Too rich. Are you adjusting the needle valves? If you are new to RC, and have never adjusted a sm 2 stroke before, ask an experienced RCer to help. If there's no one around, start the engine, go to full throttle, leave the glow igniter on, turn the needle valve in until the engine no longer increases in rpm, if you you go beyond quickly unscrew the needle 1/4 a turn, then get it to peak again and unscrew it slowly until you hear a sm drop in rpm, remove the glow starter and do it again, then pinch the fuel line real fast, the engine should jump in rpm, if it looses rpm richen some more.

Lower the throttle to idle, let it idle for 10 or 20 sec, then open the throttle, if it just dies, the low end is too lean, if it gurgles, spits and sputters, the low end is too rich. The low end needle is more sensitive than the high end needle, so move it an 1/8th a turn at a time.

After it's adjusted you usually wont need to adjust the low end, unless you make a big change in nitro or oil content.

Hope this helps.
--Rick