Weak Spark (on MGA)

[malcolm]

I put the MGA through an MOT this afternoon to celebrate her 50th birthday, then I remembered why she has been off the road for so long.

She has a ballasted 6V coil, and the 12V feed from the starter has broken so I've set up a spare feed on the dash. Returning from the MOT she started missing at low revs (below 2000rpm) and conked out on the driveway. Switching the coil to 12V got her going again, but back to 6V and she'd fail once more.

A weak spark, but I've changed everything - distributor, coil, contacts, capacitor, rotor arm, ballast resistor. I've fiddled with the leads (the silly ones located to the distributor cap by means of a screw - I've cropped the ends of the leads to make a new screw hole).

Also it's temperature related - only happens when hot - maybe 10 miles driving.

I'm baffled. Anyone have any ideas or thoughts about what could cause temperature related spark problems?

 

[resto]

have you checked for earth faults??????

 

[malcolm]

Good call - tomorrow I will check I had a very similar problem with an MGB V8 fitted with a Mallory distributor. Took me 2 years to trace that to the earth in the distributor (really bad design on those).

The earth in this distributor is an actual wire, and I had the same problem with the luminition distributor which shouldn't need the earth, but I've not checked the engine earth yet.

 

[Steve Hennessey]

Malcolm, did you substitute with a new coil or a known good one? Fell into this trap myself once when I had two duff coils laying around so then tried a brand new one and it worked!! On the MGA does the coil need to be ballasted?

 

[pepper]

The other one that got me on my Morris Oxford MO was the rotor arm, it was fine when it was cold but would fail when it got hot..

There are some really cheap nasty rotor arms on the market these days, the quality isn't there any more.

 

[angel]

I was just about to tell you about the earth inside the distributor but you thought of it earlier...very weak point on these Lucas distributors.
I don't recall the MGA standard coil to be ballasted...I suppose you had changed it to a non standard type?

 

[malcolm]

The car has been modified a little When first investigating the problem I found a fault with the rotor arm breaking down when hot. Might be worth checking the replacement. Might rewire and try a 12V coil now the starter feed for the 6V has broken.

Thanks for the ideas - you have picked up a few things I've not been careful with. I'll go around the car again and see what I can find.

 

[malcolm]

I've had a fiddle today. When the car is hot and misbehaving it can be fixed temporarily by changing the rotor arm for a spare cold one. Both are Lucas unfortunately. Chinese don't seem to have the hang of materials yet. High carbon plastic apparently.

Where do you buy the rotor arms for the Oxford Pepper? I'd imagine we are running the same distributor - 25D? Found a seller of fancy blue ones which might be worth a shot.

 

[pepper]

I normally pick up a rotor arm or two from NOS at places like the BL rally at Peterborough, BTW now you've got your MGA MOT'd did you know about this great event??

Its not publicised enough really because its a fantastic day out.

My Distributor is the same as the MG TD, its an earlier engine, (side valve)

 

[pepper]

Here's a link for that BL rally

https://www.tiscali.co.uk/entertain...r_corporation_british_leyland_owners_day.html

 

[malcolm]

I'd not make it there without an operational rotor arm. Besides 2nd August clashes with the fabulous Renault 4 BBQ in Bedford.

I'll try one of those brightly coloured new improved parts and see if they work. Failing that I wonder if I could make one Pity I had a tidy up a couple of years ago - must have had an original one I could have used.

 

[pepper]

ops: I'd hadn't realised they were on the same day!

 

[malcolm]

I mailed the American fellow with the nice red rotor arms. He suggested also not using the silly distributor cap with screws that dig in to the leads as that system doesn't really work with carbon leads puts a lot of strain on everything else.

I'm not really a fan of car meets, though did go on the Bromham (local villiage) classic car run last weekend in one of those nice reliable Renaults.

 

[Steve Hennessey]

Arrrhhh - I didn't know you were using silicon leads with a 'grub screw' cap designed to take copper cored HT wire. Could explain a lot. Nothing wrong with those caps but you will need to use the correct HT lead. It just so happens that I have some copper lead as well as a 'vintage' Ripaults kit to make up leads. If you end up going for a new cap with push in connections I can help there as well.

 

[malcolm]

Them leads might well be the main problem. The later MGB leads and cap should sort me out. Though they worked for 10 years! Bad system though I'll agree.

 

[malcolm]

Fixed it. Many thanks everyone for your help

I found a new unbranded rotor arm in a box of bits, popped it on, and the car now works when hot. The box also contained a new set of copper cored leads which I'd forgotten about - they're on now. Luminition ignition is back in place again (I'd had it on points to rule it out), and the car is now fab on hot idle.

I've ordered a pair of non-carbon plastic rotor arms from Distributor Doctor. which should hopefully be a longer term fix. (same as the American ones I found on Advanced Distributors).

 

[angel]

So you think it was most due to rotor arm than plug leads?

Thanks for this useful hint for us who work mainy on english classics for a living...

 

[malcolm]

Definitely it was the rotor arm cured the problem - I was careful to test that independently - the pattern part I tried was OK. I didn't test the ignition leads separately from the change from points, but I suspect it was changing the leads that further improved the running (points were in good condition so should have been working fine).

I suspect the replacement Lucas rotor arms have low resistance, and the resistance of carbon rather than copper leads made the rotor arm the shortest path to earth. Others complaining about them are often using "high performance" coils, so maybe they just about work as standard but fail when something is slightly different. Or my worry - maybe they are failing a bit all the time.