Rough Engine at Mid-Throttle

[bobg56]

Its a good habit to take plenty of pictures before tearing anything down, also to do one carb at a time so you have a reference just in case, glad you fixed your problem.

[D-Dubya]

Thanks for closing the loop by sharing your findings. Problem found and fixed...and fortunately, nothing really bad happened.

[Jcurry]

Thanks for the info. Mine also runs rough mid-range (has since I bought the plane) and I have not been able to figure it out. I've checked float weights,
Idle screw adjustment, mechanical sync, vacuum sync, carb to intake manifold o-ring, carbs overhauled, all with no joy. My ignition system is fine. Keeping my fingers crossed...

[Jcurry]

Found this last night on LH carb. Screws in other carb were tight. Removed screws, applied blue loctite and reinstalled. Borescoped cylinders, no signs of damage or missing screw.

[funflying]

Jack those screws should be peened over to lock them in place. It takes help to do it and I used a steel rod in a vice to back the screw. And a punch to peen the threads.

Saw it done in a Rotax video and just followed their technique.

[azonslow]

I agree with funflying in that we staked those screws when we rebuilt the Bings on our legacy RV-12. However, the latest MMH edition that I could find states:

"Install throttle valve with 2 new slotted head screws M3x6 secured with LOCTITE 603, ensure that it is reinstalled in the same position as marked at removal."

73–00–10
Page 16
Edition 2 / September 2022

[BillL]

Kudos to the OP for not only finding the issue, but reporting the cause!! Two thumbs up.

Pictures are helpful for low memory, but correct throttle plate position is clear as day. The plates are machined at an angle so they will fit the cylindrical bore when closed. Once this is learned, one should find it easy to install properly. Back-in-the -day, I used to leave the screws touching but loose, snug with the plate closed, then tighten and stake.

[Catbird]

That's EXACTLY how I did it, the second time...

[Jcurry]

After 2 years of on and off troubleshooting, I inadvertently discovered my problem. While comparing another RV12 to mine, I noticed his choke arms are 180 out from mine. I confirmed in the manual that that his are installed correctly. Flipped mine 180 and all is well with the world. I'm embarrassed that I did not catch this sooner. Dog leg should be pointing down not up. 1st pic is wrong. 2nd pic is correct.

[Piper J3]

I’m missing something in the translation…. I see your photos and understand that the actuating plate was installed incorrectly, however… it appears that the rotating brass shaft is still in same position when the plate is installed correctly. This doesn’t explain any difference in operation of the enrichment circuit (choke).