The little nipple visible in your third photo originally supplied air that had passed through the air filter to the "choke stove" in the exhaust crossover passage in the intake manifold.
Since you now have a carb with an electric choke, the nipple isn't needed or used and can be capped off.
In addition to possible accelerator pump issues that have already been mentioned, there's a little "dogleg" latch lever on the RH side of the carb between the choke mechanism and the carb that is supposed to latch the secondary throttle plates closed until the choke is fully off and the engine (hopefully) is warmed up a bit.
If this little lever is stuck, missing, or misadjusted and the secondaries can open with a cold engine, it will act EXACTLY as you describe.
Also, the (rather hidden) secondary "airvalve spring windup" setting on the torsion spring at the RH end of the secondary air valve is CRITICAL in getting the carburetor to operate properly.
If too tight, the secondaries won't open as they should, if too loose, the secondaries will open to0 early and too much causing a "bog" or "stumble".
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Today's Featured Article - Diagnosing Engine Difficulty - Diesel Tractors - by Staff. The following suggestions are listed for your assistance. You can make simple adjustments on your tractor that will improve its operation and save you the time and expense of hiring someone to do it for you. Always make one adjustment at a time, and if the adjustment made does not improve the condition, return to the original setting before proceeding to the next adjustment. Hard Starting Cold air temperatures. Insufficient fuel.
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