The flying monkeys got me...

Helis and fixed wing

AMA 957918
IRCHA 4345
AMA Intro Pilot Instructor

Thursday, May 26, 2016

RCGF-USA 10cc Walboro Carb Installed and Tuned

I upgraded the carburetor on the original RCGF-10CC with the single needle throttle/detached carb after purchasing the upgrade kit from RCGF-USA. Updating the carb itself was easy. Installing it back in the Sukhoi and tuning it took a couple days and some MacGyvering.

I didn't like the single needle as it broke in. It was touchy, and started becoming more difficult to keep tuned. She started to lag on throttling up from idle to full, and I couldn't tune it out. So, upgrade.

The single needle was a choke-less carb. The single needle was in the middle throttle body, and the control rod attached on the same side, you adjust it for high RPM, and hope the low end takes care of itself. This was on the underside of the engine when I originally installed it, and the throttle servo was set up on the top. My history with the Sukhoi and the RCGF-USA 10cc is here.

I had a plan for the re-install. It didn't go as planned, and I ended up having 2 main problems to overcome and improvised as I went along.  I had to rearrange the throttle servo and its case, and re-design the throttle control arm.

The new carb has an idle needle arm on it and it was hitting the mounting arm up there around 11 o'clock. I had to grind the arm a little to allow it to slide fully back.

The cylinder head is oriented down, so this is the underside. The throttle arm  was too long and would strike the cylinder head before reaching its limit, so I had to trim off the end and use the closer hole.  The small lever aft is the choke arm, and its not removable so you can't change the orientation of the lever. It points in.  The choke and throttle do not hit each other and are not in each other's way.

Its pretty crowded under there. The choke arm, of course, goes forward and I used some wire to secure it to the cylinder as it goes by (not seen here). This was the only way I felt comfortable securing the throttle control arm to the throttle lever. The servo had a metal control arm, and I used a ball link to attach it to the servo arm, so this is not very adjustable.

A better look at the small choke arm with its rod, and the larger cut throttle lever. The fuel inlet to the carb adds to the crowd.

Building the new throttle servo box took a lot of time. It sits sideways now, with the top aligned with the aircrafts centerline. The arm points down. In case I ever have to change it, I wanted it to be easily removed, so I had to build a Rube Goldberg case to secure it and allow removal. It works well. I also had to rebuild the hood that houses the ignition (seen in the video, not pictured).

It took about 2 hours to get the engine tuned. I was taught to set the high needle for max RPM at full throttle, then adjust the low needle to allow smooth acceleration from idle to full. The high needle is easy. But the low needle... It was like a Chinese number puzzle. Minor adjustments of the high needle on occasion, but also adjusting the throttle limits to balance a smooth acceleration with a decent and acceptable idle, and not having problems with engine stall when quickly accelerated or decelerated. When going from full throttle to idle it dips below idle and can stall, so the idle is a bit higher, around 2300-2400 rpm, and can dip to 2100 rpm.  She maxes out around 8300 rpm. And this is the final result.

The next step is cowl, and then learning to trust it.


  1. Hi Ken,
    Can you please share with more details about the initial needles setup and the tune process? I'm having hard time with the tuning ):

  2. I oo have had a lot of problems getting them tuned. I finally got out flying today and spent an hour trying to find the sweet spots. They are no where near the factory settings of 1-1/2 and 1-3/4. I think mine are out about 3-4 and a half turns!

    Ypu may know this already... Just get the engine running and once its going set the high needle (the aft most one, nearest the firewall) to get the high rpm at full throttle. It will be a little rich in a new engine. Once that is set, let the engine warm up a little. Once it is, drive the throttle slowly from idle to full throttle, and tune out (richen) the low needle until this is a smooth transition. Once it is, jam it from low to full, andd tune it until this is smooth. It may lag a bit but tune it u til this fast teansition is smooth. You will need to work out the throttle end points. Mine are Full 128% and Idle 31%. The full is set until the throttle goes full without bending the throttle control rod, and idle is whatever the engine tolderate for low rpm. Mines a little fast and its because the cylinder head is a new one. Hopefully as it breaks in it will run smoother.