There are a few things that can cause this.
First is the if you did not use a carburetor rebuild kit, the you may have to do it again.
You also can use what we call "Mechanic in a bottle" and it may help you considerably.
Now we have to look at the valves. If this is an overhead valve engine, the valves must be adjusted at least every two years. there should be a clearance of between .004" and .005" between the rocker arms and the valve stems when the valves are completely closed.
Check the engine oil and make sure that it is not LOW. If the oil was EVER low, you may have an engine that is going bad. It may even be starting to lock up.
If the engine is not the problem, you may have other issues with the tractor.
Check all of the idler pulleys. The bearings must spin without making ANY noise when the belt is NOT touching them, You may have to take the belt off to spin them
Also The bearings in the mower deck spindles must spin freely without making any noise. Spin them by hand with the belt NOT on and listen for noise. If there is noise or if it does not spin freely, then the bearings must be replaced.
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