil (on that required no external resistor) but why is the motor running smoothly when I have the leed for cylinder 2 about 7.5mm from the top of the plug?
Stefan

Have you pulled that plug to check the plug gap ?
Dean Smith ('73 RWA)

It might be a bad distibutor cap. When you pull the lead off of the plug you're changing the geometry of the wire which can be just enough to "fix" the problem where it goes into the cap. I know this sounds a bit far fetched but with these cars, who knows.

Martin
Martin

Yes, Stefan, you are correct, there should be no condenser in your installation. However, the condensers on these cars have caused so many bizarre failures over the years that it has become a standard joke to suggest them as a cause, the more far-fetched the better.

You did say that you tried this coil with the same result on your other Midget; what did the other Midget's coil do on this one?
David "no point" Lieb
David Lieb

Stefan. The coil should be good. If it is a coil, or a king lead/coil lead problem, it would show up on all cylinders, not a single cylinder.

You mention that, "I put the coil on my other Midget and had the same fault and that midget is running a Mallory optical distributor." This would seem to confirm that the coil is at fault. But, once again, if it is only happening on the number two cylinder, rather than all of the cylinders, it cannot be a coil problem. (Or, if it is a coil problem, the failure to spark should be on a random basis, affect all of the cylinders at some time, and you just happened to notice it on the number two cylinder.)

David L's first post mentions testing each of the other leads to see what happens. This might provide some information of value. At the very least, it would be interesting to know what happens when you detach each lead and hold it your 7.5mm (1/4") from the spark plug cap.

But, if you want to find out what the problem is quickly, and have the best chance of getting it right the first time, have the engine checked out on an engine analyzer to see what is actually happening. Ideally, the exhaust system would be hooked up to an exhaust gas analyzer to see what changes are present in the exhaust gases as you are making your various observations and tests.

I did this a few months ago with my daughter's car, which she had brought up to me after not being able to get it through emissions testing--it has had no problem in the past. About fifteen minutes of professional mechanic time, with the engine and exhaust system monitored by the machines, and the problem was solved. Number two cylinder was not firing because the distributor cap was not allowing the spark plug lead to fully set into the terminal, causing a weak spark. Do not know, exactly, what your problem might be. But, not being able to diagnose a problem, after four hours, and having a professional, with the right equipment, find an obscure problem in less than a half hour was an interesting learning experience.

Les
Les Bengtson

" Obviously you haven't checked how the engine runs when all four leads are held away from the plug (unless you are part octopus) "

Part Octopus won't do. You have to be at least half Octopus.

R.
Richard 79 1500

Lee,

Thanks for the advice and I agree with you that with the correct equipment this problem could be solved very fast. I need to take the car into a professional tuning shop and have the DCOE set up right as well. I guess I am more concerned about all of the other things I need to do to the car and want to put off that final "tune" until everything else is done. With my ignition problems I am thinking about putting the mallory distributor from the other Midget in the one I am having problems with. I am not driving that one (well I am not driving either currently) and I have a EDIS set up I built for the car but its sitting here on the dinner table while I am still putting the new engine together.

I was hoping it was the coil that was bad, but as you indicated why would it only run rough when the leed is connected to cylinder 2? I replaced the distributor cap last night and the leeds and it did not fix the problem but the only consistency was the coil between the 2 cars. Today I bought a new coil to replace the one I have in the car that "required no external ballast" but I did not get it running. I got it installed but the sun went down on me and I have not installed shop lights in my garage/port so I went down and bought some chicken are retreated into the house and started watching TV.

I am wondering if anyone has ever had a problem simular to this? I am starting to lean towards the coil being the problem but through all of my schooling in electronics I can not figure out why a coil with a internal resistor would put out better spark when one of the leeds is held far enough away from its contact on the spark plug.

