Strobe timing at tickover or high revs

[Ian F]

Hi Geoff, the good (nay excellent!) news, is that these old engines will run when we'll away from optimum settings. When you get to drive the thing, listen for pinking at low revs, if present, retard the ignition until it is not discernable above all the other racket! You won't be too far away from a useful setting,

Ian F

[Ian F]

Distributor advance curve from DD

Not sure whether this will be legible, but here is the distributor advance curve for my rebuilt 25D4 from the DD.
It is a little difficult to interpret as it refers to distributor rpm etc.

Ian F

[GeoffK]

Hi Ian. With regards to your chart I am assuming that the black lines are the upper and lower limits and the red crosses are where your distributor sits within those limits. I assume that the distributor revs are the same as the engine/crank revs because if they weren’t nowt would work. Do you know what the distributor angle refers to? I assume that the black lines flatten out because the distributor stops advancing which probably equates to the 32 degrees @ 3000 rpm that I referred to earlier in this thread.

[Bumpa]

Because the camshaft turns at half the speed of the crankshaft, distributor rpm is half the crank rpm. So according to the red crosses on the graph, at 1000 rpm (crank), which is 500 on the graph, the ignition timing is 13.5 degrees (10 degrees static timing as per manual, plus 3.5 advance from the distributor). The maximum advance for the red crosses is 20.5 degrees, which seems a bit low to me. The workshop manual states maximum advance to be 24 degrees. However, I'm sure the Distributor Doctor knows what he is doing!! Maybe it's to compensate for modern fuels.

[Ian F]

Apologies for the late reply. Geoff, you are correct about the black lines and the red crosses, so the max distributor degrees shown as plateauing at circa 10 degrees I think must equate to circa 30 degrees ignition timing (which is stated to be optimum). So x3.
The distributor, I believe, turns at half engine speed. So perhaps the graph is calibrated to very excessive engine RPM? For race engines?
My reading of the graph suggests minimal distributor advance below 600 RPM, which coincidentally is the recommended speed for setting the base ignition timing.

I have managed to find these data for the 18V MGB engine in a book "B Series Engine Data"

My reading of this table suggests that the distributor advance in the DD graph should be added to the static advance, giving a total of circa 30 degrees advance. I think this all adds up now, and you can read off the advance at any rpm, not forgetting to add the static advance.
Mike - your 24 degrees max advance looks like the max distributor advance (only)?.

How does this look to you both?

Ian F

[Bumpa]

I expect you are right Ian. I would expect max advance to be around 30 - 34 degrees total, as it is for my V8. I must say I find DD's graph confusing. As for setting the timing at 600 rpm - you're 'aving a larf! I have never managed, in over 50 years of tinkering, to get any engine without engine fuelling and spark management software to smoothly idle at such low rpm. If I can achieve a smooth idle at 900 - 1000 rpm on my carburetted and distributor equipped cars, I reckon I have done well.

[Vic Butler]

Bumpa, when I timed the 2.5 petrol engine in my Series 3 Land Rover by ear, I had it running so slowly that the ignition light was on, even with an alternator. It is possible.

[Ian F]

Just checked my records for the MGB (I scribble in the back of my Haynes manual), and every record states: "ignition timing set at 17 degrees at 750rpm". I think I have found this setting works best for my engine, so I just use this as my benchmark.
I thought I should check exactly what I had been doing rather than guess!!

Ian F

[Paul Hollingworth]

I plotted out the curves based on the data from Paul Hunts website. If you look it comes from impecable sources and I've seen nothing so far on this thread to contradict it.
You will see that there is only 20 degrees of centrifugal advance in the 25D and its all done by 2200 rpm for advance curves 40897 and 41288. These are the curves recommended by Peter Burgess for all but the most highly tuned engines. I can only assume that the combustion gets faster by its own accord at higher revs probably as a result of the higher air flow.

you can see why I go for 20deg at 1000 rpm. This will work for both the above curves. Easy to set as its a whole number of teeth on the scale.

[markvrs]

I have always liked John Twist of University Motors very simple view when he did an MGOC seminar a few years ago in the UK. Just set max strobe advance at 32 degrees. Everything below that falls into place. That's were you will be most of the time when driving, and then just raise the tick over a smidge for smooth idle if needed.

Always done it that way as just a few revs can quickly change advance at low (tick-over) revs on an old 25D, but advance levels off quickly over 2000rpm , so easy to rev engine up and just check it maxes at 32 degrees advance.

Now I have seen the data and graphs on here I can see the reasoning.

Cheers, Mark