how do you clean the filter? I use a soft 1" paint brush, and just lightly brush the outside. the grey little hairs in the filter fabric are keepers.
are you keeping your chains sharp?
also, any chance you are running carb on rich side ... I used to set my 540 a little too fat, and I was also getting a lot of "blowback" in the intake system, showing up at airflow dead zones in the carb connections and in the filter bend, where dust would accumulate (tiny specks of oiled dust look like shiny mountains on the adapter). I'm just guessing that my excess 4-cycling produces more blowback (any one who knows why please help if you can). At the advice of (?-walker I think?) I've learned to lean up the carb a little, and the blowback has pretty much gone away. Now, it seems I have more "black foam" (actually crud) coming out the muffler. (I'm still nervous about the noticable increase in heat that's still coming off the cylinder when I finish a couple of cuts-but so far no trace of scoring or seizure.)
As to airflow, I did just the opposite of what you are doing! Rather than restricting flow (increasing suction, velocity and dust entrainment), I cut a 3/16" x 3" slot across the back of the black top cover, to increase breathing (reduce velocities) outside the carb (as well as opening up the muffler insides). I used a box cutting knife with red-hot blade to cut the hard plastic, and I drew my lines on a piece of masking tape. With no actual scientific measurements or understanding, I thought the configuration and dimensions (net efficient flow) of the intake was almost rediculous, for the quantity and suction velocities of intake air involved. I wanted to reduce the velocity "hot-spots" and turbulence leading into the filter. I also shaved down the hard edges inside the ducting flow path to avoid dead air spots. (My next upgrade for the 540 will be the Tilly carb -- the choke baffle on the Walbro doesn't IMHO configure well with the filter/bend arrangement. With that 90° bend in place, there's just no way there's an even distribution of flow across the WT choke baffle The top of the choke baffle is in effect a dead air zone, which creates a negative vortex which circles around the choke baffle during operation. IMHO -- an unnecessarily compromised design.). The person who manufactures an air filter for the 400/520/540 series which incorporates the 90° bend into the air filter (eliminates I part and two screws), so that a larger filter attached directly into the carb, where the 90° bend attaches now, and which has a generous intake funnel in place of the bend, to transition airflow from filtration velocities to venturi velocities, will do a good thing for that design.)
slightly off topic, but decreasing 4-cycling blowback and decreasing dead spots in the intake path, and reducing intake filter velocities might help ... that was my experience anyway.
