I think it was Tom Walgmuth who said he runs his small engines on lacquer thinner to clean out the carb. I'd give that a try or just give the carb a lot of shots of aeresol carb cleaner when it's running. Sounds like the main jet has some dirt in it. You might be able to pull the main jet out and squirt some carb cleaner directly up into its seat. Lots of small engine carburetors have jets that can be removed by unscrewing the big nut at the bottom of the bowl.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13