I have the red .90 size muffler as well. I have a bolt-on/bolt-through muffler adapter from Jett on it but I think the adapter is limiting power a bit so I think I'm going to drill the threaded holes out and all the way through on the engine case to use the Jett muffler without the adapter. My .90 runs pretty well on 5% nitro fuel as well. During the first gallon or two after the bench runs it didn't like even 5% nitro so I ran FAI fuel for awhile initially. I had to clock the spraybar about 35* to lean the midrange. After a couple gallons, I had to reset the spraybar to straight in. Runs great now, and the compression is incredible. All of my ST engines run really well.