Donít guess at the cause of the problem; guessing can get expensive. And leave you breathless chasing every guess.......it could be this, it could be that......just troubleshoot the problem and leave the guess work for buying Lottery tickets. And do not buy a new part for the tractor until you can answer this question: how do I know the part on the tractor is defective?
6v or 12v? Front or side distributor?
Check for spark and fuel. (see tip # 13)
You need to answer 2 questions before you do anything else:
With the bolt in the carb bowl removed and the gas on, will the fuel flow fill a pint jar in less than 2 minutes?
Next, get out your adjustable gap spark checker * , open the gap to 1/4", hook it up, turn the key on and crank the engine. Does the spark jump 1/4"? Post back with the answers.
* Don't own an adjustable gap spark checker? Buy one! Not a test light! Until then, take an old plug, open the gap 1/4" ground it to the head & look for spark. Itís not the color of the spark that counts; itís the distance it jumps. https://www.amazon.com/AMPRO-T71240-Energy-Ignition-Tester/dp/B00A8FO87S/ref=sr_1_fkmr1_4?ie=UTF8&qid=1513273147&sr=8-4-fkmr1&keywords=ampro+spark+checker