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MG Midget and Sprite Technical - Ingnition coil

One of the teething problems I have with my 72 midget is after rebuilding the engine I find that it won't rev above 3500 (where it will misfire if pressed) and lacks power.
The rebuild was +20 thou rebore, kent 256 cam, cleaned up and skimmed cylinder head and unleaded valve seats. It has an LCB exhaust manifold and sports exhaust.
I have dynamically timed the ignition to original spec, set up the carbs (they are running a little rich, but it passed the MOT on Friday so not that bad).
I have fitted new spark plugs, leads, dizzy cap, rotor arm, replaced the points and condenser with an electronic unit. I have also rebuilt the carbs with new jets, float needle valves, throttle spindles (original needles).
I have not replaced the coil and did notice that it gets very hot whilst the engine is running. So hot that I could not keep my hand on it for long. Is this normal?
I hope it is the coil as the only other thing I can think it may be is the cam timing. I timed the cam in to Kents spec and checked it before refitting the engine. I was sure it was ok but have not checked it again since getting the engine back in the car.
Any help would be greatly appreciated.
Thanks.
D Brown

Coils run hot (if you think about it, there's current flow most of the time). If you have ign on with engine stationary (and points closed) they run excessively hot and might even have oil leak. You might want to check you have correct 12v coil and not 6v coil to use with balast resitor (like on some 1500s, AIUI).

You might want to check your air-filter isn't blocking the (small) breather hole on the front of the carb(s); this causes hesitant running when you try to rev. (BTDT GTT)
Anthony Cutler

I don't think the coil should get so hot that you cannot put your hand on it - at least not with the engine running. A replacement coil is only about £20 so I would try that. You seem to have tried everything else!

One other thing, what clearance did you set the valves to? With the 256 cam I think they need to be 16 to 18 thou. Too tight and it could cause a misfire at 3500.
Guy Weller

Thanks for the replies. I am going to get a new coil tonight. Not sure if it worth getting a sports coil or not though.
The valve clearances were set to 12 thou. when the engine was built. I will check the spec for the cam and reset them.
Dave
D Brown

The ignition coil like the condenser tend to get a lot of blame for all sorts of issues. There is a standing joke on this forum caused by certain individuals claiming every fault was simply due to poor condensers. Naturally occassionally the coil can be at fault the same as 1 in every 10,000 faults a condenser can be to blame, but I believe your problem sounds more like fuel starvation.

I think the suggestions above are all reasonable although the point made by Ant ios of particular importance

Also check the fuel supply this can be restricted by the filter gauze in the pump connection becoming blocked and would seriously restrict power.

However I may be wrong and it could be the coil or even the condenser LOL.
Bob Turbo Midget England

Thanks Bob.
It won't be the condenser as it has been replaced with an electronic gadget.
I had not thought about fuel starvation. I have fitted a new fuel pump, I hope it pumps enough fuel. I will test it. How much should it flow a minute?
D Brown

Faulty coil shows up as lack of power engine power in my experience

Coils can get very hot if you wire up the ignition switch wrong, I know as I've done it, of course I meant to take a digital photo before I took the switch off for reference but I forgot, only four connections and I still got it wrong

Nigel Atkins

You going to have to help me out here Nigel how does an incorrectly wired ignition switch cause the coil to become excessively hot?

The 2 faulty coils I have had in 40 odd years were:

1 Complete ignition failure.
2 My points kept melting! I assume this was as a result of High tension straying into the Low tension circuit
:)
Bob Turbo Midget England

It could also be that the engine can't breathe fast enough - I had this problem with mine after some tuning upgrade, and Peter Burgess suggested I fit a wider bore hose from the can on the timing cover to an overflow can, which I fitted to the inside wing. That did the job for me.
Geoff Mears

Bob explanation was me, an idiot, reconnecting the ignition switch without reference to how it was wired before I disconnected the wires by using different permintations as things didn't work

I touched the coil whilst under the bonnet and it was really hot

I must admit I now can't even remember why I was messing with the switch

Perhaps I'm wrong with the reason but I know the coil was very hot

I mentioned it as I thought others could learn by my mistake (if only to to recored where wires go before you disconnect then)

I make loads of mistakes and they usually cost me at least some money so others can save from not making my mistakes

I personally have never had a coil go in 32 years, but I do like to keep my cars in good working order, 3 or 4 years ago I know someone who's coil went and I've heard of others so it's not common but it does happen
Nigel Atkins

The switch must have been wired with power on the ignition circuit always and not switched.

