Idle Mixture Problem Solved

 I did some reading of the Stromberg NA-S3 manual and on-line.  Bob Kachergius, "The Stromberg Specialist", has a good article on how to adjust the idle mixture.  There is a needle valve with an adjustment screw on the back of the carburetor.

Rather than have a mechanical accelerator pump, this carburetor has 2 Idle Discharge Nozzles.  One nozzle is above the Throttle Valve, when the valve is closed at idle.  As long as the valve is closed it provides all the fuel needed to keep the motor running.  

As soon as you open the Throttle Valve a little, the second port is uncovered to allow some extra fuel until the motor speed increases to about 1000 RPM.  

Above 1000 RPM the Main Metering Jet provides the needed fuel, if the float is properly adjusted.

If the Idle nozzles are adjusted too lean the motor will stumble or stall while opening the throttle, my problem exactly.  

Bob's procedure has you install the Idle Adjustment Needle Valve without the spring so you can accurately count the number of turns until the needle gently seats.  He says it should be 6 1/2 to 7 turns.  I have another carb. which does need 7 turns.  The needle on the Fly Baby carb. only went in 4 1/2 turns, don't over tighten.  His recommendation is for an initial opening, with the spring installed, of 3 turns.  Then you open the valve in 1/4 turn increments, if needed to solve the idle problem.  I started with 2 turns and it runs great.  No hesitation nor missing.

By the time I was done it was a 4 o'clock and the sun was right in my eyes heading down runway 23.  I decided to go back tomorrow so I can fly earlier in the afternoon.  Hopefully it will be as nice as today.

Ignition Problem Solved - Now I Seem To Have A Carburetor Problem

 

Replacing the coil in the right magneto and carefully re-timing both mags. seems to have solved my ignition problem.  I ran the motor and taxied around the airport for a few minutes.  At 1700 RPM I have a 100 RPM drop on the left mag. and less than that on the right.  Everything seems to be back to the way it was before all this.

Well not quite everything, now the motor hesitates and sputters when you try to accelerate from below 1000 RPM.  

If it had an automobile carburetor I would say the accelerator pump is bad.  Opening the throttle allows more air through the carburetor.  At the same time that causes a slight increase in pressure in the throat of the carburetor, of if you like, less vacuum.  This means there is less pressure differential to push the fuel from the float bowl into the throat.  Both changes, more air and less fuel, mean a leaner mixture, which, depending on the mixture just before the throttle is moved, can cause the motor to misfire or lack the power to accelerate.  On an automotive carburetor they solve this with a little accelerator pump which squirts just a bit more fuel into the throat to richen the mixture.  It works, in my '63 Corvette you could slam your foot to the floor and the car would still accelerate without hesitation, while smoking the tires and slamming you in the seat.  

On airplanes we generally avoid jamming the throttle open quickly.  Also, no one wants the motor to sputter when you're trying to recover from a bad landing and need full throttle, now.  

It's possible I've got a small air leak between the carburetor and the cylinders or some dirt in the carburetor after setting 26 years.  I won't know until I get things apart.  I've ordered a carburetor kit from Fresno Air Parts.  It's coming by 2 day Priority Mail which means it should be here in the next 2 weeks, oh well.  It's already 2 days and the tracking just says it's In-Transit.  I'll get back on it after Christmas.

Merry Christmas!

Ignition Problem Still Not Solved

 I removed the magneto, cleaned the contact points and cleaned up some grease that was slung out of the bearings I had repacked.  I really didn't find anything wrong.  I found some slight wear on the cam which operates the points.  I ordered a new cam and waited, and waited.  It finally arrived yesterday, 8 days late, Oh well.  

I reassembled the magneto with the new cam and reinstalled the magneto.  As the sun set I was able to get the motor running, but there is no improvement at all.  

since this magneto does not have an impulse coupling I may be able to fix up a spare and see if it solves the problem.  Hopefully tomorrow I can remove the mag and work on one of the spares I have.  We'll see what happens.