The only thing I can think of is it requires the coil to build up a higher potential energy (voltage) before breaking through the air as an arc thus discharging a larger amount of electrons (amps) through the plasma of the spark making the spark last longer and be hotter..... or the coil could not support discharging 4 times in a row and the delay created by the longer spark gap (two sparks in series, one between the leed and the plug and one on the plug gap itself) helped it charge.

I did check the spark plug gap tonight and it was to the correct spec across all of the plugs. These plugs have at most 100 miles on them.

Stefan

Richard,

If I could ever talk that wife I ordered online into shipping freight I would make her hold the leeds!
Stefan

>>>These plugs have at most 100 miles on them.
=== ah, try swapping the plugs around and see if the fault moves with the swap - plenty of people have experienced faulty new plugs in recent times, quality doesn't seem to be what it used to be.
David Smith

Stefan. Moving the spark plug lead away from the plug increases the voltage the coil must produce to cause the spark plug to spark. When you do this with the coil lead, and the engine is hooked to an engine analyzer, it allows you to see if the coil is capable of putting out its full rated voltage, normally about 20K volts. When it is done with a spark plug lead, it allows you to see if the rotor, terminal, and lead are capable of putting out the same voltage as the coil lead. A basic test to see if any voltage is lost in transmission between the coil lead and the spark plug lead.

So, what is happening here? We know that your engine is failing to spark on the number two cylinder when the spark plug lead is connected to the spark plug. We know that when you back off the lead about 1/4", the plug seems to be firing. (Your observation that the engine idles more smoothly.) Thus, it would seem that you have to rise the firing voltage to cause the spark plug to fire. But, this, in turn makes no sense because the voltage will build up in the coil until the spark jumps the gap in the plug anyway. So, the evidence is at odds with itself.

One thing which might be happening is that the firing voltage required to fire the number two plug is excessively low--as when the outside of the plug is wet or dirty and the spark grounds out down the side of the plug before sufficient voltage is developed to cause it to spark inside the cylinder head. This would be in line with David S's mention of a bad spark plug. I would agree that changing the plugs around, and inspecting the nose for damage, would be a useful piece of troubleshooting. If the problem stays with number two cylinder, there is some other cause.

If you are able, I would suggest both a compression check and a check for air leaks around the front leg of the intake manifold. It would be useful to know that there is consistent compression among the cylinders and that there are no air leaks.

Les
Les Bengtson

Out of interest have a look at the coil in the dark, if you see lots of blue sparks flying around the coil like i did, then its time for a new one,

cheers
Brad
B Richards

I thin the likely answer will be a bad shaft bushing, causing the rotor to hit the #2 cap terminal. Check inside for signs of contact between the two pieces. Also check for lateral play in the shaft. It may be time for a distributor rebuild.
Jeff Schlemmer

Stefan,
If switching coils produced the same result than the problem is elseware. I'd think along the same line as David S or Jeff that its the plug or in the distrib. Both should be relativly cheap and easy fixes, unless your bushing is shot indicating the need for a distributor re-build.
I hope to meet you and one of the Midgets at Brit Bash in Melbourne.
Phil
Phil

The coil thing is worth a go, my spare was worse than the one fitted to the car.
cheers
Brad
B Richards

Yes, a bad Autolite plug took my van off the road for a year. It was a few hundred miles old, and who'd of thought? What an education I got looking into other areas...
Steve (Georgia)

Many (too many) years ago, I used to rally a lotus cortina, during an event I gained a nasty misfire, the engine had been serviced with new plugs, leads, etc before the event and fully tuned.
I jumped to the conclusion that I had dropped a valve as I had been pulling some high revs and it seemed likely. After much labour the engine was stripped and I discovered that the fault was actually a new plug with a moving centre electrode!!!!
Rule one, check the simple things first, rule two, don't jump to conclusions without real evidence and rule three, new things are often faulty!
Graham
Graham P 1330 Frogeye

This thread was discussed between 16/10/2008 and 23/10/2008

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