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MG MGB Technical - Timing electronic ignition

Hi,
This one has me puzzled...the car's a '66 BGT with a stock engine except for K&Ns, richer needles and Magnetronic electronic ignition. The car's been down on power and a bit rougher than normal so (amongst other things) I checked the timing and discovered it was miles out - off the scale, probably at least 20 degrees BTDC. I dynamically set the timing to the book figure, but the engine ran really badly (barely keeping going). The only way I can make the engine run reasonably is by putting it back where it was!

Now, when I installed the Magnetronic, I had some trouble with the magnetic disc. If I read the instructions properly, the small slots in the shaft of the disc go around the peaks on the distributor shaft. Try as I might, I couldn't get it to fit like that so I figured I'd fit the disc with the wider section between the slots over the peaks on the dizzy shaft. I'd then just rotate the distributor to put the timing back to where it should be. Is there any problem with this (the disc still fits tightly on the distributor shaft)? And why does the timing have to be so far out just to make the engine run reasonably? When I first fitted the Magnetronic unit last summer, the car ran brilliantly, it's just got progressively worse.

Can anyone shed any light or ideas on this!?

Thanks, Tim.
Tim Jenner

Tim - since your Magnetronic magnetic disk is not aligned per instructions, you may have a situation where the rotor is misaligned as well. If the disk is aligned in such a way that the ignition triggers between the cam lobes (as opposed to a points system that triggers at the lobes) then the spark could have a large gap to jump between the tip of the rotor and the proper distributor cap terminal. Normally, the tip of the rotor is wide enough to handle advance variations, but if you are far enough off, rotor tip wear and cap terminal wear could cause your performance to deteriorate over time. Rotating the distributor body moves the pickup and the cap but not the rotor.
Just a thought - I may be way off base here.
Paul Joly

Tim-
More information would be useful as we will be taking potshots at your problem. So many things will cause rough running. What you need to do is to do a thorough tune-up in step- wise sequence. First, re-torque the head, re-adjust the valves (this is one good bet for the problem). Renew cap and wires, re-gap plugs (better yet replace), then set the timing at the distributor. If this does not work well, you will need to verify that the timing mark is correct. (The front pulley is rubber laminate between two pieces of steel, it can shift and be wrong.) Find TDC by watching the number one cylinder come up while the intake valve is opening. You can then see if your timing marks are accurate. (Personally I tune by sound, advancing the spark on the distributor until the engine is smooth, revs well and doesn't pink under load.) After this you can then set about adjusting the carbs for balance, making sure the dashpots are working well, et cetera. During the plug change, it wouldn't hurt to do a compression check, as a burnt valve, or a recessed seat will give rough running. I tune my car every 6 months however it runs, just to keep on top of things, including my own knowledge. FWIW---Tom
Tom Lennon

Thanks for the responses, guys. More info - a new unleaded head was fitted about 2 years ago, I set the valve clearances, changed and gapped the plugs, etc about 6 months ago when I did a full service. I assume TDC is marked correctly - it's always been fine in the past, but I will check this evening. I'll also try again to fit the trigger disc in the right place!

Thanks,
Tim.
Tim Jenner

I have an almost identical setup to you (magnetronic , k&Ns richer AAA needles etc) on a 78 BGT and I too have to advance the igintion about 20 degrees for full power. Areyou using unleaded fuel with an additive. I will check the position of my 'dots' on the disc compared to the rotor
Martin Gunn

A couple of years ago I fitted a magnetronic ignition to a '72 B. I seem to remember I had to soften the disc in warm water in order to be able to fit it over the dizzy shaft. Have you tried this?
Steve
Steve Coulson

Hi guys,

I'm using neat unleaded since I have an unleaded cylinder head fitted. I tried to check valve clearances and the timing mark last night but rain stopped play - I managed to check the plugs (virtually as-new condition, correct brown coating on tips) and rotor arm and cap (both as-new condition). I also had time to try to get the trigger disc repositioned but still can't do it. I'm beginning to wonder if I'm reading the instructions correctly - I'll try to explain, sorry if it's confusing!

Inside the shaft of the trigger disc there are 8 raised ribs arranged in 4 pairs. The ribs which form a pair are approx 5mm apart, and the pairs are opposite each other (so there are 4 pairs equally spaced around the shaft). Between the ribs of each pair, the plastic is raised slightly higher than it is between pairs - thus I can't fit the trigger disc such that the cams on the distributor shaft fit between the ribs which are in a pair (because the plastic is raised). It fits easily if the cams are between the pairs. The instructions say put the cam lobes of the distributor shaft "between the raised internal ribs on the trigger disc" - is that between the close together pairs of ribs or between the further apart ribs which form the edges of 2 pairs (as I have it)? The instructions also say "align the dot with the tip of the rotor arm" - I took that to mean the dot should be at the leading edge of the rotor arm, which is only possible if the disc is aligned with the cam lobes between the small rib pairs (i.e. the way I can't get it to fit)!

This is driving me nuts!

Cheers,
Tim.
Tim Jenner

It sounds like the phasing betwen the trigger and the rotor could be the problem. Remove the plugs and the dizzie cap and mark the body where the leads are then put a timing light on the coil HT lead (which may need to be connected to a plug resting on the block). You should see the rotor frozen in four positions, and ideally the marks where the the plug leads go should be at the *trailing* edge of the rotor arm. If, while doing all this, you can suck on the vacuum capsule hard enough you should see the rotor arm appear to move clockwise, moving the marks towards the leading edge. The ideal position is where maximum and minimum advance cause the marks, and hence the spark, to be in the central section of the rotor arm contact with the same amount on unused space each side. Even with standard points, looking at the burn marks on the rotor you can see that phasing isn't perfect.
Paul Hunt

Paul,

Thanks for the suggestion, I'll try that. Looking at the rotor arm, the burn marks run from about the centre of the arm right to the trailing edge - there's no unmarked metal at all at the trailing edge. Sounds like this could be the problem!

Cheers,
Tim.
Tim Jenner

If you have an old distributor cap, you can drill a fairly large hole between the no. 1 tower and coil tower. Run the engine with this cap and look at the rotor position with a timing light. This does the same thing as Paul's suggestion - just in an engine-running mode.

Wayne
Wayne Pearson

This thread was discussed between 28/06/2002 and 01/07/2002

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