Does backing it off `mean move the dizzy clockwise or anticlockwise(cu engine)??
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Whatever is the same way as the rotor arm turns when its running
Both adv and ret
This how I see this working, please correct me if you know different for this engine, which I don't know and have never worked on:
Required Fact: When idling there is a high vacuum, progressively reducing to nominally atmospheric pressure at full throttle, pretty well regardless of rpm.
1) The Vac Retard only gets a vacuum signal when the throttle is closed, or at very small throttle settings;
2) The Vac Advance only gets a vacuum signal when the throttle is away from the closed position;
3) (1) & (2) above are achieved by the positioning of the vacuum ports within the carburrettor throat, with the butterfly or some other device masking the vacuum ports appropriately;
4a) Going on (1), the Vac Retard signal is predominantly an idle signal, and can be ignored when the throttle is opened much further;
4b) Thus we get a Vac Retard of about 12 deg. max at idle (850~950 rpm)
5) Combined with a static setting of about 7 deg BTDC, this gives a strobe reading at 900 rpm of about 5 deg.
ATDC - this is for emissions and smooth idling, AFAIK.
This 5 ATDC is the datum quoted with both vac tubes conencted and the Idle Stabiliser Unit's connectors, if fitted, plugged together.
6) As soon as the throttle is opened significantly, this signal (1) disappears, but is almost immediately replaced with the beginnings of the vacuum signal (2), which will advance the ignition somewhat.
7) Strong Vac Adv (2) signals are produced under part throttle and low load conditions, at medium to high rpm. advancing the timing for economy reasons e.g when running on a light throttle or lifting off at speed. Max vac Adv. is maybe 10~12 degrees @ 200~280 mbar.
8 ) When loading the engine up, and the vacuum reduces below about 100 mbar, the Vac Adv. reduces to zero, effectively retarding the ignition so to prevent pinking or detonation, which allows high pressures and hence torque to be produced smoothly and safely.
9) At all conditions the Centrifugal advance curve determines the basic timing advance dependent upon time available for combustion as the engine speeds up, so advancing up to a mximum safe value of 21~25 degrees at 3400 rpm.
10) For several reasons Centr. Adv. doesn't increase beyond this speed, one being that combined with a whiff of Vac Adv. at high speed due to easing off, gross over advance might occur threatening high speed detonation. Another is that cylinder filling (volumetric efficiency) is starting to decrease after that max torque rpm, and hence there is less mixture to burn requiring less advance. Also, as the rpm increases, at a set throttle opening, the vacuum increases and may bring this Advance into play.
11) Most centrifugal systems have a two stage curve, in this (2.0 CU) case, the kink point is about 1600 rpm, quite typical. Cylinder filling (state of tune, camshaft etc.) and combustion efficiency determine the basic Centr. Adv. curve chosen.
Knowing these basics, allows better intuitive and calculated adjustments to be made, particularly with respect ot the danger areas, usually over-advance. Although constant retarded running can produce more exhaust heat and threaten exhaust valves, somewhat akin to running a weak mixture that takes longer to burn (hence the popping back with a blown exhaust or missing air-cleaner)
Did they check that the centrif and vac adv/retard are giving figures in the right ball-park. Otherwise the static timing will be pretty meaningless.
Not sure about this and not looked at that myself
The above should shed some light on this... [HM]
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Providing it doesn't overheat
As above too...
Spoke to German about this the other day and he said best to get a temp gauge but failing that you could see how hot the dipstick is by touching it. Did this and its HOT!! (methinks its too hot,it hurt!!) Or am I not understanding properly what he meant???
It will always burn you I should have thought
the oil would often be above 100 C on an Aircooled?
You might ensure all the air-cooling ducts and shrouds are sealed well and fit for purpose, with no bits missing. Check the oil cooler isn't clogged (airways) etc.
it would struggle.....
