There are lots of email's flying around news groups and email lists regarding setting timing.  To many of us it seems simple but it is basic to how internal combustion engines work.

Essentially, in a four stroke engine like all of the Air Cooled engines we work with, a bunch of things have to happen in a certain order
to make the engine produce rotating power (torque).  It's all about torque, or how hard you can twist something connected to the crankshaft.  (Horse power is mathematically determined by multiplying ft. lbs. of torque times the RPM of the engine and dividing by 5252.)

If we start with the piston at the 'top' of the cylinder, or the farthest extent it will travel into the cylinder, we have what is commonly referred to as TDC.  Strangely enough, in an opposed four engine like the ACVW's we work with, it's actually RDC, or right dead center for the #1 cylinder which is the one usually referred to. Top is a reference to early design engines that were designed to have the piston actually move in an up and down orientation.  But we are stuck with the reference.

The TDC on the pulley moves to the right, or in a clock wise direction.  After 90 degrees, it looks like this:

If you have a degree wheel pulley, it would look something like this.  Now the crank shaft is rotated another 90 degrees and we get:

Notice the pulley now says BDC.  The piston is at the bottom, or least amount of entry into the cylinder.  Another 90 degrees and we get:

OK, here's the tricky part.  Notice the degree wheel says 90 again, but this time it's BEFORE TDC.  For you math students, that's also 270 degrees ATDC.

Remember that by convention we only refer to the #1 cylinder for all of this.  However, look at the this picture:

Notice that TDC is up on the wheel, and that the connecting rods toward the top of the picture are extended away from the crank shaft but the ones near the wheel are drawn in toward the crank.  These are the rods for the even number cylinders, number 2 to the right and number 4 to the left.  They are at their BDC positions.  If we rotate the crankshaft half way around or 180 degrees the BDC will be at the top of the wheel, but #2 and #4 piston will be at TDC.

Again notice that the numbers increase each side of TDC.  Numbers to the right of TDC represent degrees BEFORE TDC because when the crankshaft turns in a clockwise direction, those numbers get to the index mark at the top of the case BEFORE the TDC mark to the index.  Numbers to the left of TDC represent degrees AFTER TDC.

So, back to timing, how do you set your timing? Most articles I have read start by telling you how to set the timing at idle and then referring to the 'advance' provided by the distributor.  I prefer to think of the distributor as 'retarding' the timing from operating speed which is the way your engine runs most of the time and where you want it to run with the most power.

Air Cooled engines seem to run best with about 30 degrees of advance timing at speeds above 2500 RPM.  So if you have a degree wheel pulley simply connect a timing light and run the engine speed up while watching the marks on the degree wheel.  As the RPM increases the wheel and the numbers will appear to move.  Continue increasing the RPM until the movement stops.  Then, move the distributor until the 30 to the right of TDC lines up with the crack in the case.  Now, let the engine idle and check the timing with a strobe light again.  It should be somewhere around 10 degrees.  That's because the distributor provides 20 degrees of 'retardation' at idle.

By the way,  you can do this with any plug wire.  It doesn't have to be #1.  There were some stories about #3 being retarded a couple of degrees so it would run cooler, but I have not seen this on later model engines.

But, what if you don't have a degree wheel?  Here are some tips.

The degree wheel is 6.75 inches in diameter or 21.2 inches in circumference.  So, each degree is .059 inch.  IF you can determine where TDC is, you can measure 1.77 inch along the circumference to determine where 30 degrees is.

According to the Bentley manuals, you may have one, two, or three marks on your pulley.

If you have a 1965 or earlier pulley, there should be a single mark which is 10 degrees BTDC.  (To the right of TDC, remember?)
So, simply mark either .58 inch to the left of the factory mark for TDC, or 1.18 inch to the right of the factory mark for 30 degrees BTDC.

1966 through 1970 pulleys should have three marks.  The one to left is TDC.

1971 through 1973 pulleys had one mark that is 5 degrees AFTER TDC. (TDC will be to the RIGHT of the mark)

1974 and later engines had one mark, but depending on the year it may have been either 7.5 degrees before TDC, or 5 degrees AFTER TDC, or in some cases at TDC.  It's pick and choose at this point.

Hope this helps.
 
 

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