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MG MGB Technical - Ignition Timing Question

Hi, I have a 1978 MGB GT, which has a mild road cam, sports exhaust and K&N pancake filters.Today I fitted a Luminition Optronic electronic ignition system complete with Mega4 coil. So far so good. I was quite happy with the ignition timing before I started so I checked it with a timing light and made a mental note of the setting. I do not fully understand the timing marks, but from the right it was between marks 2 & 3. After fittimg the kit it did start up but the timing had moved to between 1 & 2 marks. I adjusted the timing back to where it had been, and again so far so good. I then read that one should keep advancing the ignition until the engine pinks in top gear, accelereating from 30-50mph. That is when the problems started. I advanced the ignition off the scale and beyond, but no real pinking? In the end I retarded it to near the original setting and gave up, hoping that is about right.
So, my question, what do the timing marks mean, and have I done it right? I'm guessing that the notch to the right is TDC, then notches left are 2deg to 8deg BTDC. The dynamic setting for my engine is 10deg BTDC, which would appear to have no mark?
Paul
P Sorensen

Does your car have the notch on the flywheel and the fixed "comb" on the pressed cover? If so the marks are TDC ,the long one, plus 5 plus 10 and the engine rotates CW viewed from the front. I had a lot of work to do on this when changing to electronic ignition, it's not surprising as you complely remove the mechanical load from the cam. In the end I just set it to where it idled smoothest. This was bit advanced as it pinked a lot, but I just retarded it a degree at time until it only pinked under provocation on gradients that I would not use that combination of RPM/throttle openeing on and left it set there, adjusting the tickover to suit. When I checked the mixture it came up as rich on the lifting pins but only by 1 flat on the rear carb and 2 on the front. It was spot on before this change.
Stan Best

Hi Stan, I have the notch on the front pulley, and above that 5 marks of equal length. The manual states the mark furthest to the right is TDC, and I ended up way off the scale to the left, and no real pinking.
P Sorensen

When changing ignition systems, even points or adjusting the gap, you will change the timing so I'm not surprised the timing changed as it did. You were wise to note the timing before you started, and should put it back to that if that was fine before.

TDC is the last pointer the pulley notch passes, the others are at 5 (and 10, 15, 20) degrees before that, 5 and 15 being lower than the others. A 78 should have the pointers above the pulley and slightly towards the right-hand (looking forwards) side so the distributor can be adjusted while looking at the marks.

Because todays fuels are so very different to originally even with the higher octane variants (and almost certainly with standard unleaded) you often get pinking at the factory timing values, and have to retard it until you don't get pinking at *any combination* of throttle opening, load and rpm, not just when accelerating between 30 and 50. My 73 will pink when climbing hills in lower gears, but not at 30-50 on the flat, for example. Originally because of manufacturing variations and manufactuirers safety factors most cars could be *advanced* (stopping short of pinking) beyond the book figures and get a corresponding improvement in performance and economy as a result. I'm surprised you don't get pinking when advancing beyond the book figure, unless your engine is lower compression than usual.
Paul Hunt

When I put the Classicheads ingnition on my car the timing marks did not stack up with the timing I measured with it. So I set the dizzi close to it's previous setting and then just blatantly twiddled it until it idled nicely as noted before, it needed some advancing to do this about 6 degrees from memory. As I say this proved a bit too advanced but not hugely and it's ended up maybe 2 degrees advanced on it's previous setting. The car goes well idles as well as it ever did (it's not cammed for low rpm :-) )and fuel consumption remains much as before. The temperature guage behaves itself so I'm happy with it. It would be interesting to check the timing dynamically and see what is really going on as the apparent advance needed may actauly just be compensating for the magnetic pick up firing at a different angle of rotation. But I have not done this.
Stan Best

"actauly just be compensating for the magnetic pick up firing at a different angle of rotation"

That's 'phase shift' and indeed what can happen when you change trigger types. If it shifts too far it can cause the rotor to have passed (or not yet reached) the cap contact when it fires. In extreme cases (like a magnetic trigger in my possession) it can fire between cap contacts with dire results.
Paul Hunt

The optronic system I have fitted works from optical sensor rather than magnets. I have left the timing set a little advanced from where it was before, and it seems OK.
P Sorensen

The same thing can happen regardless of whether it is magnetic or optical.
Paul Hunt

Thanks Paul, I assume that's why it's so important to get the right fitting kit for the distibutor.
P Sorensen

Absolutely.
Paul Hunt

This thread was discussed between 10/08/2008 and 13/08/2008

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