Ignition timing for MkII Square Four

[paul.jameson]

Can anyone tell me what the advance should be on a Mk II Square Four? Waller says TDC fully retarded, which is fine, but I need to know either the degrees before TDC fully advanced or the distance before TDC fully advanced. All contributions gratefully received.

[will_curry]

In the Electrical Equipment section of Waller it gives test data for the
Square 4 distributor.

"Centrifugal advance begins at 200-350 revs (distributor) with maximun
advance of 21 - 23 degrees at 2,300 rpm"

It's not clear to me if the 2,300 rpm is crankshaft or distributor.

[Paul_Linden]

When I was setting up my Square Four from scratch I looked at the information in Waller and read it as 22 degrees of advance on the advance-retard unit, not 22 degrees on the crank BTDC. I also took the 2,300 rpm to be at the crankshaft.

[paul.jameson]

Many thanks for that information, both of you. Working from the iron 4G advance of 5/16" BTDC I calculated that the iron 4G had an advance of 38 degrees before TDC so with the higher compression ratio of the Mk II 22 degrees at the distributor equating to 44 degrees at the crankshaft makes all the sense in the world to me.

However, the advance is governed by the size of 2 holes in the plate on the main distributor shaft. On the underside of the plate there is stamped a figure which I assume is the advance provided by the holes in that plate. On the only main shaft I have, which is the one damaged in the fire on my Healey, the figure reads 15 degrees. Now there is no certainty that the distributor originally fitted to my Healey was genuine Ariel, or from the correct year to match the engine, since the Healeys sought parts from wherever they could find them. So it could be, for example, a shaft from an Austin 7 and timed by the Healeys by ear and road test (in the same way that I have timed the Healey subsequently).

Further comments are welcomed.

[will_curry]

Although I've not done it I know of the holes in the plate being filed out to increase
the advance range. It occurs to me that the further out the bobweights have to
fly the more revs will be needed to overcome the increasing tension in the springs.

[nevhunter]

I've generally gone for about 35 degrees full advance. You'd need a well designed combustion chamber to run less. The least I've seen recommended is 28 and 26 for twin plugs. The Racing JAP KTOR's had 45. That's for a combustion chamber that resembles an orange peel. It's there to help it start and idle to a great extent so the spring tension gets involved there and the second spring is the stronger one coming in at higher rpm. The weaker one affects the idle and starting.. In the absence of a tester you might use a suitable driil (or Lathe)to spin it.(1/2 engine speed) Idle rpm about 1000 engine rpm and it HAS to drop back or the idle speed will stay high or be irregular. Advance sluggish and the acceleration will be dead compared to what it could be Nev

[david.anderson]

Paul
My mk2 is exactly the same as your 4g. I set 8mm or 5/16” fully advanced which is tdc fully retarded on my 4. I always set the fully advanced figure as it is quicker. To accurately set tdc means setting a piston stop and fitting a degree wheel. I did that the first time and then measured what full advance was and have subsequently set the timing fully advanced. It is also where my performance is best although my compression ratio is up a little. So there may be some differences in the distributor advance plates in general.
David

[Roger Gwynn]

1949 dissy 40124 A and D had 23 degree advance on the dissy
1950-54 40124 E,F,H & J 15 degree advance
1955 40264A 15 degree
1956-58 40486A 15 degree
Bruce says whichever model use 5/16 fully advanced

[simon.holyfield]

Bruce says whichever model use 5/16 fully advanced

Rather than set it to TDC fully retarded?