Small Engines (Lawn Mowers, etc.): B&S 12.5 hp starting issues, lmt, air compressor

Question
B&S 286707-0437-01 on a Snapper 281011BE RER. About 45 minutes into first cut of the season,engine would race then slow down and continue to act like this. I stopped engine and it wouldn't restart unless I removed the air filter. I replaced the filter with a new one and had the same results, first filter had only about two hours of use, With the filter off and engine running it will, at different times, speed up and slow down. I adjusted the idle mixture and got the same results. Carb. is a Walbro LMT. After I start it with no filter and put the filter back on, it will run but will surge and back down at different times

Answer
Have you removed, disassembled and cleaned the carb with compressed air from an air compressor and carb cleaner?  From what you described I would start with cleaning the carb.  Take pictures as you remove and disassemble the carb and you should have no problems putting it back together.

Let me know if cleaning the carb helps.
Eric