When I set up the throttle end points on my gas engines, I set the linkage to make the carb throttle plate to be in its maximum open position. However, full power is reached with the throttle plate at approximately 2/3's wide open. Setting it up it for the maximum open position for the throttle plate seems like a waste of the servo resolution. Would it be better to set the linkage to the point which gives the maximum power? By setting the linkage, I mean using the longest control arm on the throttle and shortest on the servo to get the full power setting.

yes. I have the servo horn at 95deg to the rod at full throttle so its maximum throw is in the last third. Then a throttle curve thrown in for good measure. Never had a plane hover under half throttle, not because lack of power.

When you set up your linkage lengths, set the throttle lever on the carb at it's half open position. Set the throttle servo at neutral with the servo arm referenced across the servo, or 90 degrees to the case, depending on the servo-arm orientation. You want the mid point servo travel to correspond with a 50% position with the servo arm. Now connect the servo end of the linkage at the innermost hole of the servo arm, and the carb end at the outermost end of the throttle lever. That connection should pretty much establish the needed linkage length. Dial min and max travels in with the radio travel adjust and then set up any curves or expo you may want.

What "Tired Old Man" said.... exactly. In fact, another approach that works very well... gets you close to 100% ATV's, and using the full trim range of your throttle...about a 5/8" takeoff point on the SERVO and about 3/4" takeoff point at the carb....works every time.

Thanks for the replies, but I'm not sure the complete answer is there. I set my linkage up basically as described by you guys. However, with the engine running, if you try to reduce your end point for full throttle, you have to reduce it significantly to hear a drop in rpm. What I'm talking about is setting the lengths of the arms on the servo and the throttle plate such that full throttle position is at that point where the engine is at its max rpm, not at its full openning. These control arm lengths would be determined using 100% on the servo end point settings. The actual movement of the throttle plate is about 75-80 degrees from idle to full open. I talking about limiting the movement to may be 60 degrees or what ever lets the engine reach full power.

Or may be the full wide open throttle is needed for obtaining full power in the air where the rpm's may be higher than those seen on the ground????

Probably.

The linkage method I described allows greater resolution at lower throttle settings before any curves are set with tranny computer. The way we did it in the old days.

Once the throttle plate opens enough to equal the venturi size no further power will result from opening it further. A slight lean to the throttle plate helps turbulate and mix the fuel air mix also. As well as draw fuel from the jets with more effect. Vertical is not really an option. Power should start to decrease at vertical because fuel draw has started to drop. And a lean run condition will develope.

So one should not set the carbs buterfly to open 100%???

Depends on the design of the venturi and where the high speed jet is. All the smaller, 1" bore or less carbs I've seen have a cast venturi. There are some larger machined bore carbs. Most of the carbs have jets on the side and as the plate rotates past them it draws out the fuel.
The plate needs to be angled so a pressue vacume is formed to draw the fuel from the jets.

Put a tach on it and see what throttle position gives you max static RPM. Usually it just max's out at approx 80% of WOT. I don't recall it actually lowering past that throttle position.

That's a good idea! The tachometer can resolve changes in RPM that the ear can't hear. I wonder how RPM varies as function of throttle position in the air. Real good argument for airborne telemetry.

CR

I've thought about this some more. Since the engine RPMcan increase in the air due to the prop unloading, I'd set the EPA's for full travel, fly it and see how it works out.

CR

Hi!
It's very simple! You open the butterfly valve as much as possible!! Nothing else!!

I have seen carbs that allow the butterfly to over travel. While I believe in the KISS principle, it doesn't hurt to make sure.

CR

If you cut the spring stop tabs off of some Tillotsen carbs you can go almost full circle with the throttle plate[X(] Most definately check that you are not doing so.

LOL @ Tired Old Man

Do that when you want to run the engine backwards only.