This would cause the coil to become extremely hot if the points were closed as described by Ant. If you had started the engine then you would never have been able to stop the engine. :) Did the fuel pump tick as well?
Robert (Bob) Midget Turbo

I replaced the coil with a new Lucas Sports coil last night. I also readjusted the valve clearances which I had originally set to the standard 12 thou but found out yesterday that they should be 16 thou for the cam I fitted. The engine runs a little better now but it still does not want to rev beyond about 3500 and will misfire if pressed.
I think the only options left now are fuel starvation (I will check that the pump flows enough fuel tonight - 12 UK gallons per hour I think), ignition timing (I will check again tonight but I am sure this is set to the standard 13 degrees @ 1000rpm), wrong needles (I am not sure that this is likely), worn distributor giving the wrong total advance (I will check against the following - 1000 rpm 13 deg, 2000 rpm 19 deg, 3600 rpm 29 deg) or the wrong cam timing.
Have I missed anything?
D Brown

Yes, my comments :o)

Mine flaked out at 3500revs. It felt like I was hitting a wall. Fitting a wider bore hose - I used one with an internal diameter which was the same as the external diameter of the old one, so I just cut the old one off level with the metal piece that comes out of the timing cover and slid it over, and ran it into a 1 litre plastic oil bottle, which I mounted onto the inner wing close by. I then blocked off the carb where the pipe used to run to. Worth considering if your other ideas don't work.
Geoff Mears

Geoff, I will give it a go tonight. I think the engine breathing is ok but it's worth a try even if just to eliminate it.
Thanks

Dave
D Brown

Just a thought, but you might want to check that the bob weights in the distributor are not seized, that would limit your ability to rev the engine. You've exhausted most other options and I doubt that it's the coil.
Robin Cohen

I have just stripped my dizzy down and the bob weights are free to move. I have noticed however, that there is 16 and a dot (16 degrees I presume) stamped on the advance mechanism. Is this the maximun amount of advance that the dizzy can give (excluding vacuum)? If so, if I set it up at 7 degrees static which then gives 13 degrees at 1000, how can I get 29 maximum at 3600?
Static timing of 7 degrees plus 16 from the dizzy give maximum advance of 23.
Have I got something wrong?
D Brown

Dave,

Just a thought, noting that you have rebuilt the engine - are you happy that the cam is statically timed in accordance with the cam manufacturers spec?

JohnT
John Turner (Midget & MGB)

Dave, this is the whole reason why you should set timing NOT by any book figure that totally relies on you having the correct distributor but by setting it at 4.5 KRPM and setting it at 32 degrees BTDC how many more times does this need saying?
Bob Turbo Midget England

By the way because the distributor turns once for every 2 revs of the engine the 16 degree plate becomes 32 (I think?) thus your static setting ought to be 0 degrees BTDC ot TDC The test above setting the ignition timing at 32 degrees BTDC at 4.5K RPM should be done with vacuum disconnected.

I haven't looked at a dissy plate for years to see the figures so I am hoping I have given the correct info.

