Ignition problems, another no sparker! Advice please FIXED

[RichardS]

Interim report

My knowledgeable friend and I have done some detective work

We turned the engine over and behaving very strangely, with some misfires and popping. Also noticed when switching ignition on (but not starter) a slight fart from no 4. So initially wondered about timing or the distributor having moved, but this seemed unlikely as the distributor was tight. Checked dizzy cap and no signs of arcing.

Still getting regular spark from coil HT lead to earth. We then established there was a weak spark at the plugs, which was visible in the evening light. We then took all the plugs out, with leads attached, and lay them on the rocker cover to earth them. Bizarrely when the ignition was flicked on and off one of the plugs was briefly fizzing with a spark (without the engine turning over). We surmised that the electronic ignition must be switching on/off randomly when powered up, and sending HT through the post nearest the rotor arm. We could still hear something when switching the ignition on and off and found that there was arcing from the base of the HT coil pillar, to the coil negative terminal.

So I wonder if the arcing from the coil HT pillar has travelled up the coil neg lead to the Aldon ignitor unit and damaged it?

So along with the new dizzy cap and leads (which were probably innocent!) looks like a new coil and electronic ignition needed. The Bosch blue line coil and Aldon ignitor were fitted about 10 years ago and have probably done less than 2000 miles, which is disappointing but looks as though time rather than useage (or lack of useage) has caused deterioration.

Am looking at a replacement from simonbbc who seem to offer a reasonably priced Powerspark unit.

Will report back when sorted

Richard

[RichardS]

Eventually sorted, with more detective work, and multiple issues that came together to keep us guessing!

1. The coil was faulty. Although it was generating a regular spark (tested by removing coil HT from dizzy and observing spark to earth) close observation showed arcing from the HT post to the negative terminal. Cleaning the coil did not cure this so coil replaced.

2. Tested the positive ignition side of the coil with a 12v lamp (good testing procedure from an old auto electric book which I will post) and found the cause of the fizzing noise (and presumably the fizzing at the plugs). When turning the ignition key to the ignition on position the light was flickering at times, suggesting problem had to be on ignition side. Removed, cleaned and nipped up the spade connector for the white (hot ignition) cable on ignition switch and another issue resolved. https://www.ebay.co.uk/itm/6V-12V-24V-C ... 635-2958-0

3. Had taken the Aldon ignitor unit out of distributor and tested () and this seemed to be switching on and off ok when the magnetic collar was moved in front of the sensor. Also doing test 3 from the book showed the 12v light switching on and off when engine turned over, confirming Aldon powering coil on and off. https://mgaguru.com/mgtech/ignition/pdf ... e_test.pdf

4. Purchased a set of 4 Lazer spark plug testers which fit in line with the plug lead and light up when the relevant plug receives an HT burst. Gives a great visual and led to another fault. (Much easier than taking plug out and testing for spark) When I reconnected the old Micro Dynamics Rev limiter which is wired to the coil the plug lights went out, so disconnected Rev limiter and all well! https://www.amazon.co.uk/Laser-2780-Ign ... B003AN1VPW

5. Still wouldn’t start and we were baffled. Had fitted new Magnecor dizzy cap which came with pre fitted numbered leads. Checked and leads seemed to be attached to cap correctly. But ….. on inspecting old dizzy cap and leads realised the plug leads were connected differently. Set engine to no 1 TDC and dizzy rotor arm points to around 10 o clock rather than the normal 2 o clock. So dizzy 90 degrees out from usual position (presumably set this way by Vegantune who rebuilt the engine in the 1980s as I haven’t altered this since. Not sure of the reason for it being fitted this way). Fitted old dizzy cap and leads back and……she fired up first turn of the key! Can hopefully reconfigure Magnecor leads to suit, or remove and refit dizzy
to normal configuration
So perseverance paid off. Frustrating but eventually satisfying. Must now go out for a drive.

Richard

[RichardS]

LT circuit tests

[HCA]

You were told earlier on that with everything else ok, the coil was breaking down! But no matter you got there in the end!

But, why are you moving the distributer all over the place? Why not just reposition the cables..?

[RichardS]

It wasn’t just the coil Hal, but a good learning experience! What fooled me was that the coil resistances seemed ok, and generating an HT pulse. It was only by looking carefully one could see the arcing. Nothing obvious to see on the coil but presume a breakdown in the coil cap insulation - as you predicted!!
Agreed, as I said I will reconfigure the new leads rather than move the distributor.
Richard