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Throttle Set-Up
Author: Clay Ramskill


One of the more vexing problems novices (and all the rest of us, too!) face is setting up the throttle to work properly.

What we want to achieve is to have full power without the servo straining, and with the throttle control on the transmitter all the way back, have a good range of idle adjustments with the trim lever, and still be able to shut down the engine with the trim also.

Now, those of you with fancy computer radios have it made -- you need to read your "computer manual" rather than this article! But for the rest of us, we'll have to explore "mechanical" ways to adjust throws until we get it right.

First, make sure that the linkage used between the servo and the carburetor is reasonably solid. For the usual nyrod or cable installations, this means "tying down" the outer sheathing so that it can't flop around, buckle, or stretch out.

The problem with the throttle is that we have a fixed throw (travel) from the servo, and a fixed throw on the engine's throttle lever, or control horn. So by changing the linkage geometry, we need to match these fixed amounts of rotation.

From full throttle to idle, your throttle servo will rotate about 90 degrees. This is fixed. And the needed rotation for the throttle is also around 90 degrees. Gee -- it sounds simple! And, if the lever arms (the distance from the rotation axis to the output connection) are both exactly the same, you're going to be close. But that would be too easy.

The general adjusting procedure:

1) Set throttle trim to center.

2) Connect linkage on servo and throttle arms, adjust clevises so that full throttle on the transmitter just gives full wide open at the carb.

3) On the transmitter, bring the throttle stick back towards idle. (a) If the carb closes completely before you reach idle, you'll need less throw; (b) if the carb is still open more than 1/32nd of an inch, you'll probably need more throw; (c) if the carb is open about 1/32" and you're able to completely close it with the trim, you should be pretty close.

4) If (a) or (b) is the case, make a throw change as appropriate. Then start all over at #1!

5) Run the engine, make further adjustments if necessary.

Assuming adjustment for total travel is necessary (it usually is!), the first thing to try is going to a different hole on the servo arm. If travel is too great, go in closer to the servo for less throw; if travel isn't enough, shift out a hole on the servo arm.

Remember, adjusting clevises only varies the distance between servo and throttle arms -- it does not adjust throw, the amount of travel. That is, if you adjust a clevis to open the throttle more at full throttle, you also get a higher idle speed.

But throw adjustment is a different matter. If you've adjusted for full carb opening at high throttle, but cannot get a low idle or shut-off, you'll need more throw. If you have an OK full throttle, but the engine shuts down every time you bring the stick back, you need less throw.

OK, so you haven't found a combination of holes on servo and throttle arms that gives the correct throw? Here are several other ways to change the throw (see diagram):

1) Use a servo saver. Setting up for a bit too much throw, the excess is absorbed by the servo saver spring at full throttle.

2) Shift the servo arm off center slightly -- this will lower the throw.

3) A metal throttle arm can be bent slightly to accommodate too much or too little throw.

4) Some servos, Hi-Tech for one, offer a servo arm which has finely adjustable length, and thus adjustable throw.

You may have guessed by now that there is no one set answer for exactly how to set up your throttle linkage and how to adjust it - each situation always has its own unique set of problems. Hopefully, the above will at least give you a clue as how you might proceed toward solving the problem in your case.


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