I'm working on my '52 Oliver 77 Row Crop gas (my grandpa purchased it new in '52) and am having a bit of a problem:

The tractor starts great - just bump the starter or 1/4 crank and it starts on the first try. I have the idle mixture set what I think is about right Almost no smoke and a very smooth and reliable idle.

As I throttle up, it's starving for fuel and requires about 1/3 choke to run properly. It makes decent power with 1/3 choke under all load conditions (but runs rich at idle if I keep the choke at that 1/3 position). 

It starts getting lean about four clicks (1/5 throttle) all the way to wide open.

Any ideas as to why I'm having this problem? Could the fuel level be too low in the float bowl?

I have adjusted the valves and cleaned the carb. All the orifices are clear and clean. I have the main mix screw turned out all the way (nearly loose at over four turns) and still can't get the top end rich enough without a little choke.

Thanks in advance for any responses.

-Joe Terry
Port Edwards, WI

(Tractor was purchased by Louis Terry, Baraboo Wisconsin and farmed the Terry farm for years. It was my dads (Warren) favorite tractor to plow with)