STROMBERG TECHNICAL INFORMATION

SERVICING THE NA-S3A1 & NA-S3B CARBURETOR FLOAT BOWL AND FUEL INLET

 

All aircraft carburetors utilize a drain plug in the bottom side of the float bowl to facilitate draining out any accumulated water, dirt or sediment.

The Stromberg carburetor uses a brass square headed ¼” NPT pipe plug to do the job. Unknown to most is that the brass plug is not at the absolute bottom of the bowl but sits on a raised shelf approximately 5/32” above the true bottom of the bowl. This does not allow all the water and sediment to be removed unless you use the following procedure. FIRST, turn the fuel selector valve to the off position. Place a drain pan under the brass plug and then remove the plug. The fuel and most of the water and sediment will now flow out into the pan. THEN, turn the fuel selector back to the on position for 10 or 15 seconds. What happens is that the fuel now flowing into the float bowl will cause a “flushing” action to get the remaining water and sediment out.

Drain PlugDrain Plug Internal Version

Now the PROBLEM… The plug removal and draining is a  MUST DO  item when performing an annual inspection. It appears that in most cases, the brass plug has not been removed for many years prior and becomes “frozen” in position. Dissimilar metal corrosion (brass & aluminum) along with water is the culprit. Attempting to remove the stuck brass plug with an open end wrench, channel-locks or vice-grips usually ends up “rounding” the square head of the plug making it’s removal almost impossible.  Thus, the plug is left in place and draining the float bowl is omitted at annual time. A “bad” practice at best because who knows what and how much trash remains in the float bowl.

Now the FIX… Remove the carburetor from the spider manifold and disassemble it completely. Take the lower bowl casting and mount it up side down in a shop vise. Be sure to use vice “jaw protectors” so the carb flange is not marred or distorted. Fire up your acetylene torch using a small tip and apply some localized heat to the top of the brass plug. DO NOT overheat at this point. Remember, brass and aluminum melt at a rather low temperature. Take your vice grips and attach to the top stud of the brass plug and gently turn counter-clockwise. A little more heat may have to be applied. You will find that brass plug is then easily removed…

HOLD ON….. what did we just find in the inside lower float bowl area? A large amount of white powdery corrosion and deep pits and pock marks. The ¼” NPT threaded opening’s threads are stripped, half gone and corroded. What to do now. JUNK the lower carb casting.  It is UN-AIRWORTHY.

1" brass hex plugNEXT, remove the large 1” brass hex plug that holds the small fuel strainer in place. You try to remove it and it takes a tremendous amount of pressure with the 1” open end wrench to even start the fittings removal. You finally break the fitting loose only to find that the brass fitting and fuel strainer fall out on the bench after only ½ turn or so of the fitting. Upon inspection, you find that most of the fine threads holding the fitting in place are “stripped out” and gone. WHAT HAPPENED???  Some not so bright mechanic grossly over-torqued the fitting dry without using a proper fuel proof sealant in an attempt stop the fuel from dribbling out at the fitting/gasket contact area. What to do now? Again.JUNK the lower carb casting. It is not repairable and UN-AIRWORTHY…

NEXT…  Take a look at the four ¼” mounting studs that the air box fits up to. They usually are in very poor shape and need to be replaced. Try to remove them and you find that the aluminum threads strip out of the carb casting… UT-OH, time to call on “Mr. HELI-COIL” and repair the stud mounting holes.  More headaches.

It appears that the “simple” little Stromberg carb ends up not being so simple as everyone assumes. What caused the problem??? Carburetor old age and  LACK of proper maintenance.

NOW THE FIX…Shop around and try to find a good non corroded lower carb casting that has good threaded openings with no defects –and- the  P R O P E R   C A R B U R E T O R   P A R T   N U M B E R  stamped on the bottom flange. Remember, Stromberg carburetors do NOT use a riveted on aluminum data plate like all others. The Stromberg folks mechanically stamped the part number (7 digits) and manufacturers reference number (4 or 5 digits) on two of the lower casting flanges.
Stromberg never did apply serial numbers to any NA-S3 series carbs.

Stromberg Part Number Stromberg Manufacturers Reference Number

GOOD  LUCK…

Remember that there are 19 “different”  Stromberg part numbers for all the different carb/engine combinations… Use the WRONG part number and ground the plane. It CANNOT pass an annual inspection with the wrong carburetor mounted to the engine

God knows how difficult it is in trying to locate a “good” complete carb core let alone a “good” separate bottom casting. Today, if you are lucky enough to locate a hopefully “good” properly numbered lower casting, you will find that many times, corrosion is already started because the previous owner used auto fuel with “ethanol” which accelerated the start of the corrosion…

What should have been done to prevent all of the above?  A) drain and flush the float chamber fairly often. B) do NOT use automotive fuel containing “ethanol. C) apply a small amount of Standard Corp EZ-TURN  fuel proof sealant to the brass threads of the drain plug and fuel strainer nut and NOT over-torque them. Let the sealant do the job of sealing the threads What I have just discussed with you are some of the many little problems which, today, are becoming so common in these great old Stromberg carburetors.

THE  ULTIMATE  SOLUTION :  Send the carb in to us here at Uni-Tech and receive a complete and proper carburetor overhaul. Your engine will then start running like a “Swiss Watch”…

Hope you found this article interesting and informative. Any questions, call me at 708-267-7111 or email at This email address is being protected from spambots. You need JavaScript enabled to view it. .


Bob Kachergius
"THE STROMBERG SPECIALIST"

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