Ed

It's because the engine has a mechanically 'fixed' timing that the ignition timing is varied by the fuel, which is why you may have to richen the fuel mixture when using a larger prop. A larger prop usually runs at a lower rpm so in order for the ignition to occur at the right time, the ignition has to be retarded. You could achieve the same thing by lowering the compression.

There are two important terms here, timing and ignition, one we can't adjust without mechanical intervention the other we can by using the mixture screw.

To all:

If the term "expert" upset anyone, as used in my initial posting, then I retract it, but it seemed like the right word when you consider the stance some contributers were taking. It is unfortunate that we speak as we find in my part of the world, this I concede can ruffle some feathers on occasions, but my intentions are honourable and no disrespect was meant to those of good heart.

Steve