I am working with an inherited Ariens 42" riding lawn mower (model 960460011) that has hydro transmission. Last year it seemed to travel faster than now. The lower speeds seem to work but the higher ones don't seem to be as fast as before and do not work when going up a gentle hill. What should I be looking for? Does grass clog up the belts or is it something else? Thanks for any advice.

I have a White Outdoor GT-2150 mower, which has recently started to run poorly. It starts fine, drives fine, cuts fine etc. but randomly starts to lose power and sputter/backfire and has actually cut out entirely a couple times. The first time I put it down to low gas, but it has since done it a couple more times with the gas at least half full. Initially it appeared to only do it when the engine was hot, but this latest time I barely even had it running before it started to do it. However, the engine did appear to be really hot even after just a few passes in the yard. I did also notice there was a little white smoke coming out from behind the air filter (when it started sputtering this morning, I stopped it and checked oil/airfilter, which is when I noticed the smoke). I believe it had some recent engine work (a family member used to use it, did some work on the valves I believe) It's a Briggs & Stratton V-Twin engine, oil level is good and the oil is clean, the air filter is clean, has a new battery, the only thing I have not yet checked is the spark plug. Any idea's what to check? hoping its an easy fix as it has been a solid mower. Thanks.