RCU Forums - GMS 120 Tuning Problems and Solutions

First off make sure that you seal and secure everything on the carb. I found the best thing for this is "Permatex High Temperature Thread Sealant". It's the stuff in the white tube. Then if you have a Bisson Pitts muffler remove the brass pressure nipple and drill it to the next size so it moves a little more air to the tank. Don’t go to big or you might weaken the thread section and cause it to break when you tighten it back in. Next get some K&B long with idle bar glow plugs. I’m also going to switch to Cool Power 15% because that is what others are having success with. I have been using Omega 15%. I burned my last tank of Omega today and the engine ran perfect. I expect it to do the same on Cool Power. Also I would not use an Easy Fueler or Kwik-Fill Fuel Valve. My engine had so much draw that it would suck air into the system through the valve. And for the prop use Master Airscrew 16X6.

---------------ENGINE TUNING--------------
The engine should start and keep running, although it may be running slightly rough at this time. This is normal. Allow the engine to warm up for 15 to 20 seconds before removing the glow plug clip. Let the engine run at this throttle setting for 30 seconds and then open the throttle to full. The engine should be running at full throttle, but running somewhat rough, with a lot of fuel/oil being discharged from the muffler. This is because the engine is running in a very rich condition–that is, more than enough fuel is running through the engine than is needed. Run the engine at this setting for at least five minutes. After five minutes, you can start leaning the engine by turning the high-speed needle clockwise. Never turn the needle more than 1/8 of a turn at this point. To determine if the engine will accept having the needle leaned, give the fuel line a quick pinch and release...just pinch the fuel line and let go. You should hear the engine increase a bit in RPM, with the sound going up in pitch. If the engine increases in RPM, you can lean another 1/8 of a turn. Allow the engine to run for one minute at this setting. Now give the fuel line a quick pinch and release. Wait thirty seconds and do it again. Do this five times. If the RPM increases all five times, lean the high-speed needle another 1/8 of a turn. Wait one minute, and then start the pinch and release series again. If the engine doesn’t increase in RPM when you pinch and release, wait one minute before you try the pinch and release series again. Don’t lean the high-speed needle until you get an increase in RPM every time you pinch and release the fuel line. Keep repeating the “pinch and release” method until the RPM does not change when you pinch the fuel line. You’ve leaned the engine as far as possible. Do not ever lean the engine to the point that the RPM decreases when you pinch the line or severe damage will result. For flying, you want to set the high speed needle so that you get an increase in RPM when you pinch the fuel line or when you pick up the airplane and point the nose of your model straight up. If you hear the engine’s RPM decrease when you pinch and release, you must immediately turn the high-speed needle counter-clockwise at least 1/4 turn. The decrease in RPM tells you that the engine is too lean, and needs to be run at a richer setting for at least a few more minutes. The entire process detailed above can be done by the end of the second tank of fuel, or part way into the third tank. It should take a little over ½ hour to get your engine ready to fly. At this point, you can adjust the idle mixture setting. On adjusting the idle screw only turn it 1/8 turn at a time. I did my adjusting by running it at half throttle and then going full throttle. If it loads up then goes full rpm it is too rich on the idle screw. SHUT OFF THE ENGINE then lean the idle screw 1/8 of a turn and restart the engine. Go full throttle until it goes full RPM then back to idle, make sure it is going to idle. Then put it at half throttle for about 30 seconds or so and then pop it up to full throttle. If it still loads up repeat the step above. SHUT OFF THE ENGINE then lean the idle screw 1/8 of a turn and restart the engine. Keep doing that and you will start to notice a difference. Occasionally check the high speed needle and make adjustments there as needed. When you finally get it to the point where it no longer loads up at the half throttle point bring it back to an idle for 30 seconds or so and then go straight to full throttle. It should go straight there with no loading up and no hesitation. If it hesitates for more than 1 second you have leaned the idle too much, open it up just a tiny bit and check it again.
This has worked for me and I am very happy with the end result. It no longer spits and sputters or dies in the middle of my flights. I hope this works for you.

Note: It is always safer to have the engine run a little rich. This will increase the longevity of the engine and make throttle response more reliable.