If that is rignt then you were over advanced by 7 degrees or there about, would that cause your problem ? not sure? but doubt it, it ought to have simply caused pinking.
Bob Turbo Midget England

John,
I am worried that I have timed the cam in incorrectly but I want to rule out everything else first. I timed the cam in on the bench according to Kent's instructions. I then turned the engine over 720 degrees and did it all again and it was correct and followed the same incorrect process twice and came up with the same result. But I am now doubting that I did it correctly (although I can't think where I went wrong). Re timing the cam is a bigger job than ignition timing and I don't want to tackle it until I am sure everything else is ok.
Bob,
2 x 16 giving 32 degree advance sounds plausible. I did a little research on the net last night and found that on a 12cc engine the timing should be set to 22 degrees dynamically not 13 for the later engines. So I tried that figure and the engine pulled to about 5000 with no misfire, much better, but still no where near perfect. I hadn't adjusted the carbs as I didn't have time and I remembered this morning that I hadn't removed the vacuum advance either. So another go tomorrow night or the weekend and we will see what happens.

Thanks for all the replies, you have all been very useful.
Thank you.
D Brown

What Bob is saying (I think unless I'm mistaken) is your trying to set a non-standard engine with standard figures which will be wrong for your engine

Plus you must allow for inacurracies and wear and tear in components (and yourself as the job gets longer and less rewarding)
Nigel Atkins

Dave, I had been following your thread as I'm waiting on my carbs to get back after a very similar rebuild. Any progress? Hope you found what was troubling you.
CJCharvet

Adjusting the timing to 22 degrees dynamic has helped a little. I am not convinced that I have the correct distributor for the engine.
I am considering a new dizzy. I need to check the advance and various engine speeds. I will let you know how I get on.
D Brown

Dave please adjust the timing to "32" degrees BTDC at 4.5thousand RPM with vac disconnected and then report back
Bob Turbo Midget England

x2 Bob!

Dave
Unless the max timing is set correctly as Bob states, over & over, you may never get it right...believe him, he's right!

I just went through this on my 1500, once I got the max set right, most of my driveablity problems went away!

Just try it & see.
Dave Rhine ('78 1500)

Bob, Dave,
Will do so tonight and report back later.
Thanks.
Fingers crossed.
Dave
D Brown

Bob,
I really annoyed the neighbours earlier this evening revving the engine to 4500 rpm whilst adjusting the timing. Got it timed to 32 degrees at 4500 without the vacuum connected. It wasn't that far off. Checked the carbs and they were spot on.
Test drove it and it was not much better. It would rev to about 5000 but that was about it. I think I could get 70mph out of it max.
I have a new coil, dizzy cap, leads, plugs (NGK BP6ES), electronic unit replacing the points, recon carbs, rebuilt engine with kent md256 cam with vernier timing gear and cleaned up cylinder head, K&N filters, sports exhaust and LCB manifold.
I am now thinking that I must have timed the cam in wrong. Any tips before I take the front of the engine apart tomorrow night?
Thanks
Dave
D Brown

I wish I had mate but you seem to have tried all the easy things. However with a vernier timing sproket in place should not be too hard to check and adjust.
Seems favourite I think :)
Robert (Bob) Midget Turbo

I have just read about red rotor arms from the dizzy doc. Sounds like the exact symptoms I have. I suppose £7.00 is not too much to waste while I am checking the cam timing. I will get my mate round to help me out with the cam so that I have a second opinion.
Dave
D Brown

I fitted the Red Rotor arm yesterday and almost all of the problems have gone. It revs without misfiring, it pulls better, it is drivable. It still needs fine tuning but that should not be a problem.
Then it is the suspension and the rattles.
Thanks for all the advice.
D Brown

Sorry I'm being wise AFTER the event here

Dissy cap, rotor arm and ignition leads are often overlooked as they are not part of a replacement schedule and it's very difficult to see if they are faulty (except the poor quality rotor arms that look and often are faulty from brand new)

Unless it's fairly new I would also recommend you change the dissy cap too as it's not expensive

I also buy new very good quality ignition leads as they will last for years and if replaced at the same time as the rest of the ignition components the whole replacement package gives good running and maintainance results, preventative maintainance and piece of mind
Nigel Atkins

This thread was discussed between 28/06/2010 and 10/07/2010

MG Midget and Sprite Technical index

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