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MG MGB Technical - 25D dizzy timing

Hi,

Swapped an old 25d dizzy on a 1972 car (will refurb or get a new one)for a seemingly better one. Car would not start without a great deal of persuasion. Checked the timing, this was now at about 15/16 deg dynamic timing 1500 rpm. But the car would not start easily even when hot; it also pinked at acceleration. Retarded timing in stages to 5 deg, no pinking, and seems to be running well again and starting on the button when cold.

Without taking dizzy off again to get the numbers, is this a mini type of dizzy and should it be ok to run the engine on tis kind of dizzy?

Ta,

Iwan
Iwan Jones

It sounds like the timing marks, on your harmonic damper, have moved due to delamination of the rubber between the two halves. To be certain of your timing, bring #1 piston to TDC, on the compression stroke, and observe where the timing marks align. RAY
rjm RAY

Iwan. Without knowing the specification number of your new distributor, neither you, nor any of us, can determine if it is suitable for use with your engine. Both the mechanical advance curve and the overall advance need to be within the proper specifications for your particular engine. This must be determined by looking up the specifications for your model year of engine, then comparing the performance of your distributor to the factory specification. (The distributor can be tested either on a distributor test machine or, if you have a dial back timing light, on the engine.)

Ray may be correct in his suggestion that the harmonic balancer is delaminating. I paint a thin white line on mine when I rebuild an engine to form a future reference. If the line begins to break up, the balancer is delaminating.

The best way to determine top dead center is with the use of a dial indicator used on the top of the cylinder when the cylinder head has been removed. Some use some form of bump stop which screws into the spark plug hole, but this is not as accurate a method. One thing to note is that top dead center is top dead center on both the compression stroke and the exhaust stroke--the piston reaches top dead center twice per firing with a four stroke engine.

Right now, you do not know the specification of the currently installed distributor. You do not know the mechanical advance curve of the distributor. You do not know if the timing marks on the harmonic balancer are correctly aligned with the crankshaft.

Under these circumstances, I would not drive one of my cars until I knew, exactly, what I had and had ensured it was close to being factory correct.

Les
Les Bengtson

Hi, thanks for the info. Unfortunately, this car is some distance away from my home. Also will not have time till the weekend after next to haul out this 'old' dizzy and look at the numbers. I'll come back to you when I do.

Ta

Iwan
Iwan Jones

Pretty well any engine should be capable of running with any 23 or 25D4 distributor - up to a point. There are hundreds, with different centrifugal and vacuum curves, and how well any engine will run depends on how well the curves match the engines requirements. You could have a lack or power, or it could pink, depending on whether the curve is less or more aggressive respectively than your engine would like. As even the original curves and specs are not really ideal anymore with today's fuels, especially if you use 95 octane, the best you (on a high-compression engine) is run the most advance you can without getting pinking at any combination of throttle opening, revs or load.

You say it was an old distributor, what condition is in in? What about the points both as far as condition and gap goes? What about the condition of the springs? These go slack and give a more aggressive centrifugal curve as they age. Do you know what your old distributor was set to? You can't just bung in a different distributor and expect the timing to be the same even if you have noted where a certain point on the old distributor body was in relation to the block and position a different one to be the same, you should still have done a static timing check before trying to start it.

The static for a UK 72 was originally 10 degrees and the dynamic 16 degrees at 1600 rpm. That's close to what you got, but you really need to have the vacuum pipe disconnected and the carb port blocked as at 1500 rpm you will be getting some vacuum advance as well as the centrifugal you need to be measuring. Removing an element of vacuum advance would *reduce* the figure you saw, but with an unknown distributor you could be getting significantly more than the maximum of 20 degrees at 2200 rpm.
Paul Hunt

Thanks for your help all. Had time to have a look at things on this car. Its not mine so I don't know how it was set up previously. Was told it ran well before. .... Humph!

Car ran very rough and would not idle properly (too high) with 'original' dizzy. Had set static timing at 10 degrees & checked lots of basic stuff valve clearances, coil, leads, plugs etc. etc. Still not good.

Put in 'new' dizzy, I don't know where the other dizzy that in I put came from i.e. which car. Ran a bit better but could not back off idle, car still backfired etc.

Recently had time to look at things. Got a good 'known' dizzy, reset static timing, checked valve clearances. etc. etc. The same problems still not working that well. So decided to go back to basics.

Found that a few things were creating problems and masking each other. First dizzy was grimy and full of quite large gritty bits; how they got there I don't know.

Second dizzy, seemed to be 'catching' on the mechanical advance weights. Also, think a poor internal wire in the dizzy was causing intermittent grounding

Also, Carbs gummed up on the outside, they looked clean, but years of deposits would not allow idle to be backed off properly and also caused Carbs to sometimes stick.

Now Sorted.

Iwan Jones

This thread was discussed between 30/09/2013 and 23/10/2013

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