Petrol engines aircooled Ignition 02

Some basics on Vac Adv Retard systems

NB. Ignore these basics and goto specifics if that's all you need. But this will stand you in good stead if things don't work out when setting the timing up 'to the book', as its not a particularly good 'book' i.e. they tell you what to do but not why - bad in my books, easily get things setup wrong.

HarryMann: This how I see vacuum timing working, across most vacuum advance/retard systems

Basic Fact 1: All petrol engines - when idling there is a high vacuum, progressively reducing to nominally atmospheric pressure at full throttle, pretty well regardless of rpm.

Basic Fact 2: Many earlier engines have just vacuum advance, soome performance engines used to ahve no vac adv/retard (to avoid dangers of over-advancing) but all the later carbed air and waterboxers seem to have both. Vac retard is basically for emissions when idling, vac advance is basically for part-throttle economy.

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. [But please see cumbriankeith's specifics section below]

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 tuning adjustments to be made, particularly with respect to the danger areas, usually pinking or high-speed detonation due to over-advance. 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)

The above helps to explain why there are several ways to set the ignition timing, whether disconnecting vac pipes or not, whthere using strobe or not, and why a basic ingition timing at idle can end up retarded After TDC...

Specifics for aircooleds

cumbriankieth: After a lot of work on dizzies and a lot of words on the forums... What I said last time was wrong!

Whether the ISU is used or not is immaterial to the timing which should be 5ATDC, provided a distributor with both vacuum advance and retard is in use, working properly and left connected.

Another approach, which should arrive at the same setting if everything is working, is to do the basic timing adjustment with just the mechanicals in use - disconnect the vac pipes (plug the retard one - the one that comes from the idling supply - behind the airbox), time it at 7BTDC at 850rpm, then check that it advances to no more than 30BTDC at 3500rpm. If it does then the weights/springs in the mechanical advance are suspect.

To check the vac stuff - reconnect the vac advance (goes to the cone-shape part of the vac unit) and when you open the throttle there should be a virtually instant advance of about 10 degs. Then reconnect the retard pipe and the idle timing should move to 5ATDC. If one or both of these fails to occur then either the vac unit is faulty or the basic throttle setting on the carb(s) is wrong (too far open - the retard vac at idle relies on the partial vacuum beyond the throttle plates - called "manifold vacuum" and this collapses when you open the throttle. The advance vac relies on "ported vacuum" taken from just above the throttle plate - this quickly develops when the throttle is opened).

All this has been gleaned from various sources, including The type4rum and the Samba - if any corrections or clarifications are needed then I hope others will contribute...

Thanks to Steve Shaw and Simon Baxter for allowing Keith to bend their ears to get to the facts behind this#

Thanks to toolsntat and camper for much prompting and contributions in a club forum thread that turned out to be a voyage of discovery - for all of us - [HarryMann]!'

Follow-up comments

Toomanytoys: Oh and dont be tempted to fit an 009 as they are useless for a type 4 engine.. 
not enough advance and not fast enough advance ramp..

From what I can remember reading, its not unusual to see 40 deg with all hoses connected correctly on a very light throttle... heard from a type 4 guru.

30 degrees sounds a bit too retarded for max advance.. I would have it at about 32-34 max mech (only, disconnect and plug vac hoses) advance, especially if running on LPG.

If you have a head temp gauge, then you can do a few tests to find the optimum setting for your bus.. but then most dont have...

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Dont think an 050 distributor is good either.. unless you reset the advance curve.. stick with a proper points or electronic type 4 one.. or a mallory.. ££££.. :shock: if seriously tuned...

Simon Whitmore