massey harris 50 tractor

marvel schibler carburetor wick screw is air and which screw is high idle one is small other is much larger thanks crowspond

With respect, Carbs do not have a high Idle screw. That is a governor function. Low idle speed is a screw on the linkage from the governor. It screws through a boss in the upper casting and controls the speed of the slow RPM with the hand throttle moved to slowest engine speed.
There is a larger brass knob top front (if I am looking at the right carb model) that is the main jet richness control. It is used to set the amount of fuel delivered under working loads and at or near the engines maximum power. Under load pulling an implement, (warmed up) it should be set so that all traces of dark grey smoke are gone from the exhaust, but just that. Too open and grey smoke. Too closed, and it will stumble and be too lean. It must be adjusted when stopped, and set in increments of 1/4 turn or less to finesse the full power richness.
There is a smaller brass screw near the flange where the carb bolts on. It is low idle mixture. It is adjusted in tandem with the low idle speed screw. This screw should be adjusted in or out until the engine runs fastest when the hand lever is set to idle. then the idle speed set screw is adjusted to the desired slowness RPM. Then the Idle richness screw is adjusted again to slightly higher RPM, then using the idle speed set screw it is again set to the desired low idle speed. This back and forth setting results in a good idle mixture and easy transition to full speed and power when needed. Jim

What Jim said, throttle low idle speed screw, low idle mixture screw, and a full load adjustment screw.

The small screw up top is the idle mixture screw.

Turning it in richens the mixture, out leans it.

It must be adjusted at very low RPM, down around 400 or so, about as slow as it will idle without dying. Be sure the engine is up to operating temperature.

The larger screw is the main jet adjust.

Turning it in leans, out richens.

Start by turning it in 1/4 turn, then open the throttle suddenly.

If it stumbles, back the screw out 1/4 turn try again.

Keep adjusting the screw until it will just take sudden throttle from idle, a single puff of black smoke is the goal.

You may have to fine tune the main setting under real load conditions.

Of course, the carb can only work as well as the rest of the engine. Good compression, spark, timing, plugs, all are necessary to run properly. There is a wire mesh filter inside the air cleaner canister, often overlooked. If it is clogged the engine can't breathe. If the gas tank is dirty, water contaminated, the carb will be also.

Many an innocent carb has been blamed for other engine problems!