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MG MGB Technical - Powerspark electronic ignition

After 10 years of changing points I decided to fit powerspark electronic ignition to my 1970 MGB, this is a simple points replacement in the original distributor.
Before starting I made the following observations @ 900 RPM. Dwell tachometer showed a dwell of 54 degrees, points indicator showed points as "bad" (half way into the red band), timing was set at 20 Degrees before top dead centre (vacuum advance disconnected), car was running okay.
It became obvious that the replacement electronic ignition was incorrect for my distributor as the point location peg fowled on the mounting plate of the electronic ignition mounting plate. I decided to move on past the point of no return and cut of the peg and secured the electronic ignition with one screw.
After making the necessary connections to the coil I was very pleased with myself when the car started. I dived under the car to set the timing with timing light and found that optimum running is when the timing mark is showing 70 degrees (approx.) after top dead centre ????
The dwell tachometer is showing a dwell of 18 degrees and the points indicator is showing "Bad" off the scale, car is running okay.
In simple language (I failed electro tech. in my engineering course Friday night class after the pub) what is going on? is my distributor now 90 degrees out of position
David Levy

It is not uncommon for electronic ignition to change the 'trigger phasing' by a significant amount. Phasing is the relationship between the timing of the spark and the position of the rotor relative to the cap contact. This relationship varies with vacuum advance (but not centrifugal), which is why you usually have burning along a significant portion of the rotor edge, and it may go to one of the edges with points. If another system changes the phasing, the rotor can have left (or not reached) the cap contact when the spark occurs, which means the spark has to jump a much bigger gap than usual, which means the HT voltages in the system are higher than usual. I've seen examples where the spark occurs when the rotor was between cap contacts, so it was a matter of luck which plug fired, and the engine ran terribly.

The dwell dropping from 54 degrees to 18 degrees is probably showing the the unit you have fitted has a variable dwell feature, and as such is fine. Whereas points - which are fixed dwell - give a coil energising time that is way too long at idle, variable dwell gives a relatively short pulse, which is plenty long enough, and helps keep the coil cool. Your points indicator is almost certainly expecting something in the 45 to 65 range.

If you are still clipping your timing light onto the lead that goes to No.1 plug, then it's difficult to see how the engine could run with the spark occurring 70 degrees ATDC instead of 10 to 15 degrees BTDC. But if you are going by the cap contact, then it's possible the leads have been moved round one position on the plugs themselves.

David, I've had one of these kits for 4 years on my 18v engine and have had no problems at all. I'm not sure what engine you have in your car or distributor, but the attached link to a video may help you with the installation.

Andy Robinson

Thank you for your responses, I have spent a number of sleepless nights trying to understand what is wrong with the timing and at this stage I have given up as the car is running okay.
Another problem that I have experienced since fitting the electronic ignition is on start up the starter motor labours as if the battery is dead before spinning up to normal revs and starting as a result I replaced my six year old battery with a new one no change, I removed the distributor cap and started the engine and the starter motor immediately went up to full revs, replaced the distributor cap and again the starter motor laboured like a bad connection or a dead battery, any suggestions.
It looks like I will be making a trip to the auto electrician and may have to fit a new distributor, coil or both.
Moral to this story, if it ain't broke don't fix it
David Levy

Just a thought, your first timing was 20 degrees BTDC your second 70 ATDC = 90 degrees. Haven't worked out what that means yet!!
Struggling on the starter might indicate that there is a cylinder firing really early on the compression stroke. This will create very high combustion chamber pressures hence the reason why the starter is struggling.
I think Paul has it and the electronic triggering is out of phase. Also the fact that you had to "adapt" the unit would suggest a misalignment, possibly the wrong unit for you dis.
Don't run it, you could do a lot of damage. Put the points back on for your trip to the electrician.
Allan Reeling

Like Allan, it sounds to me like it is over-advanced, so combustion is almost stalling the starter.

Is it low compression or high? I've heard of low compression being so far advanced it will stall the starter, but running doesn't cause audible pinking or detonation. On a high compression this should be very obvious - and potentially damaging as said.

This thread was discussed between 09/07/2016 and 30/07/2016

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