Throttle While Cranking

So, why will a carbureted auto engine start up with the throttle open? Because an automotive carb has an accelerator pump that squirts a charge of gas into the manifold. On a tractor, with an updraft carb and no accelerator pump, the only thing getting gas out of the float bowl on the carb is the vacum created by the pistons going down in the cylinders. Open the throttle, lose the vacum, and- no gas. Unless you have the choke shut, which will do the same thing and usually give even MORE gas since it shuts off the intake tract on the upwind side of the carb.

Sorry so long-winded!

1 - Unlike a carburetor on a car, your tractor's carb is pretty simple. Consequently it only has two "sweet spots" where it delivers the exact fuel/air the engine needs: At idle and at wide open throttle. Now if you crank the engine with the throttle open the carb tries to deliver the wide open mix. But the air flow thru the carb is so low at cranking speed the carb feeds little (or no) gas, and the engine behaves like it has no fuel. However setting the throttle at idle results in a fairly decent F/A mix and thus permits the engine to start.

2 - The lousy, overpriced gas the oil companies are now peddle doesn't burn worth a ^#@&!. Unless the F/A mix is close to ideal it's amazing an old tractor can be started at all.

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Note also that different carbs/tractors behave differently. My Super H (same motor as your 300) is somewhat reluctant to start with the throttle wide open, especially when warm. But it lights right off with the throttle pushed in. And my Minnie Mo U absolutely WILL NOT start warm unless the throttle is set to dead slow. But the Super M starts no matter where the throttle is set, hot or cold. Go figure...