Some, but not all OS 1.60's run lean on a Bisson Pitts style muffler because the muffler is free breathing and provides insufficient back pressure to pressurize the fuel tank. This seems to be a borderline condition where certain 1.60's run well in warm weather but they go lean when the cool season comes and you find that you can't richen them no matter how far out you screw the needle valve.

I prefer to use the Perry VP-30 pump to take advantage of the increased power allowed by the low back pressure mufflers rather than crimping the muffler's exhaust tubes to create back pressure. The pump also allows you to place the fuel tank anywhere you want to.

The pictures show where I mounted Perry VP-30 pumps on a Giles and a Katana. One picture shows that the rear of the Giles' fuel tank extends well behind the wing tube. The remaining picture shows the O.S. 1.40 EFI pressure fitting that I use in my I.60 engines. The Perry pump comes with a little pressure fitting that you may find easier to install because you drill and tap the crankcase cover for it.