Curtiss Jenny Restoration

Welcome! We hope you enjoy following the restoration process of a 1918 Curtiss JN4D Jenny. Once completed, the aeroplane will be flown and displayed at the Candler Field Museum in Williamson GA (30 miles south of Atlanta). You can contact me below by clicking on "VIEW MY PROFILE"

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Location: Peachtree City, Georgia, United States

Monday, August 10, 2015

Jenny update - carburetor

These old engines fascinate me.  We got the Jenny back in the air with the overhauled engine, but I could not get the Hisso to idle correctly.  It was too fast.  I had the throttle back against the stop and the engine chugged away at 800 RPM.

The engine was was smooth...and happy...but without brakes and a tail skid, we could not stop the airplane on the ground.  The idle was just too high.

But, the arm of the carburetor was back as far as it would go. Something was not right.  It was time to dig into the Hisso manual.

The manual showed a neat diagram following the path of the fuel at idle (red color).

The Stromburg NAD-4 carburetor is a unique creature.  At idle, fuel is drawn up an internal tube to a small idle port in each venturi (there are two).  There are two bleed screws there.  Unlike modern carburetors which adjust the amount of fuel, the NAD-4 adjusts the amount of air.

The book identifies these screws as "idle adjustment needles".  Even better!

With the engine running, we tried rotating the bleed screws.  The idle speed stayed at 800 rpm, but smoke poured out the stacks as the mixture enriched excessively. Turn the screws the other way.  Too lean.  Engine ran rough.  So much for that idea.

Air comes in around the bleed screws.  Perhaps it's not getting enough air.  As you can see, there needs to be a gap in the threaded bleed assembly.  Small holes allow air to enter and the screw meters it even more.

We adjusted the assemblies until the holes were fully exposed.  No change.

There was much head scratching and frustration.  How could we fly this thing if we can't get the idle low enough for the plane to stop on the ground.  I know what you are thinking.  No...I'm not installing brakes.  No way.

I jumped on the internet and tried searching every conceivable combination of "Stromberg NAD-4 carburetor idle problems" but there was little information available.

Again, I checked the physical stop on the throttle arm.  Indeed, the butterfly valve was closing.  Whatever we were hitting, it was a physical stop.

Another good thing about old airplanes is the plethora of help out there if you just ask.  I sent emails to Frank Shelling, who has way more experience with these engines than I do, Glen Peck, Paul Dougherty, Kevin Connor, John Saunders and David Cretchley in New Zealand, Andrew King......I fell fortunate to have many friends.

I was looking for a smoking gun, but the advice they gave me was clear - pull the carburetor and look around.

We pulled the carburetor. The butterfly valves moved smoothly and hit the stop.

We pulled the fuel screen and checked the spring that operates the float arm.  It was fine.

Then we noticed something unusual.  The bleed assembly was flush in the right side of the carburetor venturi.

But the assembly on the left side was protruding.

Here was the problem!  The butterfly valve was hitting the protruding bleed assembly and it wouldn't close all the way.  There was a gap between the valve and the venturi wall.

So the throttle was never fully closed...even when it was "closed".

Very gently, we pushed the bleed assembly back flush with the venturi and reassembled the carburetor.  I am happy to say the engine idles perfectly at 400 RPM now!

I'm posting this because someday, someone will search "Stromberg NAD-4 carburetor idle problems" and rather than finding nothing, this will show up to help them.

Time to go flying.




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