Step 4: Carb ajusting.
We got the theory part of the engine under control. We can't really tune a piston or adjust a crank-shaft, at least not in your every-day engine maintenance and adjustment. So without further delays lets dive into the 2nd phase of this project& The Carburetor. What good is a 1.2 HP engine if you can't keep the dam thing running? That's exactly my point, it does not matter how little horse power your engine has, if it can stay running for the entire duration of the main then you will have a real good change to at least get one of the top three positions. They say that before you can win a race -first you must finish. The first part of finishing a race it to have a well tuned engine. In this article we will go over how a carburetor works and how to adjust it. Without any further delays lets get busy!
The carburetor has one main function, to regulate engine speed. It accomplishes this by metering the amount of air and fuel as required, to sustain combustion per the input of the throttle servo. Thus for a low-speed idle you would have a small amount of air and fuel entering the engine. This would in effect lower the chemical energy entering the combustion chamber and thus lessen engine power and subsequently lower the RPM. As we open the throttle the carb will allow more air and fuel into the combustion chamber, thus increasing engine power and RPM's (revolutions per minute). Now that we know what the carb. has to do lets explore the underlining fluid mechanic properties that allow the carb to function effectively at different throttle settings.
What allows the carb to pull fuel from the fuel tank is the venturi-effect. This states that in a converging funnel the entering fluid velocity increases as it passes through a reduction in the funnels throat diameter. This increase in fluid velocity decreases the localized pressure at the venturi throat to below atmospheric pressure. This low pressure region is precisely where fuel enters the carburetor throat. This is what allows the engine to "suck" fuel from the gas tank. The truth is that the venturi-effect is all that is needed for the engine to get fuel. Pressurizing the fuel tank is really only done to decrease the effects of fuel level on the mixture setting of the carburetor.
Fuel Metering Devices
The venturi-effect draws fuel from the tank but does little to regulate it's flow. It's true that as the engine accelerates the amount of air that moves through the engine increases. The increase in air velocity also increases fuel flow into the induction port, this helps the engine self regulate the fuel up to a certain point.
This is not the only means for the carburetor to meter air and fuel. Engines need a metering device to help regulate the amount of fuel that enters the carburetor. This is accomplished with an adjustable orifice, typically we call them needles or jets. Most engines have a second adjustable needle that helps regulate fuel at low throttle settings. By adjusting these two needles we can control the transition from low to high speed operation of the engine.
How do we adjust a carburetor?
The carburetor is typically adjusted with a long flat-head screw-driver. Carb adjustments are then done by rotating the needled in, our out of the needle seat. The idle speed is adjusted by a screw at the base of the carburetor. This allows the throttle barrel to only close to a preset position.
The carb has three main adjustments that allow you to set the following:
1. Set the idle speed.
2. Set the mixture at idle (Adjustable on 2-needle carbs only).
3. Set the high speed needle mixture and control engine temp
How to make carburetion adjustments:
The throttle stop screw or idle-speed screw (same thing) determines how far the carb barrel will be able to close when the servo is in the neutral position. Typically you set the servo/throttle linkage so that the carb will go from fully open when the trigger is fully pressed to fully closed when the trigger is in neutral. Then you would adjust the idle-stop/speed screw so that there is a 1-2 mm gap when the servo is in the neutral position. You might need to readjust the spring collars on the throttle linkage to force the throttle arm against the idle speed screw.
Tip#1: If you completely mess up the carb setting and you want to go back to the factory recommended needle setting then you must have the carb fully (Yes I mean fully closed) before you can set the low-speed needle to whatever turns the engine manufacturer suggests. Before you close the carb fully back the low-speed needle a bit to make sure you wont put un-needed stress on the needle seat.
Tip#2: There should be no speed change whatsoever when the car is in idle and when you hit the brakes. If the engine's RPM drop either your linkage isn't set right or the idle-speed screw is set too loose. Tighten clockwise until the carb barrel doesn't move when you go from neutral to full brakes.
Tip#3: Some RTR kits have servo horns that are too small. There is not enough servo throw to open the carb barrel, if you use servo trim to be able to open the carb fully, then when you go to neutral the carb doesn't close enough. To compensate for this the novice engine tuner opens up the low speed needle to drop the engine RPM so the car will stay still when at idle... The drawbacks of correcting the linkage problem with the mixture control is that now the low-speed is too rich and the car won't idle for more than a couple of seconds before the engine sputters and dies.
To fix this problem you need to get an after market servo horn that is larger yet still fits your particular servo brand. Now you can go from fully open to fully closed, without using trim. Now you wont have to compromise the carb settings because of lack of servo throw.
At this point you would start the engine warm it up and commence tuning. Adjust the low-speed needle clock-wise until the engine doesn't sputter when at idle. You want a fast idle, if the car wants to move forward a lot, then turn the idle-speed screw counter clock wise to lower RPM until the engine just barely want to engage the clutch. It may take a little time to get the settings right.
Remember you want the fastest idle you can get away with. It will make the engine more stall proof. Some engine will overheat if the idle isn't rich enough, you need to experiment to determine what's the right setting for your particular engine. When every thing is set right the engine will be able to idle through an entire tank without missing a beat.
The high speed needle will control fuel flow into the carb from 1/2 to full throttle. Typically the high speed needle is set to allow the engine to reach it's peak power point, then you open the needle slightly and go racing. On very hot and humid days you will probably have to make a compromise in the tuning department. For most this will mean you will richen up the high-speed needle to lower engine temperatures to acceptable levels. Everyone has their own interpretation of what an acceptable engine temperature is, for me anything under 260 is acceptable. Going higher will typically mean shorter engine life-span and less reliability.