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MG Midget and Sprite Technical - Timing

Musing on this one after tinkering with various bikes in the shed, but it applies equally to the midget. Let us assume that we can statically time to a reasonable degree of accuracy, and for the sake of argument, let us also assume that we agree that to dynamically time with a strobe or a dwell meter would offer a tad more by way of fine tuning.

To statically time we align the marks by rotating the base plate just as continuity breaks with ignition off or when the lamp just illumintaes when ignition is on. So far so good. To set dynamic we use the strobe at whatever rpm, and rotate the baseplate a smidgen more CW or ACW accordingly until the marks align.

Therefore, as we now know how much further to rotate the baseplate from static to obtain the dynamic timing, can we not translate that amount of rotation into a linear deviation from the aligment of the marks at static to obtain the correct timing at dynamic. So for example an added rotation of 5 degrees CW/ACW at the baseplate to obtain correct dyanamic could translate as the firing mark being 1mm to the left or right of the indicator mark once static is achieved?

The point being we may easily achieve something closer to dyanamic without actually undertaking dynamic timing. Some universal adjustment of the firing mark from the indicator mark at static could perhaps be obtained to fine tune further from a general consensus of how much rotation is required, or is every car so different that this would be impractical.

Oggers

I think I follow and I don't see why not. There are plenty of ways it can be done.

You could quite accurately measure, say, 32 (?) degrees on the crank pulley by measuring the circumference of the pully and divying it up. On A/1500 engines, you would then make this mark anti-clockwise from the TDC pulley mark.

Then time it up at 3000+ RPM, using a standard strobe and getting this new mark to line up with the TDC mark on the timing cover/block.

Malc.
Malcolm

Actually... maybe I don't follow!

If all is correct, you are able to accuratley static time things (say to 10 deg BTDC) and the dizzy advance is correct and functioning (adding 20 degrees for example), you shouldn't then need to move things about any more to achieve correct dynamic timing. You will have your 30-whatever degrees.

And in reality, everything is different, old and a bit off anyway.
Malcolm

Yes I think Malcolm is right. I recently fitted a refurbished Dizzy to a 1275 engine which had also had a new timing gear set and chain. We set the timing statically to get it started. The timing was spot on when we checked it dynamically!
Bob Beaumont

Malc

Understood about the advance etc, but is not accurate timing done dynamically to account for the slight difference in firing point when the engine is running at a nominal rpm? Advance then allows for deviations from this nominal rpm - normally above it?


I still think a nudge of the aligned timing marks at static could be obtained to achieve something a tad closer to dynamic. Obviously it will not be spot on, but possibly better than simply leaving it at static.

I guess I could revert to my useful long hill again! Set at static, test up the hill, and rotate baseplate a wee nudge accordingly. Test again and lock off or repeat.....
Oggers

Bob

Yes - appreciate that happens, but surely not usual that static and dynamic concur?
Oggers

Hmm
My first little issue with this is on setting the timing at 3000rpm
Ok, that's fine but you need to know what the distributor is doing or how it's advance is set up
Setting at 3000 is ok if the dist. has stopped advancing at that speed which usually isn't the case, they're still advancing up at that
eg
Say for example a dist is set up for 30 deg full advance but doesn't get to 30 till it's doing 4000, If you set it at 30 at 3000 it can easily have more left in it and you can end up with 40 or some other engine destroying figure
If you're going to set it at 3000 you then need to give it a good rev with the light on it to make sure there's nothing more happening
I don't understand this fascination with 3000
now, static timing--
Static timing is just that, timing done with the engine not running
If the spec is, say 10deg. then, as soon as the engine starts this will increase, depending on idle speed and wear and tear
An MGB for example will increase about 3 deg at normal idle and if the idle speed is up a bit it will be more
If you timed this at say 10 dynamic it would reduce the static back to 7 or so which is a backward move
Best to set it static, at the 10 and then use the dynamic light to check the rate of advance against the specs for the distributor being used, then you'll know for sure if maybe the springs have gone off and the dist is overactive down low or the advance mech. is stuck and won't advance or if the thing has too much total advance etc
Dynamic timing is a checking tool
William Revit

Wille

Arguably dynamic is also a calibration tool which could be used to calibrate timing at static by the method I stated or some other perhaps?
Oggers

Oggers, maybe I am missing something here, but if the dynamic timing was a recognised/ consistent additional linear adjustment of the dizzy, then why wouldn't the handbook figures just add the necessary extra to the published figures? Surely the reason they don't is the variation in components, due to wear, different designs and different engine set ups. The static should get you a running engine but the dynamic adjustment is needed to get that specific engine running properly.

