This is the sixth topic in the series about the 100 hour / annual inspection. This series focuses on maintenance performed during an effective 100 hr. inspection on R1340 and R985 engines.
- Oil Change with filter/screen & sump checks.
- Valve adjustment – Positive or compression.
- Ignition timing check – Spark plug servicing.
- Compression check – differential.
- Air filter and carb – heat system check.
- Fuel System Screens.
- Cylinder Head Checks.
Fuel System Screens: There are at least two fuel system screens that should be checked during an annual inspection; the aircraft fire-wall fuel strainer or gascolator screen and the carburetor inlet fuel screen. The Pratt & Whitney engine maintenance manual calls for carburetor screen inspections on a 50 hour basis making the inspection part of any 100 hour inspection!
These engines are typically equipped with Bendix-Stromberg float type carburetors; NA-Y9E1 (1340) and NA-R9B (985) which have inlet screens constructed of a fine mesh bronze screen element soldered to bronze bosses. One boss on each screen assembly is threaded and equipped with a square head to facilitate removal, use an 11/16 inch wrench or square head socket. The screen traps contaminants from the aircraft fuel system and engine driven pump, deteriorating hoses, etc. on the inner diameter of the screen.
The photographs below are identified by carburetor model; R-1340 / NA-Y9E1 and R-985 / NA-R9B.
Clearance adequate for use of a socket may be an issue on the 1340 requiring that it be removed with an open-end wrench.
Fuel enters the 985 fuel screen from the end opposite the square head and flows downward through the center of the screen, through the needle-valve and seat into the float bowl. Fuel enters the 1340 carburetor fuel screen through holes in the upper boss and flows downward into the center of the screen, through the needle-valve and seat and into the float chamber. Aircraft fuel pressure indicator ports are “downstream” of the carburetor fuel screen.
Everyone routinely checks the aircraft fire-wall / gascolator fuel screen or filter but the carburetor inlet screen generally gets ignored! If your carburetor inlet screen has never been removed it is possible that the screen may be damaged or destroyed during the removal. Dissimilar metal corrosion of the screen bronze boss to the aluminum carburetor body can cause the screen bosses to seize in the carburetor if left in place over extended periods of time. If your carburetor’s screen has never been checked you would be wise to obtain a replacement screen prior to removing yours!
The gasket for both the NA-Y9E1 and NA-R9B carburetor screens is made of stamped aluminum and is not a crush gasket- It can be easily over-torqued! It is recommended that the gasket be thinly coated with a non-corrosive, fuel-proof sealant just prior to installation. The screen should be torqued at 160 to 175 inch-pounds and safetied.
Check with your aircraft’s maintenance or service manual for information on your particular airframe / firewall fuel filter requirements!
If you need some MRO assistance for your radial engine, contact Covington Aircraft!