Hi Arthur;
Yes, altitude makes a difference in the rise seen as the mixture control knob in the cockpit it pulled aft to shut the engine off. Reason? There’s less atmospheric pressure at 5,300 feet than there is a an airport that’s at 1000 feet or sea level.
Lower atmospheric pressure translates into fewer oxygen molecules entering the carburetor at higher altitudes than will enter at lower altitudes. Less oxygen means that less fuel is required at idle rpm to get the desired 50 rpm rise.
The 1 1/2 turns I mentioned in an earlier reply are a starting point. The engine should run at that adjustment at both altitudes. But the idle mixture adjustment screw in the carburetor must be either screwed in by turning it clockwise (to lessen the amount of fuel if the rpm is greater than 50 rpm at shutdown) or screwed out by turning it counterclockwise (to increase the amount of fuel if the rpm drops straight off at shutdown).
Since atmospheric pressure has an effect, the correct idle mixture setting at 5,300 feet will not be correct at 1,000 feet or sea level.
I’m just slightly confused. You ask in the last sentence “what would you think the usual position on the mixture knob be from full in after the setup is complete?”
The position of the mixture knob on the panel when the mixture is full rich must have some “cushion” That means the mixture arm on the carburetor (I’m not talking about the idle mixture screw) is against the full stop on the carburetor, the knob on the cockpit mixture control should not be bottomed out against the panel. This “cushion” should be at least 1/4 inch. This is important to make sure the mixture setting on the carburetor hits full rich before the mixture control in the cabin hits full in.
I hope that’s clear.
Best,
Steve
