I send a mail to AXE ROSSI asking how the break in should be done...and they anwer me the following (it's true, the break in is really long!!!)
ENGINE BREAKING IN INSTRUCTIONS
AXE Rossi high performance engines must be correctly broken-in for maximum performance and life! Rushing this process can result in premature engine failure.
The carburetor of our AXE Engines is set at the factory at a rich setting for an easy start of the engine. The factory initial setting is the following: Main needle 3.5 turn open from closed position β Idle needle 2.5 turn open from closet position.
For a good break-in run the engine while the car is sitting off the ground at that rich setting for 3-4 tanks and then for the next 3-6 liters (1-1,5 gallons) run it on the ground at a low speed and gradually speed it up while you progress through each tank and leaning the high speed needle 1/8th of a turn at a time until it runs close to peak performance. Please don't take short cuts and run less tanks.
Make sure the engine stays within a reasonable temperature range 110-120Β°C (230-248Β°F) and make sure you see a good amount of blue smoke coming out of the exhaust throughout the throttle range to insure you are not running the engine overly lean. Do not run the engine too lean or you will over heat it and cause permanent damage to the internal components of the engine.
Remember always run the engine with a clean air filter.
The fuel is very important for the run of the engine.
The correct fuel needs to contain:
Synthetic Oil 6-8%
Castor Oil 2-3%
Axe Rossi recommends the following Fuel brands:
AXE by ROGA