He said he had dirt in the carbs. Where did the dirt come from? I would be looking at some crap floating in the fuel tank blocking the pickup line.

Had a similar issue on a big loader one time. I know, not an apples to apples, but the result is the same. Crap was floating in the fuel tank and it would run great some times and other times it wouldn't run right at all. Finally had to take the fuel tank off, clean it out and all was well. Just my 2 cents.