Petrol engines aircooled Ignition 02

Some basics on Vac Adv Retard systems (Harry)

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' (Bentley/Haynes etc.), 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 have no vac adv/retard (to avoid dangers of over-advancing) but the later VW carbed air and water-cooled boxers have both.

Basic Fact 3: 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...

Other factors - mixture, engine condition etc.

1) Weak mixtures burn slower, rich mixtures burn faster - within reason.

2) Weak mixtures thus need igniting earlier, to avoid overheating exhaust valves, and vac advance at part throttle economy cruise aids this; Rich mixtures are generally more prone to detonation, but gross overfuelling cools the cylinder - proving that spark ignition combustion is a highly complex phenomenon.

2a) Weak mixtures are thus one cause of popping back through the exhaust (not backfiring as such) - particularly on the overrun when an over-run fuel cut-off is not part of the carbs settings. Excessively retarded ignition is another.

2b) Weak mixtures can be caused by air-leaks in either the inlet tract or exhaust system (leaky or too 'open' a system that hasn't been tuned in properly)

2c) Restricted inlet or exhaust (air-filter, exhaust or blocked cat) will produce rich mixtures, sometimes disastrously so at higher engine speeds - to the point where the engine will literally cut-out!

3) An engine set to the detonation advance threshold in winter, will likely be dangerously over-advanced in summer - don't push it. Think very cold air, maybe weak, very hot, maybe rich (and think detonation at high inlet air temps).

4) Acceleration (sudden transient load increase) requires mixture enrichening (accelerator pump or equivalent) and a reduction in spark advance (usually achieved by manifold vacuum dropping rapidly - vac. advance falling)

5) Poor engine condition usually requires a reduction in spark advance (to minimise light pinking); breathing less well, also requires less advance.

6) Some engines (good comb. chamber design) can be quite resitant to pinking and detonation - and therefore can be (and often are, erroneously or for economy) advanced too much without problems but to detriment of performace, especially pick-up and pulling power at low~mid rpm. Always try a small degree of retard before trying more advance (but always keep a good mark of the datum ditributor timing position to work back to)

7) Remember the interdependencies of timing and mixture, both need to be fairly close to right to be able to make clearly drawn conclusions

Specifics for aircooleds (Keith)

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

Whether the ISU is used or not is immaterial to the timing which should be 5 Degrees ATDC, 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 (Simon)

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...

Centrifugal Advance Curve

Centr. advance curve plotted from Bentley manual data (for California 2.0 litre air-cooled vanagon)

Red vertical bars are tolerance limits (as interpreted from given data)