By far the steepest learning curve for a nitro rookie is learning how to tune a carburetor. It can be awkward at first, but with a little effort, you’ll get it and never forget how to do it. Before we start adjusting needle valves, let’s consider the role of the carburetor and how it works. Properly tuning a carb is much easier once you understand how it functions.
A rotary carb contains a horizontal “barrel” that has a vertical hole drilled through it. As the barrel is rotated from the open position (the hole in the barrel is aligned with the hole in the carb body) to closed, the airflow that is pulled into the engine is reduced and, as a result, the engine speed is reduced.
In some cases, the barrel not only rotates, but it also moves slightly from one side of the carb housing to the other – an action similar to a screw being twisted from a threaded hole. This sideways motion operates the low-speed needle by pushing it into or out of the spraybar.
The slide-valve carb components are similar to those on a rotary-barrel carb, but they’re just arranged differently. Instead of having a vertical hole drilled through it, the slide valve is a solid piece of plastic or metal. To regulate airflow, as the piece slides into and out of the carb’s venturi, it allows air to pass by. As with the rotary type, the slider contains the low-speed needle valve to control the fuel/air ratios for idle and low throttle settings. As the slider is further closed, the low-speed needle valve reduces fuel delivery and the air flow is reduced; as a result, the engine slows down.
WHAT CARBURETORS DO
All carburetors control engine speed by varying the amounts of air and fuel that enter the engine. In RC 2-stroke engines, either a rotary-barrel or slide-valve carb is used to control this vital air/fuel mixture; the valve controls the amount of air fed into the engine, and the fuel-mixture needle, or needles, controls the fuel delivery. As your vehicle’s transmitter trigger is pulled, the throttle servo either rotates the throttle barrel (on a rotary carb) or retracts the slide valve (on a slide carb) to feed more air into the engine to increase power and speed. Similarly, when the carb is closed, air and fuel intakes are restricted and, consequently, the engine slows down.
This cutaway picture of the carb illustrates how fuel flows through the needle valves (shown by the blue highlighted areas). Note how the tapered needles (circled) fit into tapered opening to control fuel flow.
A slide carb; it’s easily recognized by its accordion-style rubber boot.
A rotary-barrel carb. The long control arm on the right is the giveaway.
This cutaway shows a rotary carb in action. at full throttle, the incoming fuel/air mixture makes a straight shot through the carb.
The barrel rotates to make the opening smaller, thus reducing the engine’s rpm.
SINGLE-, 2- AND 3-NEEDLE CARBS
All RC engines use either rotary-barrel or slide-valve carburetors, but there’s an additional level of distinction within these groups: the number of tuning needles they have. The most basic engines have a single needle that adjusts the fuel mixture, but the majority of engines use two needles: one for a low-rpm mixture setting and another for high-rpm adjustments. Some high-performance racing designs have three independent needles to adjust the low-, midrange- and high-rpm mixture settings. Next, we’ll look at the pros and cons of each needle configuration and focus on tuning advice for all three varieties.
SINGLE-NEEDLE CARBS
Single-needle carbs are often billed as “easier to tune” because there is only a single needle to adjust, but tuning a single-needle carb for good performance over a broad rpm range can actually be a challenge. Single-needle carbs have only one setting, so the engine generally runs well in a narrow rpm range, but any engine speeds above or below that range will suffer from a less than ideal air/fuel mixture.
An engine that’s tuned as well as it can be with a single-needle carb may run reasonably well in the midrange, but it’s generally very rich at idle and very lean at high speeds. If the engine idles for too long, it can stall because the mixture is rich at low rpm, and running in the high rpm range may cause excessive heat buildup because the mixture is too lean at those speeds. You can adjust the needle to the rich side to compensate for the lean high speed, but then the low speed becomes even more rich, and that makes it difficult for the engine to idle for even a couple of seconds. Tuning a single-needle carb is a question of compromise. Here’s what to do.
On a single-needle carburetor, there is only a high-end needle. The needle usually has a tall, knurled body that makes it easy to twist by hand, and there is often a slot for a screwdriver tip in the top of the needle body for body-on adjustments.
Tuning a single-needle carb. The best way to adjust a single-needle carb for all-around performance is to start with a rich but drivable setting and then turn the tuning needle clockwise (more lean) in 1/8-turn increments until the engine’s performance feels well suited to your running conditions. If the engine misses, sputters or stops at full throttle, richen the mixture 1/8 turn at a time until these problems cease.
Getting an engine to run at its best is relative for those that have a single-needle carb. The final setting is always a compromise between allowing the engine to idle for a few seconds and yet still have enough fuel to prevent a lean condition at high speed.
Engine temperature should be your guide when tuning engines with single-needle carbs. That’s not the case when tuning multiple needles for performance, but there’s little choice with a single needle, and it’s best to err toward the side of caution so the engine will live to fight another day. Keeping temps below the 280-degree F mark is a reasonable goal.
2-NEEDLE CARBS
Two-needle carbs offer an enormous performance advantage over their single-needle counterparts. Two-needle carbs allow better acceleration and idling because the low-speed mixture can be leaned independently of the high-speed mixture. When properly tuned, a 2-needle carb engine will feel more on the pipe through the whole rpm range than any single-needle carb engine you’ve ever driven!
The low-speed needle is on the rotary lever side (on rotary carbs), or on the slider side (on slide carbs) of the carb body. It travels with the slide valve or rotary barrel and gradually enters the spraybar as the throttle is closed. The low-speed needle valve affects fuel delivery from idle to approximately 1/4 throttle, and then the high-speed needle valve takes over. When given an option, always choose a 2-needle carb over a single-needle carb.
