Tuning up a 912
Tune up of a 912 is a set of tasks that should be done in the following order:
Valve adjustment
Set points & ignition timing
Adjust carburetors
Valve adjustment has been documented in an article on Pelican’s website . Perform that procedure first, and then proceed with setting the point gap as shown below:
Point gap is sometimes expressed as dwell angle, and the point gap will affect ignition timing. Pop off the distributor cap and remove the distributor rotor. Look at the points and rotate the engine with the generator pulley, watching the points carefully so as to stop at the maximum opening – top of the distributor’s cam lobe. The point gap should be 0.016 “. Your mini feeler guage should fit in there nicely. If the point gap has changed, reset it by loosening the screw and moving the plate so as to establish the correct gap. Re-tighten the locking screw and confirm correct adjustment. Replace the distributor rotor and put the cap back on. Now we’re going to static time the engine.
Let’s just look around here for a minute. The engine rotates clockwise and you’ll be turning it using your wrench on the generator pulley. When you did valve adjustment you turned the engine backwards – not here ! Familiarize yourself with the case index mark and the marks on the crank pulley. These should be somewhat familiar to you from valve adjustment. When setting the ignition timing, what we are doing is making sure that the spark is delivered to the cylinder at just the right moment – we’re adjusting the phase angle of the distributor and the crankshaft. That bolt at the base of the distributor is the locking bolt that holds the distributor in place and prevents it from spinning around. Loosening this bolt allows you to turn the distributor slightly and adjust the ignition timing. I have installed an allen bolt and Nylock nut here for ease of adjustment– your bolt may not look exactly the same.
The engine wants the spark to be delivered at some number of degrees “before top dead center” (BTDC). Special note: Incorrect setting of ignition timing is a quick way to burn up your engine. Be careful here and make sure you understand what’s going on. I assume no responsibility for any errors or misstatements made in this procedure. Read the fine print for details.
Now stick your head down in there and look at the pulley, assuming you have set the engine to top dead center following valve adjustment. You’ll see at least two and possibly more marks on the pulley – or if you have Will’s cool degreed pulley, a very nice engine divided scale measuring off degrees more accurately than you can possibly file a notch.
The first mark is 0 degrees – top dead center. Then about 1 ½ inches to the right – there are the advance marks, usually 28 and 30 degrees. All of these are measured at the case reference mark, that groove in the 3rd piece that indicates the position of the crank.
What we are going to do first is set the timing of the distributor with the engine stopped – so called static timing. Using a simple light bulb connected to the coil, we’ll rotate the engine with the ignition on, and the light will come on when the points close, allowing current to flow. This will give us the static timing setting.
So, here’s what you do. Connect your light bulb between the cold side of the coil, and any convenient ground. I use the bolts on the fuel pump. Turn the engine counterclockwise so the 30 degree advance marks on the pulley are well to the left of the case reference mark. Now turn the key on until the ignition and oil lights on the dash illuminate. DO NOT START THE ENGINE. Using your wrench, SLOWLY rotate the engine until you see your timing light’s bulb illuminate. NOTE the position of the pulley relative to the case reference mark. It should be in the range of 0 to 5 degrees before (to the right of) the pulley’s zero mark. Repeat several times to be sure.
Now, if this is not the case, loosen the distributor locking screw and EVER SO SLIGHTLY rotate the distributor body as shown, then turn the engine to the left (counterclockwise) and then back again clockwise towards top dead center. Turning the distributor towards you retards the timing – the light turns on closer to 0 degrees. Away from you – the timing advances. It’s VERY sensitive and there is some mechanical hysteresis – so keep turning the engine back and forth, watching where the light turns on relative to the pulley’s position. When the timing light illuminates at about 1-3 degrees Before Top Dead Center, you are in good shape. Tighten the distributor locking screw, check the timing once more, and turn the key off.
Next Step – measuring total advance with the stroboscopic timing light. This includes the centrifugal advance that increases with engine RPM.