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RC Engine Tuning Tips

Article by Kenny McCormick

Most everyone knows engines need to be tuned to run their best. But a lot of people are scared away from nitro because they think it's some sort of black art. Perhaps they tried one and didn't get it right, perhaps their engine wasn't made properly, perhaps their fuel is bad. Doesn't really matter. Getting these little marvels of engineering to behave isn't some mysterious black art, it's actually quite simple.

The first step in tuning your radio controlled car engine is to get it running in the first place. This will mean resetting the carb to the factory pre-sets, which can be found in the instruction manual. If you have just completed break-in, your needles are right where they need to be. Either way, your engine should be running quite rich at this point. Once your carb is set, you will need three simple tools: A small pocket screwdriver, your ear, and your eyes.

Before we start turning needles, you need to know what to look for. There's four states of tune an engine can be in: Overly rich, slightly rich, absolute perfect, and lean. Most people should aim for the slightly rich setting, avoid both extremes at all costs, and only go for 'absolute perfect' if you absolutely need to. Setting your rc engine to 'slightly rich' will ensure you don't have to tinker with the carb constantly, the engine lives a very long and happy life, and it makes tons of power, so this article will focus on achieving this setting.

Rich Nitro Engine Tuning

rc engine closeup

A overly rich engine you are likely familiar with. Just for a refresher, it's going to have absolutely no power, wolf down fuel like crazy, and smoke like a chimney. It will also sound like it's barely running at all, and it won't idle very well. Break-in is done at this setting because the extra oil flowing through the engine helps flush out small metal particles shaved off during break-in, among other things. However, continuing to run at this setting will severely stunt your fuel economy, you'll have no power, you will go through glow plugs like crazy, and you will damage your engine. Glow engines used in rc cars have a cone-shaped cylinder that is designed to expand more at the top than the bottom. The temperature range at which it is the proper shape, a perfect cylinder, is between 220 and 280 degrees Fahrenheit. Below this range, the cylinder will wear out much faster, and you risk breaking the connecting rod. Above this range the cylinder expands too much, which just leads to a stall.

Lean Nitro Engine Tuning

A lean engine is a starved engine. It doesn't have any fuel going into it, which makes the RPM unstable. Lean engines don't like to idle down properly, and they accelerate almost instantly. They also lack lubricant, which will make them wear out VERY quickly. They also run very hot. You'll hear a popping and crackling noise coming out of the exhaust at idle, and it will barely smoke if it smokes at all. The engine also gets a very high pitched wail to it, and it just won't sound right. Lastly, a lean engine is VERY difficult to get started, and often won't do more than pop and snort for quite a bit. Overpriming a lean engine always gets it fired right up, and it will often die as soon as it burns the prime off. Do NOT let your engine run here for very long, as it will QUICKLY lunch the engine. Detonation will put the combustion chamber and may melt a hole in the piston, and it may melt/dislodge the glow plug element, which will then fall into the cylinder and damage an already oil starved cylinder and piston. In short: This is death mode. Do not let your engine run here. If you run it lean it's lifespan will be measured in minutes. Maybe a couple hours if you're lucky.

Absolutely Perfect Nitro Engine Tuning

Absolutely perfect seems, at first glance, to be like a lean rc engine. It does act like one in some regards, they tend to run a bit on the warm side of the range and they smoke very little. The key difference is they do smoke somewhat, usually when you nail the throttle, they idle down properly when you lift off, and they sound like they're pulling hard. They behave themselves, they won't pop and crackle at idle, and they do not eat glow plugs. An engine here is making the most power it is capable of making, yet is still adequately lubricated. Chasing this tune, however, will require constant attention to the carb. Engines this small are highly sensitive to the atmosphere. The tune will creep about minute by minute, and a perfect engine in the afternoon is running lean by evening.

A slightly rich engine will sound like a perfect engine, but it will be a bit softer. Throttle response will be slightly sluggish, but not severely so. The engine will smoke, but it will not release liquid droplets of fuel out of the exhaust. It will tend to run on the cooler side of the temp range. Performance is excellent, but slightly slower than perfection. Glow plugs will last for a long time, on average three to five gallons of fuel. The engine will be very easy to start, but may not idle properly until it's warmed up. Idle speed is nice and stable, and it idles down quickly and smoothly. This is where you want your engine to be. An engine set here will not care about the atmospheric changes morning to afternoon. I keep my own engines here, and I only adjust them twice a year to compensate for summer and winter temp differences. That's it. Between those two adjustments, it's a matter of putting the fuel in and firing it up.

Tuning The Engine

high speed engine needle

The tune on your engine is controlled with three needles. One is the high-speed needle, one is the low speed needle, and one is the idle speed screw. They are ALWAYS tuned in the order I just typed them, the only reason you should deviate from that is if you're making a minor correction based on atmosphere. If you have a midrange needle, set it to the default and do not touch it again. Adjusting them is simple, leaning the radio controlled engine is done by turning the LSN and HSN to the right, richening to the left. Idle speed goes up when you tighten the idle speed screw, and drops when you lower it. Keep in mind you will need to readjust your throttle linkage after you have the engine tuned, as the idle gap will drop significantly.

Now that we know what to look for, where to set the engine, and how to set it, let's actually get it tuned. You need somewhere with the room to let it scream at top speed for a good five to ten seconds. A large, empty, paved parking lot is ideal. You also need your three tools, a pocket screwdriver, your ear, and your eyes. Start the engine and get it warmed up as best you can with the current settings.