Some caps have a check valve to let air in but not let air (and gas) back out, which can be a real problem with a full tank and a hot engine!
And even a vented cap can get be clogged. I once bought a new 'vented' cap, it wasn't! Try a run with the cap loose.
It is possible but unlikely the pressure in the tank would be enough to over ride the needle valve in the carb. If that were the case, once the engine were shut off hot, the carb would begin to overflow and the engine would be severely flooded on restart.
I would look more for a weak spark, points out of adjustment, worn distributor, bad condenser, bad coil, worn plugs, bad wires, cap, rotor, etc. Check the spark at the end of each plug wire. It should give a 1/4 inch spark to ground.
Could also be fuel problems, check the fuel supply available in the carb bowl, remove the carb drain plug and check the flow. Engine off, fuel valve open,should give a steady flow with the plug removed. Not slow to a drip or stop. There is a screen in the carb inlet elbow, a common place to clog.
Catch what is caught in a clean glass, look for water or trash. If the sample is contaminated, so is the tank and carb, both will need to be cleaned. A rust flaking tank will be an ongoing problem.
How is the air cleaner? There is a wire mesh inside the canister, it is often neglected. If hard packed with dirt it will have to come apart and be replaced. Not an easy job, but if clogged it will not run top end.
Finally, the main jet may need to be adjusted. Last thing to do after everything else is right. Start about 1 1/2 turns off the seat, idle the engine, then suddenly accelerate. It should take sudden full throttle without hesitation. If it balks, back the main screw out 1/4 turn. If steady black smoke, turn it in some. Keep adjusting until it will accelerate smoothly. A single puff of black smoke is the goal.
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Today's Featured Article - Tuning-Up Your Tractor: Plugs & Compression Testing - by Curtis Von Fange. The engine seems to run rough. In the exhaust you can hear an occasion 'poofing' sound like somethings not firing on all cylinders. Under loaded conditions the tractor seems to lack power and it belches black smoke out of the exhaust. For some reason it just doesn't want to start up without cranking and cranking the starter. All these conditions can be signals that your unit is in need of a tune up. Ok, so what is involved in a tune up? You say, swap plugs and file the points....now tha
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