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MG Midget and Sprite Technical - misfire at speed, coils and ballast


Hi
Just starting to use my 1977/78 1500 midget it had been of the road many years before I got it, it has had new plugs points condensor leads and dizzy cap, I have installed all of these and done the timing. Also cleaned and balanced card and sems fine (but I have not set the mixture)

Car was running fine then yesterday it started missing at 3,000 revs plus (tacho bouncing around so i think it must be ignition related)
The I realised that i had put some 95 ron unleaded in instead of 97 super plus.(Set timing on super plus)
Put more 97 in this morning, miss now at higher revs under hard accelration but slightly better.

Is this likely to be down to fuel or something else?
I have not changed the coil, could it be due to the coil breaking down?

Also where is the ballast resistor on the car and what does it look like, I as k as ther is a small barreled shaped thing with a black wire mounted on the coil bracket, it has a black wire leading fron it to a female spade connector the wire is short so probably went to the coil? It is currently diconnected.
1978 wiring diagram doesnt mention a ballast resistor as far as I can see but manual says ballast type coil required, can anyone explain?

Thanks
Jon
Jon Warren

Hi, Jon -

The ballast for the coil is actually a resistance wire in the loom, and not a discrete "resistor" per se. There should be two wires sharing a connector on the positive lead of the coil - both white, with a light green stripe. One of those wires is the ballasted feed, and the other supplies full 12V, but only while starting. If you have both of those wires connected properly, you're receiving the reduced voltage via the ballast wire once the engine is running.

I'm not sure what that cylindrical unit might be!

Cheers,

-:G:-
Gryf Ketcherside

Jon,
The cylindrical thing is most likely a capacitor that someone has added to suppress radio interference. There may also be one on the alternator.
Guy Weller

Jon. "ther is a small barreled shaped thing with a black wire mounted on the coil bracket," This is a condensor which was used to suppress radio frequency interference from the ignition system.

It would be good to know what distributor you have in the car and whether it has been modified. The factory distributor used in the 77-79 period was the Lucas 45DE4 distributor, also called the Opus system. On a new, 79 MGB, the Opus system began giving the same problems you describe within the first six months of ownership and had to be replaced under warranty. You still find a few cars, which have been in storage for many years, that have an original Opus distributor in them. Most, however, have been changed out internally with either an Ignitor or a Luminition conversion. Thus, to begin the sorting process you need to know what you are working with.

Do not know where the "ballast resistor" was located on the Midget. On the B, there was not one. Rather the full time voltage to the coil was reduced by the use of a resistance wire which served the same purpose. There was a resistor associated with the Opus system and looked like a finned, rectangular box next to the coil. But, the wiring went to the distributor rather than the input (Positive) terminal of the coil.

Ignition timing does not cause misfiring. It can allow pinging/pinking when a lower grade of fuel is used than the engine was set up for. I agree that the actions of the tachometer tend to indicate a low tension ignition circuit problem and that would be the first thing to investigate.

Les
Les Bengtson

Jon,

I had the same symptoms as you with my Midget, misfiring and the tacho bouncing around.

It turned out to be points related, gap closing up etc.

Remedied by fitting the simple and inexpensive electronic ignition as supplied by Britpart. It is intended for Land Rovers but fits any Lucas 45D distributor.

Never had any trouble since.

Dave
Penwithian

Jon

Just had the same problem last week with my '78 1500...misfire at 3000rpm & above, tach bouncing around.

All I did was replace the terminals (connectors) on the wires that connect to the coil and the distributor.

You must remember we're dealing with cars that are OLD! I just bought mine 6 weeks ago and have been fighting corrosion and rust in harness and connectors since.

Don't overlook the fuse box...I was going down the road last weekend and all guages, including tach and turn signals, quit working...pulled over and spun all fuses in the box and magically all was back to normal...ordered new fuse box!

What I'm finding is: if the wire, terminal, etc. is suspect...don't clean it (like I've unsuccessfully done)replace it!

The distributor in my car should have been an electronic unit, but it's been replaced with an earlier points style unit (as yours apparently has been), that I haven't been able to positively identify.

My distributor is very worn, I'm planning to replace it with a new after market electronic unit, so at least I'll know what I have and be able properly tune the engine.

Good luck and try terminal connector replacement first...it's cheap and easy!

Dave
DL Rhine


Thanks for the replies
Went to start car after work and it would not fire... ,
eventually traced to no spark from coil..
But the coil wasn't the problem the fault was the small plastic thing between points and condensor, it wasn't done up properly so LT circuit was shorting to earth rather tha going to the coil!
Ran home ok touch wood etc

Dave the britpart kit looks a bargain is it easy to fit and set up?

Jon
Jon Warren

Good for you! It always seems to be something simple with electrical problems...it's just finding that simple thing that wears you out!

Good job.



DL Rhine

Jon,

Fitting the Britpart ignition system is a doddle.
It comes with simple instructions.

Even I could do it :o)

Dave
Penwithian

This thread was discussed between 28/07/2009 and 29/07/2009

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