Before doing a muffler mod, lets do first things first, just to make sure you are successful in removing the limiters. If you can't remove the limiters, don't do the muffler mod. Get yourself a 3 1/2" Deck Screw in size 6, and have a size 8 standing by. Screw down into the red plastic limiter until you seem to be hitting metal and the plastic is about to strip (stop before you do that). Get yourself a vice grips and firmly lock onto the dry wall screw. I wear gloves while doing this, and pull up with steady and strong pressure. Sometimes I pull up and the red cap doesn't come along. I then go to the other limiter cap and have at it. Usually the aluminum positioning plate comes up with a limiter and the worst is over. You might have to use a size 8 deck screw if the 6 screw stripped out, but you will get them out. It is a good idea to have a set of hemostats available for finesse work. Everyone talks about aligning the slots, but I seldom have luck with that, as when I screw in the dry wall screw, the plastic stops move, regardless of how I had them positioned. I just bully the limiters out. Works almost every time, and when it doesn't, I remove the recoil mechanism and outer plastics to gain better access to the limiters. I use a carbide cutting tool to remove the aluminum spacer, and then I have complete access to the limiters. Once the limiters are gone, I reset the low (closest to the carb) to about 2.5 turns out, and the high to about 3 1/4 turns out. I turn up the idle screw about 1 turn just for starters. This is just an initial adjustment, and you will likely be too rich, but you can lean that out later.
Once you know the limiters are out, you can remove the muffler. There are two nuts holding it on, 5/16 socket. Remove the nuts, then slide the muffler off the two muffler retaining bolts. Study the muffler and locate the catalytic converter biscuit (you can't miss it - looks like a dense catacomb). I would recommend using a face mask or respirator when drilling the catalytic converter, as it is supposed to have cancer causing properties. Drill a small pilot hole into the center of the catalytic biscuit. Now use larger drills to open the hole diameter up to about 1/2" to 5/8" in diameter. I wouldn't take anymore out than that, because what remains of the biscuit helps to quiet the saw but doesn't materially add to increased power should you completely remove it (some might disagree, but the extra removal of cat isn't going to make that much mower power at this point). The other side of the muffler is a breeze. Remove the screws that hold the deflector in place. Remove the #30 screen and the deflector. You want to grind out the lower level deflector blocking material, leaving the higher rear deflector exhaust scoop. Use an air hose to blow out the muffler, and wash in warm soapy water, rinse, and blow out again. Let the muffler completely dry and reinstall the deflector and screen. Reinstall the muffler with #25 torx driver. Now you can start your tuning process. If you use a tach, keep your maximum rpm to around 12,200 - 12,300 once warm. Run your saw with 40:1 2 cycle synthetic oil, mixed with premium non-ethanol gasoline. Your saw will become a little breast.
