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MG MGB Technical - Definitive Answer Please

Ok, final throes & trying to get the car on the road. 1975 mgbgt rebuilt hif4 carbs, pertronix distributor module & red distributor arm, manifold vacuum. I am reading lots of differing views so here goes. What is the dynamic timing at 850 rpm. Haynes has one value & lots of other sites say different. I would like to get the ignition timing correct before I move onto the next step. Compression is fine btw, & I'll recheck tappets as well. I'm trying to set up the carbs but am struggling until I have a definite answer.
Pat Gregory

If it runs I would go to the other end - set the advance at 32 deg @ 3500 rpm vacuum line blocked using a dial back timing light or extended degree scale on the cranshaft pulley. Then hook up vacuum advance and adjust carbs for mixture and synchronization maintaining rpm at 1000. Once done you can reduce idle to 850 if you wish.
Mike
Mike Ellsmore

Definitive, ha ha!

As long as your ignition system is working _correctly_, it doesn't need to have the _perfect_ timing before you can move on to setting up the carbs. In fact, until the carbs are set up correctly, you can't expect to set the ignition to its ideal figure.

The original timing figures - and more importantly the curves - were chosen to produce the best performance. These are not precision engines, and given the manufacturing tolerances on many components some engines will go into pinking and pre-ignition before others. For that reason there was also a safety factor applied to the figures which retarded the timing slightly so even the worst case wouldn't experience damage.

Also for that reason, with any particular engine you may well find that it will run perfectly safely with more advance than book, and gain both performance and economy. I found I could do that on an A-series engine in the 70s, in a new car which was regularly dealer serviced, and it always came back running like a dog because they had set the timing back to book. But in my 1973 MGB I found I couldn't run any more than book without pinking. If you read Haynes you will see that they recommend advancing the timing above book in small increments, accelerating at full throttle from 30 to 50, until just a trace of pinking is heard. However I also find that pinking can occur on part-throttle acceleration up steeper hills whereas it doesn't on full throttle, so you have to bear that in mind.

All that was on the original 4-star leaded which was 99/100 octane, I believe. With basic unleaded at 95 octane and super unleadeds at anything from 97 to 99 octane the situation changes again. I have found pinking occurs very much earlier on 95 octane, and even though I usually run on 99, when touring in remote areas where I can only get 95 I've had to retard the ignition at the road-side to control pinking, which saps performance and increases running temps. For that reason the 25D4 distributor with its vernier adjustment has come back into its own.

And none of that takes account of the timing curve! The mechanical distributor is pretty crude, and is very limited in how it can alter the timing through the rev and throttle range, basically two springs on weights that result in two straight lines with a 'knee' rather than an actual curve. The effect of that is the actual curve probably only touches the ideal curve in one position. Added to that even those 'curves' were designed for leaded, which we means we are now further and further away from the original, even ignoring changes for component wear, replacements with non-standard parts, and modified engines.

Basic electronic ignition does not change any of this. The very expensive 123 overcomes the issue of distributor wear, but if you bought the original one for the MGB it only came with the original curves designed for leaded, and some vendors recommended the generic version as being better for today's fuels. It's only if you buy the Tune version, or one of the other (significantly cheaper) programmable ignition systems available, that with the aid of a laptop you can develop the ideal curve for your engine and fuel, which involves much belting up and down roads (for most of us) listening out for pinking for each rpm band, and tweaking the curve accordingly.

Without that, you are back to the original Haynes advice.
paulh4

one little thing I'd like to add here is if using the method of 32 deg at 3500
That's ok BUT after setting it at that you have to rev it up past that point to make sure the mechanical advance has reached it's limit- There is always the possibility that there is heaps more advance left in the dizzy which could cause engine damage
William Revit

I refer to my previous thread were I plotted out the advance curves from PaulH's Bee & Vee site. I agree if it doesn't pink then sneak on a few degrees more advance. I wouldn't go more though 5 though.
The curves are for vacuum pipe disconnected.


Paul Hollingworth

That shows exactly what I meant-
If that 45D4-41610 (red line) was set at 32 at 3500 it would go out well past 40 at higher speed----BANG
William Revit

The curve diagram makes a lot more sense now, Haynes gives 11 deg for 18v779/780, but the car was happier at closer to 20 deg when running at 1000rpm, which is borne out by the curve for 25D4 which is fitted. I will start at 20 deg approx & work from there. i was lent (lended?) a compound meter which the made things worse as i couldn't get a decent vacuum figure. I will persevere as is & update accordingly. Thanks for your help.
Pat Gregory

As Willy suggested, make sure you are reaching maximum advance by revving way past 3200 rpm. Maximum mechanical advance is positively limited. A worn dizzy can go significantly past the intended limit however and may need modification to get back to original specification. Quite often just a thin "sleeve" on the spring post.
Allan Reeling

This thread was discussed between 15/06/2018 and 24/06/2018

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