After working on this 3.7 Sachs engine (old engine low hours) for several months I cannot get the midrange to smooth out in the air. After changing the ignition three times, putting on a new walbro carb, new plugs, and endless tweaking of the HS & LS needles the high end is strong and smooth and the starting and idle are good, and transition is OK. However, the needle adjustment is very sensitive and the midrange sounds like Porky Pig stuttering along when the plane is in the air. The needle settings that sound best on the ground are not often the ones that fly the best and it takes nearly a minute at WOT to smooth out when cold. The new velocity stack and air ram seemed to help a little.

I don't know if the engine has any porting work, but at times it seems happier/smoother when pulling uplines with the 22 - 6/10 Zinger. The one flight that didn't stutter ended dead stick because it was so lean, but other times running leaner results in more stutter. This Byron Christen Eagle does NOT like dead stick landings.

The gas has been changed several times and different oils/ratios have made no apparent difference so at this point I'm out of ideas.

Has anyone run into this kind of stubborn behavior? How did U solve it.

Forgot to mention the exhaust in above post. It looks like a homemade Pitts type with header leading into an approx. 1 1/4" square manifold with two tubes coming down. Don't know if the exhaust may make the midrange sensitive but the WOT is no problem.

Your problem may be due to disturbed air around the carburetor. Namely the the atmospheric sensor diaphragm. You may know about this and have already tried it but if you haven't...... Take the cover with the four screws off the side of the carb. If there is only one vent hole then insert a piece of brass tubing and solder it. You may have to remove some of the plating to get the solder to flow. If there are more vent holes, plug them. Run a piece of fuel tubing from the brass tubing that you soldered in to a place behind the firewall that will have undisturbed air. This may fix your problem.

Your sensitive needles and excessive four-cycling are often caused by low pop-off pressure. Low throttle four-cycling is a fact of life and is most annoying with loud mufflers. Is it causing problems? If you couldn't hear it would you know it was doing it?

Kweasel, low pop-off pressure is a new one on me. What is it and how do I check for/solve it?

Pop-off pressure is a measure of the resistance of flow between the pump and venturi outlets. It effects the final position of both needles and the fuel/air mixture around 40% throttle. It is easiest to adjust by changing or modifying the spring under the lever inside the carb. The spring tension is the only adjustment for 40% throttle, neither of the two needles will have much effect in this range. You may not have a problem, they all should four-cycle at some point as the throttle is closed. Try stretching the spring and see if it cleans up the mid-range and makes the needles less sensitive.