I have a Magnum .91 4 stroke that is giving me fits,i can't get to idle down very low, if i try it will die.
I do not have a tach so i can't say what the RPM is at idle but the plane will start to roll from a stop at idle.
I started to mess with the mixture screw inside the throttle arm and now it starts to backfire and throw the spinner.(when starting)
I get very good transition from idle to full throttle.
Also at idle it is running very rough.
I have a 13X8 prop on it.
What could be my problem?
Does anyone know how many times from all the way in i would turn out the mixture screw on this engine?
Any help is appreciated!

Check the manual, it should have the needle setting start points, and most likely a troubleshooting section.... Other than that... I can't help you sorry man!

If the engine was running well before, put the needles back where they were and put a little tension on the nose wheel or main gear to keep it from rolling.

A prop with a pitch of 8 will still pull tons of air at a low idle. Try a larger diamiter and lower pitch.

Just a thought.

Dru.

Open the low speed screw 1/4 turn, and try it again.

If you are flying off pavement, it is going to be hard to not roll at idle with a 13-8 prop. Your enginle lower as it gets fully broken in, probably in about 3 gallons of fuel. Try a 14-6 or an APC 15-4W and see it this helps.