Mike, I have my LA .40 in a Brodak Oriental arf. I have 3 of these and was warned against buying all of them. The concensus said that the .40 doesn't run well for stunt. I disagree as I like all of them. I use a Randy Smith tongue muffler on mine opening the holes up a bit. I didn't add any additional holes. I also removed the remote needle as I found that it needles far superior to the stock one. Having my engine fully cowled, I don't find any overheating or runaway problems. I would think that your engine is retaining too much heat either through the stock muffler your venturi could be slightly larger. I haven't used Sig fuel as I still have an ample supply of Powermaster 5/22. I recently purchased the Thunder Tigre prop your using but, have yet to use it. I use the APC 10.5x4.5. I will say that the Thunder Tigre is an extremely heavy prop over the APC. I don't think that is the problem as I know several people using that prop on the .46 and also .40. Is your engine getting leaner as the flight progresses? If so, could this be a tank issue in which your starting out rich and then it goes overlean. I found the best obtainable runs for me was using a uniflow setup but I run muffler pressure to the uniflow pipe. I would think the things that would make a engine run hot would be too much prop load, not enough fuel, restricted muffler. I have successfully run the stock muffler though. I used it for 2 seasons , its just its weight is a big penalty especially on the Oriental which was designed around the Fox. The La weighs almost 2.5oz more as is. I know Brett Buck has mentioned more than once that the stock set up is usually the best bet. I just went with the tongue due to weight and I found I could set the needle better due to the engines note change when I hit the inbetween 2-4 cycle. The stock set up was more of a monotone sound. I would make sure the engine is tight in its mounting. I would then make sure your not suffering from the case of the bad vibes which could be foaming the fuel causing a lean run. If so and its minor, a squirt of Armor All in the fuel may surely help matters there. One squirt will do in a gallon jug. It would be imperative to check all fuel tubing connections. I've had small splits in tubing and also chaffed fuel lines which went undected which can give you fits beyond belief even though they don't appear they're leaking fuel. This can lead to harder starting and surely leaner runs. All my engine have the stock plastic backplate. I was told they're typically not a problem until you take them apart. Well, I did and reassembled without any gasket or sealers and never had a problem to date. This is going on 3 years now. I still keep an eye on them though and you should do the same checking for leaks and also cracks. Especially keep them snug and not to overtighten them. With the glow plug removed, is there any kind of bind in the engine? I'm just shooting in the dark here so try anything. Ken