All nitro engines have an idle adjustment screw. This adjust the motor speed when the throttle is in the idle position. If it is set too fast, the tires will want to move. If it is set to low, the engine will cut off. The factory setting is usually right on, but if you need to adjust it, you want the throttle opening to be about 1/8" at idle speed. If you are new to nitro motors, you may easily get the impression that the motor is always running too fast. If the motor is actually running too fast, the clutch will be engaging and causing the tires to turn. You want an idle position that is as slow as you can get without the motor Nitro TC3 Idle Screw shutting off. In most cases, the factory setting is right on as you want the throttle opening in the carb to be right around 1mm. This is not likely to need adjustment.
Low Speed Adjustment
If your carburetor has a low-speed adjustment needle like show on the Nitro TC2 motor to the right, this needle is used to adjust the fuel mixture when the throttle is between 0 - 50%. This is the needle to be adjusting if your motor is difficult to start. Some motors are very sensitive to this setting and a difference of a 1/4 turn may be the difference between good performance and not being able to get your motor to start at all. Once you have this needle set, you should rarely, if ever, need to adjust it again. Turning the screw Nitro TC3 Low Speed Needle clockwise will lean the setting while counter clockwise will richen the fuel mix. A good method of testing the low speed adjustment is to get the motor up to operating temperature and pinch off the fuel line that goes from the fuel tank to the motor. If the motor dies abruptly in under 3 seconds, the low speed setting is too lean, if it takes 4-5 second or longer to die, then the low speed setting is too rich.
High Speed Adjustment
The high speed needle adjust the air/fuel mix when the throttle is between 50% - 100%. This is the needle you will be adjusting to get the top performance out of your car. If the high speed adjustment is too rich, you will hear a chugging sound when romping on the throttle before the motor revs up and takes off. If the setting is too lean, the motor may sound like it is slowing down or hesitating before revving up.
Single Needle Carbs
Some motors only have a single needle adjustment which means the low-speed and high-speed are eliminated by a single setting. While this is easier than a dual-needle setup, it is not as efficient as you often are sacrificing acceleration for top speed or vice versa. The best way to tune these is too start with as rich of a setting as you can get the motor to start under and keep leaning it out 1/8 of a turn at a time until you find the motor's sweet spot.
Is It Set Right?
The best way to tell if the motor is tuned properly is by testing the temperature of the motor. Each motor is a little different so consult the motor's owner's manual or website to find out what the optimum temperature of the motor is. This is usually between 230 - 260 degrees. There are some motors that run under that range and even some that prefer hotter temperatures, so again, check with the motor manufacturer. Without a temp gun, how can you tell if the motor is too hot or cold? One simple way is to put a drop of water o the cylinder head. If it evaporates in 5 seconds or less, the motor is too hot and needs to be richened up (counter clockwise adjustment of the high speed needle). If the water takes longer than 7 seconds to evaporate, the motor is too cool and can be leaned out After each adjustment, run the vehicle as you normally do for at least 2 minutes before adjusting it again in order for the temperature to equalize. Always make small adjustments.
Your sitting there yanking on the pull start for an hour and getting blisters on your hands and your motor still wont start. What do you do? The best thing to do is to put your settings back to a point where the engine "should" run even if poorly. First, make sure the idle adjustment isn't set too low, make sure there is at least a 1/8" gap for air to flow through in the throttle body. If the idle adjustment is not at fault, then the low speed mixture needs to be adjusted. Start by turning the low-speed needle (or single needle adjustment screw) all the way clockwise until it stops, be sure to not tighten this screw down. Next turn the screw counter-clockwise 2 1/2 turns. Almost all motors will start within a 1/2 turn of this neutral setting. To test the low-speed setting once the motor is running, pinch the fuel line. If the motor dies within 2 seconds or less, the low speed setting is too lean. If the motor take 4 second or more to stop, the setting is too rich.
Rules of Thumb
While every motor is a little different and will take a little effort to get dialed in perfectly, there are a few things to keep in mind. First, it is always better to run a motor rich than have it be too lean. If a motor is run too lean, it will run hotter and wear out the components quicker. Secondly, keep in mind that at the temperatures these engines run at, they can easily cause nasty burns. Always think safety. Remember that when you adjust a setting, the motor needs to run for at least 30 seconds for the motor temperature to adjust to the new settings before you can tell if that setting works better or worse. The single most important thing when tuning a nitro motor is patience. Take your time, learn the "personality" of your new motor and soon will have it dialed in and working perfectly.