The pinch test is probably one of the best nitro engine tuning tips than I can offer. When the engine is at full throttle, simply pinch the fuel line feeding the carburetor for a brief second. It’s best to pinch the fuel line that connects the carburetor to the high-speed needle valve. This would be the line labeled “Silicone Tube” in the above drawing. If you pinch the fuel line before the high speed needle it will take the engine longer to react.If the engine is running rich, the RPM’s will increase slightly before the engine sags. If the engine is lean, the RPM’s will immediately decrease. The high-speed needle valve should always be slightly rich. Slightly rich is good for your engine! Slightly lean will destroy your engine. The engine will lean out while flying your airplane due to the higher elevation and emptying of the tank. You will need to compensate for this by setting the engine slightly rich on the ground. If in doubt, always err on the side of being too rich. Your engine will last longer and there will be less of a chance of your engine stopping in mid flight.