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The effect on performance of the vacuum advance on your distributor, is quite different, when the vacuum pickup point is on the manifold (manifold advance) as opposed to connected to the carburetor point (call ported vacuum). When connected to the carby, the vacuum point is basically blocked off by the butterfly at idle. That is probably why your car "revs so freely". As Altezzaclub has mentioned above, the K series engines will accept a large amount of advance at standard compression ratios, without detonation & pinging. I've been running "manifold advance" vacuum into a transducer which produces a 0-5Vdc signal "mapped" to a programmable ignition, and I regularly obtain 45 deg of advance, without a sight nor sound of a ping. Never under estimate the difference even a small change in ignition timing can make to the overall performance of your normally aspirated K engine, as this video clearly demonstrates. All good fun. Cheers Banjo

We all talk about static timing as being 8 or 10 or 12 deg BTC, as being important, but really it is only an initial "static" setting, that allows us to get the car started. The most important ignition setting is the dynamic advance setting, which is the advance whilst the car is running, under various load conditions. However, few of us ever get to check the dynamic setting. We just assume it gets there. The maximum dynamic setting, is the sum/total of the static setting (rotating the distributor at rest until the points just open) plus the maximum centrifugal advance, which is the function of the little "bob weights" under the dizzy plate, that don't get looked at often. If you do ever pull your dizzy to bits one day, you will notice that the little bob weights, or the adjacent plate, have a little number stamped on them like 14 or 18 or 21 or something similar. This number is the maximum number of degrees of advance, those particular centrifrugal bob weights & springs can provide, Some people play with these weights, end stops & springs, to increase the maximum advance possible. You will also notice that the two little springs are different, one being "lighter" than the other. This is so one bob weigh moves out first, The other heavier one then adds to it, as revs increase, to provide a 2 stage advance curve. So lets say your static advance ignition setting is 12 deg., & lets say the bob weight/s have 14 stamped on it. The total advance possible is therefore 26 deg. The maximum advance in our little K series engines is all achieved by the time the revs have reached say 2500 - 3000 rpm. So the most important thing to determine, to my mind, is to ensure that when your engine is running at 3000+ rpm, that the advance reaches this possible maximum. So what about this vacuum advance you/we are discussing here. As vacuum decreases, with the engine under load, with W.O.T. (wide open throttle) the vacuum actuator on your dizzy, actually retards the advance. This is because under load, the engine "sucks" harder, and with greater density, it ignites easier,earlier, (not as lean), so not as much advance is needed. That's it in a nutshell, but there are plenty of places on the net where you can read about that in more detail. So this is why when you set your static advance setting, you always take off the vacuum line & block it, so it has no effect, on the setting. So once you get your motor running, you should set the dynamic advance to the maximum recommended possible. As we can't change the bob weights action, without modifying them, the maximum achievable is still adjusted by rotating the dizzy, with the clamp just nipped slightly. However this is done at 3000+ rpm, rather than at idling. The reason this method is not used widely, is that you need a timing light to check that you are achieving say 26 deg, as in the example above. However, a timing light is a very good addition to your toolbox, if you are serious about getting the most out of your K series motor. The problem with this technique is that to read 26 deg of advance on the timing chain cover is difficult, as the markings usually only go to 20 deg BTDC. Now you can guess where say 26 deg is, or pre-mark it on the case, or get a adhesive degree strip, to stick on the edge of your crankshaft pully, but it must be a strip designed for that precise dia. pulley. The timing light manufacturers have overcome this issue, by adding a knob you can turn to retard the flash, until it lines up with TDC instead of say 26 deg. The 26 deg, is then read out on a little LED screen. From your description, it sounds like the current p/u point on the inlet manifold is causing your problems, or maybe the vacuum actuator on the dizzy is faulty. Try the sucking on a plastic tube connected to the vacuum actuator, & see if the dizzy contacts plate moves. You might also have an air leak on the inlet system somewhere that is causing the engine to idle fast. The first bob weight usually starts to move around 1200 - 1500 rpm. The idle must be down around 800-1000 rpm, to ensure that the bob weights are not advancing the timing at idle. The possibility is that a bob weight spring has fallen off and there is nothing holding back one or both of the bob weights. Let us know what you find. Sounds like your teacher is doing well, and created a great interest in you to question why things work as they do. Cheers Banjo

What you is quite correct. Cars used to run no vacuum advance at all, it was added as an anti-pollution device in the 70s. The first ones came off the inlet manifold like yours, and the timing was set as you did. Take the vac line off and block it. (or the massive air leak into the inlet will lean out the mixture and the motor dies) Then set to 8deg advance, put the vac line back on and adjust the idle speed on the carb to suit. So you have it idling at 25deg- this is needed as a lean mixture is difficult to burn so you start the fire earlier. THAT is why there is a vac advance- you boot the car to accelerate and the vacuum collapses from the open throttle and the ignition retards. Then when you have speed up you back off the throttle, lean the motor out and the vacuum advances to burn the mixture thoroughly. If it is too advanced and you get pinking as you boot it, it won't be the vac advance, it has no affect when the throttle is open. The weights do the rest of the advance curve, and the usual way to find optimum advance was to floor the accelerator and listen for pinking. If there was none you advanced it a few degrees and did it again. You timed the acceleration with a stopwatch and picked the fastest time, as over-advancing will slow it down overall. Finally, 4Ks will take massive advance, I had one of mine idling at 18deg without vac and it drove fine.