Carburetor reed valve body gasket misalignment

by Had Robinson
updated November 12, 2020

The gasket between the carburetor and the reed valve body on engines with an internal pulse port e.g. Top 80, newer Moster 185's can become misaligned.  When this happens, the pulses from the crankcase that operate the fuel pump inside the carburetor will be weak or non-existent and the pump will not deliver sufficient fuel to the engine.  It will slow down, stall, and or hiccup.  The fuel/air mixture can lean out and the engine overheat, even seize.

In the photo below the misaligned gasket completely covered the pulse port on the reed valve body of a Top 80.  The pilot could have saved himself $100's by fixing this himself.

misaligned carb to reed valve gasket

Special tool needed: Top 80 – #32 drill bit (2.9mm) A 7/64" drill bit  For other engines use a bit that is just smaller than the pulse port.  It can be slightly loose in the pump port hole.

Use the following method to perfectly align the gasket between the reed valve body and the crankcase.

1. Remove the carburetor from the engine.

2. Carefully remove the exposed gasket.  Note the small hole in the gasket and in the face of the reed valve body (in the lower right hand corner).  These holes must line up perfectly in order for the fuel pump inside the carburetor to function correctly and optimally.

3. Replace the gasket and observe if the holes are perfectly aligned.  Often, it may be possible to move the gasket around so that the holes are misaligned.  It can be so bad that the gasket will cut off the passageway to the carburetor and the fuel pump will work poorly.  Remove the gasket.

4. Place (2) small dots of RTV on the side of the gasket that contacts the reed valve body (the RTV will help align the gasket).  The dots should be right next to where the gasket contacts the long studs that go into the reed valve body.

5. Put the gasket on the reed valve body and visually align the hole in the gasket with the pump port hole in the reed valve body.  If needed, place the drill bit through the gasket and into the pump port hole.  The bit will perfectly align the gasket.  With the bit in the hole, use your fingers to push firmly against the gasket where the dots of RTV are.  Wait a minute or so.

6. Carefully remove the drill bit.  Make sure the holes are still perfectly aligned.

7. Immediately reattach the carburetor to the engine and tighten the nuts.

If you did it correctly the pump port holes will be aligned so that the pump can work at maximum capacity.