I have an 91' B&S 18 hp twin cylinder engine(model 422707). Since I've had it, I've had to run it at half choke. The coil went out, so while I had all the tin off replacing it, I found 2 bolts on the right head backed out against the cover, so I also replaced both head gaskets and de-carbed the piston tops and heads.
I pulled the plug off the carb bowl and pulled the low speed jet, and sprayed carb cleaner in everything, now I need to run it at almost full choke, and I have almost no torque. I'm guessing I moved some dirt around inside, but I could be wrong. While I wait for a response, I'll take the carb off and give it a proper cleaning to see if that clears it up.