Morris Minor Owners Club

I have just read, with great interest, a series of posts from nearly a year ago about ignition timing. I have some questions.
As I understand it the vacuum "advance" actually retards timing under load to prevent pinging. Is this right?
If so, initial timing is set X degrees + or - TDC and once the engine starts the diaphragm pulls the timing ahead. Can anyone tell me:
How much advance occurs?
Is this adjustable?
Is the correct static setting TDC?

Darrell McDonald

Hello Darell,

the vacuum advance, (Some, I think mostly American versions have vacuum retard for emissions) advances the timing.

You experience pinging under load which drops the vacuum right back so the vacuum advance mechanism reverts to no advance.
Timing if set statically is quite straightforward, using a light or meter to whatever degrees before TDC is specified say 5 degrees. Few cars are set at TDC or after. If you set it with a strobe light it's possible the vacuum advance will affect the timing, so is normally disconnected to remove that variation.

The advance from the vacuum unit is set and there are a variety of them. They are stamped with figures for vacuum to start advancing, maximum advance in degrees and maximum vacuum to give full advance, and no they are not adjustable.

Alec

The static timing for the side valve engine is TDC and adjust by road test. However that was in 1948 when petrol was 80 octain. These days you could probably go for considerably more advance and you have no vacuum advance to worry about, that would be the road test bit. It would be useful if someone could come up with some figures for an engine using today's fuel.With the low c/r of a side valve the advance until it pinks method does not really work, you just get a fall off in performance
My engine has just been rebuilt but is considerably modified so would probably produce different figures on the rolling road but would give an indication. I don't know what the figures for an Alta are, I expect Graham knows

So - for example - my 1098 engine (with 940 head - so ~ 10:1 comp ratio) running along in top gear with light throttle - no pinking. I come to a slight hill and open the throttle slightly - and the engine will 'pink' lightly - but if I floor the throttle there is no pinking, because the manifold vacuum has collapsed and the vacuum advance has fallen away. I consider this setting to be as near as I dare set it - it's possibly just a tiny bit too far advanced. I have absolutely no idea what the static setting is - why would I want to know ?? Any figures given in old books were for engines running on very different fuel to today's blends - and so are worthless. Any time I have an engine in bits I just get it running by twiddling the dizzy - then set the timing by road test. There is no other satisfactory way - unless you have access to a rolling road!!

Thank you gentlemen,

Mike, of course, knows that I have an MM with an Alta head but my question also relates to the 1967 1100 that I am trying to buy.

I say "trying" because while the owner and I are in good agreement the authorities are being obdurately stupid because 40 years an "S" became an "8" while recording the serial prefix. (MA2S5 xxxxxxxxx)

There, I've got that off my chest, I hope those "authorities" don't read these posts.

Darrell

Hello BMCE,

"why would I want to know ?? "

So that in the future when doing any work it can be reset pretty close to what it should be.

But I suspect you already know, even if it's not in figures but in the position of the distributor when installed?

Alec

I always set the ignition timing by disconnecting the advance vacuum, then gradually rotating the dizzy from TDC at tickover to a point where the engine revs cease rising and the engine sounds a little less than happy.Then retard it just a bit (a smidgen). The engine then sounds happier again. Lock the dizzy. This always seems to work although it is advanced on what my tuning friend with the light has as correct. If pinking results then one can retard a little (its never happened with mine).
I suspect that the old recommended settings were given because customers expected engines to pull like trains from low revs, whereas we are happier nowadays to drop a cog when accelerating.
Its difficult to define how an engine sounds "happy" but you can certainly learn it quickly enough by trial.

Yes - 'best idle' usually works - but always make sure it doesn't then pink - or kick back on the starter in cold weather!

On a newly rebuilt engine best idle does not seem to be precise enough as internal friction seems to affect the idle speed so that best idle and best performance do not necessarily co incide.

Indeed - that could be the case - which is why I then suggest road test trials with 'advance till pinking - then retard slightly'. I accept that with stupidly low Comp Ratio engines - there may not BE any obvious pinking - in which case you can only adjust to where you feel you get best performance.

I have to agree with both mike.perry and bmcecosse on the last previous posts. In my own experience newly rebuilt MM engines don't stay tight for long but once they are broken in I can not get one to "pink" with modern fuels. They seem to tolerate a very wide range of settings and finding the sweet spot is a seat-of-the-pants exercise.

I wonder if there is a case to be made here for taking about a millimetre off the head to get more use out of the fuel? I haven't done the math to figure what is required to get the ratio up to perhaps 8.5:1 or dare I say 9:1, how much iron is available in those heads?

Darrell McDonald

Just make sure you leave enough space for the valve heads........

If you take too much off the head you can get too close to the water jacket and get stress fractures in the head above the pistons.
Therefore it is important to know if the head has previously been skimmed. Over skimming will also move the ramp away from the valves, effecting the gas flow between the valves and piston

I guess that was the point of my question, how much can SAFELY be removed without risking a cracked head or valve contact. Not that I have any immediate plans but what's the point of having a discussion without talking about what you "might" be able to do. Previous skimming should be evident with machine marks unless it was done on a belt. Does anyone know what is the index surface on an MM head? The first surface that is machined from which all other dimensions can be taken? On the block it is usually the crankshaft bore and associated machining from which the cylinder bores and the top deck can be measured, or do I need further education on this?

I have just been looking at several MM engine blocks that I have and, compared to modern castings, they are amazingly rough. One example in my possession had the opening for the starter covered by casting flash nearly a millimetre thick roughly knocked out with a hammer and left with the ragged edges.

BTW - in a different discussion on the morris_minor site I have heard that the engine number is stamped into the face where the starter goes. This is in the USA. I have never seen the engine number for an MM anywhere except on the little brass medallion. Have I something else to learn here?

Darrell

The water pump engine has the number stamped on the face of the block, in front of the head, above the water pump.

In a previous thread BMC said: "Measure the thickness of the head - they ALL start life at 2.750" . Any less - then that's what's already been skimmed off."

Aye - but that's for A series engines - they seem to talking about the flat-head side valve engine. I have no idea about the head thickness of these engines - although at one time I had three of the blighters !

My mistook I guess, I thought they were talking about "A" series.