I just bought  the used os max. In order to convert it to controline one needs to remove the carburator and add a venturi and a nipple(since the nva is in the back of the motor). I wonder if I can just leave the carburator and fix it wide open, and adjust the needle accordingly?

That's exactly what I do.  I fly the plane with the carb open and the needle leaned comfortably for good fuel mileage.  Then I begin lowering the prop pitch until I get close to the lap speeds I want.  It's usually still a little fast, so I reduce the carb opening until I get the RPM down to a comfortable lap time.  Now I've got good lap times and excellent fuel mileage.  I bolt a wire from a motor mount bolt to the carb arm and just adjust the carb arm to get the RPM I want.  Works very well for me.  I'm flying a TT 42GP, OS 45FSR and OS 46LA.  Works great on all these engines.

Paul

Or Fernando, you can use standard OS parts to convert it. Take the rear NVA off and use the FP25 Venturi, Tower No.LXCW99, and the front NVA, Tower No. LXCR49.

It's a straight drop in.

...And if it's an early .40LA with the integral RNVA you can replace the backplate with aluminum one from a .40FP. (+ a gasket)