RE: Newbie to Gas General Information
37. Throttle Linkage, Trim, and Adjustment - Our carbs are really intended to be nearly "full on" a majority of time...not to be continually varying them in the middle range which is our main usage. So, to get things "right", it's important (maybe VITAL) to properly setup the mechanical *and* electrical setups of the throttle and carb arms. Here's what I suggest:
Method A - Basic Approach
Full DOWN throttle trim is "cutoff", closing the carb completely. This is the full ATV adjusted on the bottom end. I set this by fine tuning the ATV so the control rod isn't binding the servo, but the throttle plate is completely closed.
Middle "detent" throttle trim is the "landing" and "taxi" trim setting. That is determined by a couple of things that inter-relate. First, in the System setup screen, set the Throttle trim authority to 50%. It is far too sensitive otherwise, for a gasser. Then, assuming that you've set the carb arm length to be slightly more than the servo control arm take off length, the 'half trim' should be where the plane will sit motionless when engine is fully warmed up. Finally, set the Throttle Curve function ONLY if you cannot get the low end sensitivity/linearity mechanically. Reason I suggest that is that the mechanical rotational and linear geometry of the arms can go a long way toward getting it "right".
Finally, the "full" trim is the "hot restart" setting, because my carbs tend to be leaner on the low end, so a hot restart needs just a bit more travel.
Method B - The Real Deal
Now - the "real deal" is to get the correct pushrod length. There are, again, many opinions on these mechanical setups, but I'll just describe mine, as it is consistent, repeatable, and reliable.
Use a Servo arm where the "take off" or "ball link attach" point is about 5/8" from center of shaft. Purpose of that distance is to ensure that you'll be close to needing 100% ATV on both ends of the throttle channel, making the "clicks" of throttle less profound. You don't want big differences per click, and a longer arm will be too much. Shorter arm requires too much ATV diddling, and introduces mechanical expo which may not be in the "correct" direction.
On the Carb, use an arm with the ball link take off point at 3/4" from shaft center. This permits more rotation of the arm, and helps solve sensitivity issues mechanically at the carb. Walbro carbs are HORRIFICALLY non linear and overly sensitive...
Now, with both arms installed, set your ATV to about 90% on both extremes. Open the carb arm to "FULL THROTTLE" (where it hits the mechanical full throttle stop), and move throttle stick to FULL throttle. Measure the straight line distance between the centers of the ball link attachment points. Now, go to full CUTOFF low throttle and low trim, and rotate the carb arm to FULL CUTOFF. Measure that straight line difference. If not the same as the full throttle length, take the average. Create a Carbon fiber, or stiff metal pushrod of EXACTLY that length.
Install. You will be very close to "perfect". Tweak the ATV end points as needed, you perhaps will need to adjust the Throttle trim authority again, but you will be very close to a good linear setup. If it isn't like you want it, THEN setup the Throttle curve function, but not until after all the mechanical stuff has been taken care of.