For best engine performance throughout the usable rpm range, a 2-needle carb is the minimum requirement. In the carb body, the low-end needle is opposite the valve mechanism. On slide carbs, there is often another screw head on the end of the slide valve, but this screw is not a needle; it is used to set the position of the ball joint.
Tuning a 2-needle carb. First – and most important – tune the high-speed needle for best top-end performance. Because the high-speed needle also controls how much fuel gets to the low-speed needle (the fuel line feeds fuel into the high-speed needle, which in turn feeds the low-speed needle), this is the most important adjustment. When your car or truck performs well at full throttle, you can begin to adjust the low-speed needle valve for reliable idle and crisp acceleration. To prevent the engine from dying under braking, use the idle-stop screw to maintain approximately a 1mm gap in the carb opening even when it’s “closed.”
Warm the engine completely by running it hard for a few minutes. If, when it has warmed up, its idle speed gradually increases on its own before it stalls, your low-speed-needle setting is too lean. Similarly, if your engine stutters and misses when given full throttle from a complete resting stop, your high-speed and low-speed needles are set too lean. When in doubt, richen both needles considerably and start over by properly tuning the high-speed needle. Then re-tune the engine until the idle rpm level off.
If you bring your vehicle in and idle it for a few seconds, and during that time the idle rpm gradually drops off until the engine dies (BWAAAAAaaaaaaeeeeerrrr – to nothing), your low-speed needle is probably too rich. Lean the low-speed needle 1/8 turn clockwise until a reliable idle is attained. Properly tuning the low-speed needle will produce a reliable idle and crisp acceleration without much bogging.
3-NEEDLE CARBS
For the most part, 3-needle carbs are the same as 2-needle carbs. They function identically, but the 3-needle carb has an adjustable spraybar in addition to an adjustable needle, so the third “needle” isn’t really a needle; it’s an adjustable spraybar.
What does it do? A 2-needle carb gives you two ranges to work with to provide the engine with the proper air/fuel mixture – low-speed and high-speed adjustments.
A 3-needle carb allows a weak third option; it’s a little weak because it isn’t actually a needle. The spraybar and the needle are adjusted at the same time to determine at what throttle position the spraybar (third needle) is exposed to direct airflow, and that creates a subtle increase the amount of fuel flow.
A 3-needle carb’s high- and low-end needles are in the usual locations, and the third needle is on the valve side of the carb. Three-needle carburetors represent the ultimate in tunability but take more time to dial in properly. A correctly tuned engine with a 2-needle carb will outperform the same engine with an improperly tuned 3-needle carb, so let your skills and experience guide your carb selection rather than adopting a “more-needles-are-better” approach.
Tuning a 3-needle carb. Initially, you should tune your 3-needle-equipped engine in the same way as a 2-needle carb is tuned. When you’re pleased with the engine’s performance, pay very close attention to the transition from low speed to high speed. Does it sputter, lag or “fall off the pipe” in the middle of the rpm range? If so, you should adjust the third needle.
To adjust the third needle, turn both the low-speed needle and the third needle the same number of turns in the same direction (1/4 turn at a time). This retains the idle setting but alters the midrange response. Run the engine for a few minutes and re-tune if necessary. When tuning, remember to make small adjustments, so that you’ll know which change affected performance.
It’s a good idea for inexperienced tuners to leave the third needle setting alone, as it requires a very experienced driver to even feel the difference at all. There’s a risk that the engine will run poorly if you tune the third needle incorrectly. It’s best to treat your carb as if it’s a 2-needle until you gain more experience.
NOTHING TO FEAR
Tuning an RC carburetor is not an exact science; you won’t find anyone at the track saying “Let’s see: it’s 65 degrees, relative humidity is 70 percent, and I’m running 20 percent fuel, so I need to be two turns out on the low end and three turns out on the high end.” Understanding how the carburetor functions is an invaluable asset in your quest for perfect power, but what really delivers results is when you couple your knowledge with experience. The more time you spend experimenting with your engine, the better you’ll become at tuning it. There’s no need to be afraid of needles!
ENGINE TUNING TERMS
LEAN To lean an engine means to reduce the amount of fuel (relative to the amount of air) entering the engine by turning a needle valve clockwise.
RICHEN Richening increases the amount of fuel (relative to the amount of air) entering the engine, by turning a needle valve counterclockwise. Richening an engine to obtain the proper air/fuel mixture will generally reduce engine temperature and increase lubrication.
NEEDLE VALVES These are the small screws found on the upper portion and sides of the carb body. These are used to adjust the fuel-to-air ratio.
SPRAYBAR This is the metering jet inside the carb body that mates with the low-speed/idle needle (when equipped). They fit together like a tapered peg into a tapered hole.
ON THE PIPE A phrase that indicates an engine is running with an ideal tune and producing excellent power.
IF YOU REALLY WANT TO GET TECHNICAL
A fuel curve shows the ideal air/fuel ratio at any given speed on an engine’s rpm scale. Because ideal proportions vary with rpm and load changes, it’s important to tune your carb so that the air/fuel mixture is optimized as much as is possible.
This graph shows what most engine tuners already know: when the engine is started, it needs less fuel because its air intake is lower. Acceleration increases engine load, and a higher rpm means that more air goes into the engine; this means the engine needs more fuel to maintain the correct ratio of air to fuel. The increased fuel requirement tapers off only as the engine reaches the upper limits of its rpm range and transitions into a “cruise” mode during which hard acceleration isn’t possible because rpm have maxed out.
Understanding your engine’s fuel requirements can guide your approach to fuel-mixture adjustment.
