I had the same problem with my 190402 tiller engine (on a 1978 Sears RT-8 that my brother gave me). The carb leaked like crazy when I got the tiller, so I rebuilt the carb. Bought a new float, but the old one was fine (the hot water test you can read about if you do a Google search).
After rebuilding the carb and checking the float system (carb apart, hold carb top upside down with the float and needle installed, then blow into the gas inlet to see if you can pass some air. My carb top passed that test, and I guess it's a valid test. Never tried that one before . . .
Put the engine back together, and the carb leaked gas fairly steadily, but at a much lower rate.
I read a comment from someone that sounded flat-out wrong, but the Briggs rep. who responded to my email said to do the same thing. In fact, the rep. sent me a Briggs document showing what to do. It's like a tech sheet. Nice customer service (and I never even bought their stuff directly!!).
The problem I had was with the emulsion tube and its seat inside the carb. You probably have the same problem. You need to buy a new one emulsion tube because you are going to ruin the old one. The tube seats against the carb at a place that is below the fuel level -- can you see the problem?
Take the old tube, grind off the threads (I used my Dremel tool). Get some lapping compound (I used water-based stuff from the local auto parts place). Put some compound on the seating part of the tube, insert the tube, and use a screwdriver to lap the tube against the carb seat. Check it every once in a while. I think I probably did it for 3 to 5 minutes, not very aggressively, and added compound one time.
Look for a smoothness to the carb seat. Thoroughly clean out the carb. I used spray carb cleaner for the initial rebuild and the lapping compound clean-up. Put the carb back together and check for leaks before starting the engine. Start the engine, and hope that the leak stays gone.
I haven't had any problems yet, and I've used the tiller for about 4 to 5 hours so far. Maybe 6 or 7, but I've been using it for about a month off and on.
If you want me to send the pdf version of Briggs' document regarding this fix, send an email to me at [email protected]. Just put in the subject line: Briggs Carb Fix. I'll shoot you a copy no problem.
Good luck.
John
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