Gasser Throttle Linkage Installation and Set Up
Servo selection:
Use a standard sized servo. That's the initial basis of a good throttle servo. If you want precision, speed, and accurate throttle position every time, use a good coreless or digital servo. If you don't care about a wandering throttle position or difficulty in establishing a consistent idle, an analog servo will do. Servo quality is predicated on the choice of manufacturer and price. The low end servos provide low end performance and a lot of frustration. They don't center well, if at all, and rarely hit the same position for the same stick input twice in a row. You will be the one to decide what you prefer. Servo torque should be a minimum of 48 ounce inches. There is no maximum but 48 to 50 oz. in. is plenty for any throttle servo. Micro servos are a
very bad idea. The Hitec hs225mg is as small as I would ever consider for a throttle servo. Metal gears generally last longer than nylon gears, and carbonite gears are noted for breaking when used with a gas engine. Nothing lasts forever. That should be enough for you to make a servo decision.
To start a linkage installation, set both the throttle servo and the carb lever to 50% positions. So the carb is opened up half way and the servo is at midpoint. Use a clamp or other means to secure the carb lever in position. Remove and discard the idle stop screw that may be installed on the carb when new. That's used for yard equipment that are normally operated at two speeds, idle and wide open throttle. Hopefully you use a wider range of throttle positions than just those
DO NOT remove or disconnect the throttle return spring. It's on there for several reasons, all of them good for what you want to use the carb for. If you are using a servo of a size suitable to be a throttle servo you will never have a problem because of that spring. If you are using a servo unsuitable for a throttle servo all the problems you will have were self-induced. You know how to fix that. Get a more appropriate servo.
Before going any further determine that travel adjust for the throttle servo is set at 100% both directions, no trims have been input, idle trim tab on the transmitter is fully closed, no mixes are in the radio, no rates have been set, and use the shortest servo arm that came with the servo. You will use the innermost hole on the arm. You may find you will want to make a hole inside of that one for best resolution. You will want maximum servo throw to provide minimal linkage travel. That doesn't make sense now but it will later.
Measure the distance between the servo arm and the carb lever hole. That's your beginning linkage length. Make the linkage and install using your favorite and safe attachment method at both ends. I'll suggest that at least one end be a bolt on ball link, that no pinned clevises be used, and if a pinned clevis is used it will be a metal pin with a secondary method of securing the clevis. 4-40 ball links can be a pain because of size and screw heads. Dubro makes a 2-56 bolt on ball link for 4-40 threaded rod. I'll let you look up the part number
No metal to metal contact is ever allowed, although it's perfectly acceptable to use a metal or carbon throttle rod as long as it's electrically isolated from any metal levers or servo arms with something like a ball link. Zee bends in a metal rod connected to the carb lever will be a disaster. Don't do that.
Hopefully the carb lever is at least 1" long. you may have to change or modify the throttle lever at the carb. There are bolt on products available to complete that task. It's better if it's 1-1/4" or so because you want use an outer hole (farther from the throttle plate rod center) to obtain maximum throw at the carb end. You want an installation where the servo moves a lot to move the throttle plate a little, with the total rotation of the servo being enough to obtain full range of motion at the carb. The short servo arm with the long carb arm helps you do that. That's resolution of the finest kind since it does not require mixing and expos. Starting to make a little sense now?
If using a decent servo you can check the amount of throw from one end to the other and how that impacts the carb lever without powering up the radio. You should be pretty close and need only to increase or decrease servo travel a few percentage points to go from full closed to full open. If it requires that you adjust travels more than about 15% in either direction move the attachment point at the carb lever in or out on the lever to determine the effects. The linkage may still be a little too long or short. Also work with adjusting the ball link/clevis in or out a little bit. This is where making a new hole on the inside of the servo arm becomes helpful.
Ultimately you want the throttle plate to achieve a fully closed position with the throttle stick full back and the trim tab full down.
THIS is the absolutely correct throttle stop/engine shut off position of the carb throttle plate because it provides full control of the throttle for idle trim and engine shut down. It also makes establishing your idle position extremely easy. Closing the plate will cut the engine every time. The trim tab will be what you will use to set idle throttle position. You can play with transmitter multiple idle positions later for those that like the idea of high and low idles. I don't, but some do. If you let the engine warm up before setting the idle it will likely be the same when you land as it did when you took off. That's another subject of its own.
You do not want the servo or the carb level to
EVER hit a hard stop at either end. The carb will be providing the engine the ability to make 100% power long befere the throttle plate is fully open. The throttle plate does little for power/RPM after it passes somewhere between 75% and 80% open. That has to do with the plate and rod dimensions/geometry and it's impact on throttle fraction area. Don't worry about this because I'm certainly not going to write all that is required to define it all.
If your servo is moving a little and the carb lever is moving a lot, the linkage geometery and installation is
totally wrong. You will always have trouble with throttle sensitivity when things work that way. You want a little servo travel to make a little carb lever motion, and to be able to use 100% of the servo travel to achieve 95%-100% of the carb lever movement without hitting stops. It's ok to use 100% or a little more or less of the servo travel, but it is never ok to introduce so much travel or linkage so long that
any bind will be present. That kills servos in a big hurry, and drains flight batteries.
Hopefully the above will get you off to a good start and make throttle linkage on a gasser a bit easier for you to set up. Copy and past this post into a Word or text document on your computer for future reference. I did
