Carburetor Adjustments Story

As I've been teaching myself to fly my new (to me) RV-9A, I've noticed a few peculiarities, like, with full flaps (32 degrees) and idle power, the airplane only descends at 500 feet per minute. Sure, it's clean and has a big wing, but... And when I land on a long runway in the touchdown zone and let it roll out, 5,000 feet later the plane has only slowed to 38 knots. Takes lots of braking to slow her down.

Yup, it's the idle speed. Or was. Cold, the engine idled at 700 RPM, but warmed up, the idle was more like 950. The idle mixture was too lean because there was no RPM gain pulling the mixture back to cutoff. And there was a jam nut missing on the idle adjustment clevis at the carb.

A friend fixed all those for me yesterday, and the RV-9A now should fly a whole lot more like an airplane. The idle speed (warm) is now 550 RPM with a 40 RPM increase when the mixture is pulled back to idle cutoff. Need to get over the last symptoms of this stupid bronchitis before I go try it, though.

The RV-8 has the same out of adjustment symptoms, but with the short wing and constant speed prop, it has no problems descending like a brick when I ask it to.

With a new (to you) homebuilt, you always need to be careful about everything, and I've found some electrical quirks in the new bird -- but I wasn't expecting to find anything as generic as the carb being misadjusted. Or maladjusted, perhaps...

Yes.

It's amazing to me how many airplanes I inspect that have this very same problem--- the carbs and/or the fuel injector servo throws are not adjusted properly. I just inspected an RV-8 last week that had been flying for 250 hours and 5 years and the throttle arm was only going to 2/3's throttle, at best! Clearly missed on the initial airworthiness inspection and many Condition inspections since then. I doubt the rings are even seated properly.
I told the owner to be prepared for some performance increase once it gets properly adjusted!

Vic

I run across these problems on the RV-10 more than all the other models combined.

Ed,

Sounds like some good diagnosis work.
...

Did you mamage to sort out your climb CHT issues on the same airplane by better sealing of the baffles?


Best Regards,

-Paragon
Cincinnati, OH

Agreed

Mel said:

I run across these problems on the RV-10 more than all the other models combined.

Click to expand...

Yes. I see it mostly on models that have a throttle quadrant, especially 8's and 10's. Most builders do not realize that you might have to drill a different hole in the quadrant arm in order to achieve full throw at the other end.

Vic

There is more to the story....the idle screw was turned 3/4 of the way in. Simply backing out the idle adjustment screw only lowered the RPM down to about 850 because the throttle cable ran out of travel and could not close the throttle all the way against the idle screw. The throttle cable housing jam nuts had to be adjusted to get the range of travel centered so the throttle arm would hit both stops before the internal cable stops made contact.

The mixture cable was also grossly mis-rigged as well. When the blue knob was pushed in to the full rich position, the knob was still about 1 3/4" from the panel. The mixture arm on the carb was barely making the lean cut-off stop. Adjusting the cable housing jam nuts at the engine bracket corrected this issue as well. Now when the mixture knob is full rich there is about 1/8" of "cushion" before the knob contacts the bezel when the mixture arm on the carb is against the full rich stop.

After the control cable rigging was corrected it was time to set the dile mixture. It was apparent that the idle mixture was tool lean since there was no RPM rise prior to flame out after the mixture was pulled to the cut off position. I started the adjustment by turning the idle mixture screw all the way in until it seated to see what the initial setting was. The mixture screw was only slightly less than 1/2 turn from the seat, I am kind of suprised that the engine actually got enough fuel to idle as well as it did! After several adjustments and tests we got the mixture set where the RPM rise was between 40-50 RPM before flame out.

It's amazing that the rigging and adjustments were out this far, for this long. I would have thought that the previous two owners would have made these simple adjustments rather than having to pitch up to get the airplane to slow down below max flap deployment speed, or drag the brakes heavily to keep the airplane below 38 knots on the ground.