Mr Cajun Gator

This info I got from surfing the net, it is very good

I am using the following method for running in and am offering it as a
guide for those cases when the engine manufacturer or supplier does not
give recommendations
Fuel
The same fuel as intended for the engine during normal / race use. Both
the low and top end needles should be set up rich, lots of blue smoke, oil
etc in the exhaust and poor pickup.
Tuned Pipe
Set this up long initially, this will help stop the engine revving too
hard during the running in phase.
Prop
Use a smaller prop for running in as this will keep the engine load light.
Running in procedure
For monos and cats I use the following procedure:
For the first 1/3 to 1/2 of the running in use the set-up outlined
above, the engine will have very poor throttling response etc and may
even struggle with idling as the mixture setting is so rich. I keep to
no more than about 1/2 throtle.
For hydros
Again with the engine set as above but with the water cooling
disconected I run full throttle on the staights and as slow as possible
through the corners. This cycles the engine through hot and cold relying
on the fuel as a coolant. I ran my CMB 21 in this way and it seems a
superb engine.
From this point onwards progressively move towards the normal running
settings:
slowly lean out the needles to get good low speed and medium speed
running, at this stage try to keep to low throttle settings. For
further details see my page on Setting the needles.
increase the prop to a normal sized prop
for the last third of the running in start using short bursts of
full throttle to get the engine up to running temperature.

After completing running in then the pipe length can be shortened, prop
increased and mixture set properly.
At all stages of running in attention should be paid to the cooling of the
engine, and its condition after each run.
As a guide I would suggest an hours running in for a 45 raising to one and
a half hours for a 90.