(*Note:  The RO Jett series of engines are not all timed the same.  Check which model you have.)

Starting points for the RO-Jett and PA AERO 40-65 engines:

Select a prop diameter (in inches) of around:
        11.5 for the 40
        12 for the 51
        12.5-13 for the 61
        13-13.5 for the 65

Select (or set) the propeller pitch to around 4 to 4.25 inches

Launch RPM for all engines should then be around 10,000 to 11,000 RPM

The pipe starting point should then be around 16.75 - 17.5 inches.

Move the pipe out (lengthen it) if:
        the engine run is too hot or it comes on too strong in maneuvers.
        the plane speeds up during the flight.

Move the pipe in (shorten it) if:
        the plane does not come on strong enough in maneuvers    
        the plane slows down during the flight.

In general:
        2-blade props, shorter props, and those with narrow blades will use a slightly shorter pipe setting.
        3-blade, longer props, and those with wider blades will use a slightly longer pipe setting.

When you find the proper pipe setting and run characteristics, you can also:
        add pitch to the prop if the plane is flying too slow
        decrease pitch to the prop if the plane is flying too fast.

Remember, all of the above are starting points, not absolutes.
We are trying to tune the pipe length to match the in air rpm of the engine.
Venturi size, nitro percentage, number of head shims, temperature, all play a part
        along with YOUR preferences and style of flying.