But I watch this with interest as I still haven't strobe-timed my frog.

I have it in my mind that the safe total advance for an A series is 32 at peak revs, in practice around 3500 will get all the advance elements added in. So vacuum, bob weights and dizzy setting add up to 32. Is this correct?
GuyW

Guy

I agree that such a proposal as mine may well be impractical given all the variables you stated. No issue with that, all perfectly understandable.

However, if it could be established that there was some viable general relationship which is normally advantageous - perhaps using statistical analysis with an sufficient number of owners - between the timing points at static and dynamic, and if that could be used to modify static timing somewhat, then perhaps there may be an advantage here.
Oggers

Yes I see your logic. Perhaps there is but the A series engine was in development and production for a long time. I imagine over that time they would have factored in the real-world experience and testing of many thousands of cars both on the road and competition. The published static figures change with the engine development through to the 1275s and presumably take account of this learnt info from a much bigger sample than we could muster now.

In ideal conditions static figures shouldn't be too far out. In other words, once static is set then checking the dynamic setting shouldn't result in very much need for adjustment. But with individual engine variations it is clearly still well worth doing.
GuyW

No Guy
The 32 is total mechanical(only) advance, "the vac hose should be disconnected" while checking mechanical advance timing
The 32@3500 would be about right but after that's been set or while setting you always need to rev up past there watching the timing marks to really make sure that all mechanical advancing has topped out

Oggers
We're going to get in deep here i see
I can see where you're coming from but worry about confusing others here--Yes it can be a calibration tool--if you were on rollers, the method for setting up a dist is to go in steps, for example, for a basic setup you would go 2000,3000,4,5,etc
Starting at 2000 move the timing to get best power then retard it back till power just starts to drop off then just back up a tiddle to that retarded end of best power, then measure this setting with the light----then same at3000, 4000 etc
Then you have a plotted curve for the dist
If it's a hotty you might end up with something that runs fine with a dist that has something like 20 degrees or so at 2000 and runs out best with a full advance of 30 deg from around 3500 onwards

So then if this dist hadn't had it's total advance modified/limited and set on 30 total then it's static would be way back at 8 or so--no good--this engine would probably start fine and easily take something like 16-18 degrees static so would need the total advance in the dist reduced to 12-14 instead of the normal 20-22

In this case yes the dynamic timing light can be used as a tool to calibrate the static timing setting point for the distributor-as well as what i was suggesting earlier for using it to check specs of a std. distributor
William Revit

Guy

Ys agreed, static should not be a million miles out and plenty of old bike owners at least swear by static and simply ignore dynamic - and they have a point - pun intended.

Yes MG and others refined their static values, as did the bike guys, but if we assume dynamic settings still offer a tad more accuracy in timing, then maybe worthwhile.

For example, if it transpired from a sample of results that generally speaking an accurate timing setting for dynamic fell between 3 and 5 degrees advanced of the static setting, we could feasibly adjust to suit.
Oggers

I guess the manufacturer plays safe with their static, and its up to someone else - owner/ garage, to carry the blame if they then adjust it to far advanced at a later date.😵
GuyW

Guy

Possibly, but I would be willing to take the risk were it justified from a meaningful set of results. I can always put it back if on the road it proves not to be advantageous.

I seem to recall from some old Ariel/Triumph bike workshop manual that as a rough guide dynamic is invariably a nudge advanced from static.

Guess I can do my own research using my handy hill or just desist from being a tight-fisted skinflint and invest in a strobe/dwell meter!
Oggers

<<dynamic is invariably a nudge advanced from static>>

yes, agreed. Manufacturer playing it safe.
I hadn't picked up that you don't have a strobe. But then when these cars were made then neither did most garages!
GuyW

This thread was discussed on 22/09/2020

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