How to tune r/c nitro engines

Written by derek odom
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Adjusting the settings of the nitro engine in your radio controlled car is important. Even slight setting changes can affect the engine in big ways. The most important tune is getting the air to fuel mixture correct, followed by making sure that nothing has air leaks. Even the temperature outside can affect how your nitro engine runs, so it is good to know how to get a decent adjustment.

Skill level:

Things you need

  • Small flat head screwdriver
  • RTV silicone

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  1. 1

    Adjust the low-end fuel mixture. The low speed needle (LSN) is the smaller of the two flat head style screws on the carburettor. It adjusts the air/fuel mixture from idle to the engine's mid-RPM range. If this needle is tuned too richly (excessive fuel going into the engine), large droplets of raw fuel will dump out of the exhaust pipe at idle, and the car will hesitate badly when it accelerates. To improve this situation, the LSN needs to be turned clockwise a bit so that the air/fuel mixture on the low end becomes more lean (less fuel, more air). Too much fuel chokes the engine and can even cause it to stall. If the idle is too high and cannot be set with the idle screw, it is possible that the LSN needs to be turned counter-clockwise so that the air/fuel mix becomes more rich. Adjust the screw incrementally, only a quarter or an eighth of a turn at a time, to prevent going too far the other way. The engine's low-end is tuned when only a small amount of fuel exits the exhaust at idle, and the car accelerates from a stop very powerfully and with no hesitation.

  2. 2

    Tune the high-end fuel mixture. The high speed needle (HSN) is the bigger of the two flat head style screws on the carburettor, and it adjusts the air/fuel mixture from the engine's mid-range to top speed. You'll know if this screw needs to be adjusted to lean the mixture if the engine cuts out at higher RPMs or if excessive fuel is spurting out of the exhaust at speed. Likewise, if the engine gets too hot (usually above 121 degrees Celsius), it could be running too lean, requiring a slight adjustment. Like the LSN, turn clockwise to lean the mixture and counter-clockwise to richen. Leaning the mixture (adjusting for less fuel and more air) always produces more power, which produces more heat, so a happy medium is necessary. Colder weather produces a dense fuel charge, and warmer weather creates a less dense charge, so slight tuning may be necessary even from morning time to afternoon in some areas.

  3. 3

    Seal any air leaks. Nitro R/C engines absolutely hate air leaks, so it is imperative to find and eliminate them. One common place for them to leak is the carburettor seal, where the barrel slips into the engine. There is an O-ring there on most models, but it may be necessary to add a small amount of RTV silicone if the seal is worn . The fuel tank must be sealed correctly and without cracks. Because nitro R/C engines work on a vacuum fuel feed, even the smallest crack in the tank can cause a lean condition (too much air, not enough fuel) and cause the engine to run erratically. The exhaust coupler between the header and the pipe must be tight, and the fuel line cannot have pinholes in it. If your line is leaking, replace it immediately. If you locate any leaks in the areas mentioned, seal them up and performance will increase.

Tips and warnings

  • The engine may not tune correctly if it has a bad glow plug so be sure you are using a new one.

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