Recently I modified my Osborne tip tanks to feed into my main tanks as opposed to feeding directly to the engine. This change necessitated removing the 5 way fuel selector valve and installing a rebuilt, standard 3 way fuel selector valve (Part #36-380026-1). At the same time I changed out the air filter. I have a turbo-normalized IO-520 that has been on the aircraft for 150 hours of Hobbs time.
The engine starts fine, idles fines, and accelerates normally until approximately 2200 RPM at which point adding fuel results in no additional RPM but reduced RPM because. Turning on the boost pump at this point reduces the RPM even more.
I’ve checked all connections for fuel leaks and found none. I checked the mixture at idle and found 25 RPM rise as the mixture is slowly pulled out. All cylinder temperatures are similar and normal. The character is the same on both the left and right tanks. I am starting to suspect an air leak that gets worse as RPM increases, but see no evidence of a fuel leak when I pull the mixture off and run the boost pump to circulate the fuel through the system.