Al,

If the carburettor is still fitted on the engine, you could hold the throttle in a fully open position by sticking a wooden dowel, or the blunt end of an ordinary pencil into it.

That way you can apply full force to lock the arm screw.
It is best to remove all parts completely and wipe them with an alcohol swab, to make sure they will not slip free.

As Joe wrote, you will need the arm sticking about 40-40 degrees forward of vertical, in relationship to the engine, not to the carburettor body (because it is itself mounted at an angle to the engine).

That way, when you turn the arm back to 35-40 degrees backward, the throttle will be at the idle setting and closed completely at 40-45.