Nitro engines often need fine adjustment of the carburetor in order to get the most out of the engine and to avoid possible damage. Weather conditions and other variables can affect the tune so it’s a good idea to check the tune on your nitro engine when you fire it up for a day of bashing or racing. You may find this intimidating, but it’s actually very easy to do and doesn’t take a lot of time to complete.
Check the glow plug and make sure the glow starter is fully charged
Before you think about starting your engine, make sure that your glow starter is fully charged and that your glow plug is good. If there’s an issue with either one of these items, your engine won’t run properly or won’t start at all. You may be confused and think that the tune is off on the engine. Before running your nitro vehicle, make sure you put your glow starter on charge while you’re getting ready to head out. By doing this, you’re guaranteed to have enough juice to start the engine. If you don’t have a rechargeable glow starter, make sure you carry a spare battery for it in your toolbox. Clean around the glow plug area with a splash of fuel, compressed air or a brush and remove it from the engine. Make sure you have the engine upside down so none of the contaminants make their way into the engine. Touch the glow plug to the glow starter—it should glow a bright yellow/orange color. If it has a dull glow or no glow at all, it’s time to replace it. If you have an electric starter you can test the plug by making sure the starter wire is attached and grounding the plug against the heat-sink head. Turn the engine over and look at the plug to see if it’s glowing.
Start the engine and clear it out
When starting your engine, it will be cold and run a little on the rich side. You may have excess fuel in the combustion chamber due to over priming of the carb. When starting, make sure you give it some throttle in order to bring the engine’s rpms up once it fires; about a quarter throttle will do. Once the engine fires, give the throttle consistently quick half-throttle blips a few seconds apart while the tires are off the ground. You can prop the vehicle up on a block of wood, starter box or simply hold it in the air. After about a minute, the temp in the engine will start to come up and you can give it some full throttle blips to clear out the engine of excess fuel.
Get it up to temp and tune high-speed needle
Now that your engine is fired up, you can start checking and adjusting the tune. Place the vehicle on the ground and run it to get the engine up to the proper operating temperature. If you’re bashing, simply run it around the area you plan on playing in for two or three minutes. If you’re racing, run a few laps on the track. Bring the vehicle over and immediately check the temp of the engine. If you let it sit longer than a few seconds, the engine temp will change and give you a false reading. When using a temp gauge, place it directly on the heat-sink head pointing down at the glow plug. Engine temps will vary, but a good temp to look for is 230 degrees. If the temp is lower, the high-speed needle can be leaned out slightly. If the temp is higher, you’ll have to richen the high-speed needle slightly. If you don’t have a temp gauge you can use the “spit” test. Just use a drop of water or saliva on the top of the engine’s heat-sink head. It should sit for a few seconds and then evaporate. If it sizzles and evaporates right away, your high-speed needle is set too lean and if it doesn’t evaporate at all, the high-speed needle is set too rich. You should only make fine adjustments to the high-speed needle and once you have made your tweaks, run the vehicle again and bring it back over for another check.
Tune low speed needle
Now that you have the high-speed needle setting where it needs to be, you can move on to the low-speed needle. Again, run the vehicle to get proper fuel flow and temp where it should be and bring it over to you. As soon as the vehicle stops, you’ll want to pinch the fuel line—make sure you use your fingers for this. If you use a pair of pliers, you can poke a hole in your fuel tubing and then you’ll have an air leak in the fuel system, which will give you more of a headache when trying to tune your engine. When you pinch the fuel line, you’re looking to see the engine rpm slowly start to rise and the engine will shut off. If you see that the engine dies right away, your low-speed needle is set too lean. If it runs for a while before shutting down, your low-speed needle is set too rich. Just like the high-speed needle, you’ll want to run the vehicle around after making changes and recheck the setting.
Adjust the idle
With the high and low-speed needles set to the proper tune, you’ll need to adjust the idle. You need the engine to idle high enough so that it doesn’t stall when you let off the throttle and you don’t want the rpm set so high that the clutch is engaged when you’re off throttle. So, you’ll want to find a happy place in between those two. When you turn the screw in, the idle goes up and when you turn it out, the idle goes down.
You’re ready to go!
Remember that when you turn the high- or low-speed needle in, you’re leaning the mixture and the opposite goes for when you back the needle out. Make minor changes each time you make an adjustment and see what that adjustment did. If your engine ran great for one tank of fuel and then doesn’t want to run the next time you fire it up, chances are that the tune on the engine isn’t the problem. Be sure that you check other components such as your glow plug, glow starter, clutch or fuel system before making adjustments to your carb.