Misfire(?) in my NSU

rugbyman

Hello Folks, looking for some advice.
I recently had the good luck to buy a 50 year old NSU 1200.

The car runs well, with a very slight wee annoying thing, driving along i experience a slight pull back, as if the ignition had been turned off for half a second.
last week after a ten mile run the car started misfiring and stopped.
A good mechanic replaced the coil, all seemed well. After twenty miles misfires returned. At this stage the coil was too hot to touch.

I limped along, finally got towed home.

Any suggestions?

thanks in advance

kadman

Thankfully being a pre " every god damn thing is now electric" car, it narrows the field to a few
possibilities, instead of 100's.

So its down to coil, high and low tension circuit, condensor , points and wiring.

On a 50 year old car the wiring has to be a candidate.

Testacalda

Most of the time with a problem like this I would suggest changing the rotor arm or the condenser, and cleaning the points first. Do these things one at a time and test drive each time to isolate what the problem actually was.

Many times I have seen electrical components function fine when cold, but start to fail when they heat up after driving for a while, including old wires!

In my personal experience the root cause of the problem has rarely been a coil, but as yours is heating up, there is an issue causing that.

Is your new coil a ballast resister coil by any chance? If a coil designed for use with a ballast resistor is used without one it will over heat and then stop working. Might work ok once it has cooled down again.

noelf

Just my two cents ( not being a expert ) has the engine been converted to run on unleaded fuel ..

rugbyman

Testacalda wrote: »

Most of the time with a problem like this I would suggest changing the rotor arm or the condenser, and cleaning the points first. Do these things one at a time and test drive each time to isolate what the problem actually was.

Many times I have seen electrical components function fine when cold, but start to fail when they heat up after driving for a while, including old wires!

In my personal experience the root cause of the problem has rarely been a coil, but as yours is heating up, there is an issue causing that.

Is your new coil a ballast resister coil by any chance? If a coil designed for use with a ballast resistor is used without one it will over heat and then stop working. Might work ok once it has cooled down again.

I dont know if the coil is ballast resistor.
the v. good mechanic told me to ask for one that did ignition only, like for points.

I should have mentioned that last week my son moved the car and left the ignition switch on. This flattened the battery, charged up and re started.
I did have in the back of my mind that this could damage the coil. We replaced it, the supplier did not have a condensor. looking back that coil might be ok.

Thanks again

rugbyman

The regulator also got quite warm, though nothing like the heat of the coil

kadman

Old bakelight distributor caps and rotors are notorious for being the culprits
in poor running engines. Hairline cracks in either get worse with a warmer engine.

There is also a smal carbon bush pin in the early distributors at the inside top,
which also leads to running problems. My own beetle and fastback coils get hot if the ignition
is left on and the engine not started, but does not have adverse effects.

Main earth leads at the engine and gearbox also cause major headaches in running early classics.
Particularly gearbox/engine to body mounts. Absolute clean connections are needed here.
Old earth cables need replacing, its a must here. The rest of the electrics will not perform
correctly otherwise.

Testacalda

No harm to check to see if your charging system is ok too. Put a multi-meter on the battery, you should be seeing 13.8 - 14.4 volts approximately at a high idle (assuming its a 12 volt system)

If a dynamo & regulator box is fitted, if the regulator has failed, the dynamo could be giving out anything up to 19volts, which would give lots of problems, including overheating your coil. Some alternators have external regulators, but it's quite rare, normally only seen on French or Italian cars.

Most likely you got a standard coil, which is a normal 12 volt coil and widely available. You could google the part number and check to be sure. Is it an Intermotor or Lucas part?

A Ballast Resistor coil is designed to work at a lower voltage, eg: 9 volts.

In this case, there will be two power feeds going to the coil, one is just a straight 12 volt feed and the other is going through a resistor, bringing voltage down to 9 volts

When cranking over the coil gets its power from the 12v wire, giving a stronger spark than normal to aid starting and negate issues with voltage drop.

When the car is running, the coil gets its power through the resistor, meaning the coil is working at the voltage its designed for. The few seconds working at the higher voltage does no harm to the coil.

rugbyman

Thank you all for your informative info. I had seen you in action Kalman with the points of your vw restore.i have been quite a while on this planet,and was aware of a lot of this, but it's great to get exact advice

rugbyman

well, the mechanic decided to clean out the carb,knowing this was not linked to
the hot coil. Car running well, did a fast ten mile trip,knowing this was not far enough to bring on the symptoms. The coil was very hot and the regulator was quite hot.

I am away working for a few days. he hopes to check the charging system for overcharging

kadman

Old style early regulators used to run a bit hot, it was nor uncommon,

coils to may get warm, but not hot enough that you could not put your

hand around it. Also the early regs could be reset to give the correct voltage output.

It was a common thing, at least on early VW's to adjust the settings and clean the contacts in them.

w124man

I'd be looking at ignition leads.

rugbyman

Hello Chaps,

my mechanic friend checked the earth s, he was not of the opinion that they were bad ,but he cleaned and tightened them anyway.
he says he measured the output of the dynamo and it was 15 volts, the output from the regulator was 14.5 volts

On an 18 mile trip the other day, the coil was quite hot but not too hot to burn me.

Testacalda and Kadman mentioned wiring as a suspect, assuming ye meant the wires from the dynamo to the regulator and from there to the coil(is that the path)and coil to distributor. Do you mean a close visual inspection or perhaps some test using battery /and/or a meter to test conductivity.


On another note Kadman, did you come to any conclusion on what rust eater/chemical is one you would recommend. i intend to sandpaper off, use chemical rust eater ,and then rust preventative . Can you recommend a rust eater and a rust preventative.

Thanks in advance

Testacalda

The resistance in old wiring can be higher than new fresh copper wiring. If the wires are suspect I would replace them, they can look fine externally but have green corrosion between the strands or a black oxidisation of the copper inside the insulation. This can have a small effect on resistance.

Small core diameter wiring won't be too expensive so IF all other avenues have been exhausted, try replacing it. Old brittle insulation on wiring is kinda dangerous anyway!

Also, did you confirm that your new coil is actually a standard 12v coil, ie: not designed for use with a ballast resistor.

14.5 volts output from the regulator is a touch high but probably ok, I would expect it should be less at low idle though, many dynamos barely charge at all at low idle