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Reed valves

by Had Robinson


Two cycle engines do not have intake or exhaust valves like in a four cycle engine.  When the piston moves up, a vacuum is created in the crankcase which is also connected to the carburetor and air/fuel is sucked into the crankcase.  When the piston goes down, pressure is created in the crankcase.  If it were not for the reed valves, the air/fuel would be forced right back out the carburetor instead of being pushed up into the combustion chamber through a port that is opened by the downward movement of the piston.  The reed valve is located between the carburetor and the engine crankcase.

As reed petals age, they loose their straightness, may be chipped, and bend more easily.  As a result they will not close and open at the right moment.  The result is an engine with less performance.  The engine may also spit at the carburetor or drip fuel excessively.  Polini recommends that pilots replace the petals every (100) hours of engine time.

One ingenious mechanic (not a Miniplane blogger) figured out a way to test for major failure of the reed valves without removing them from the engine:

I've found [reed valve leaks] by listening. Remove the spark plugs, the air box over the carburetors, and open the throttle butterflies all the way. Now get someone to spin the motor, either with the starter, or by rope pull, while you listen though the carburetor throats. Use a piece of clear plastic aquarium air line tubing for a stethoscope and put it down into the carburetor throats. On a good reed, you will here a single air intake event per crank rotation. With a bad one, you will hear what sounds like breathing in and out with two air movements per crank cycle. This only really works well if your motor has a single carburetor throat for each cylinder.

Valve body leaks in the Top 80

A loose or improperly torqued down reed valve body can leak air.  This may cause the fuel pump to fail because the pulse port is integrated into the reed valve body such as in the Top 80.  The engine may run OK at idle because the inherent suction of the carburetor will overcome a failed fuel pump.  The moment the throttle increases past 1/4 open or so for a few minutes, the fuel pump fails and the engine will stop abruptly due to lack of fuel.  I have also seen defective carburetor – reed valve body gaskets that have the hole for the pump port either misaligned or too small to do its job.  The Polini Thor engines have a small piece of tubing between a port directly on the crankcase and the fuel pump on the carburetor.  These engines and most other paramotors will not experience fuel pump failure if the reed valve body is loose.  A failed fuel pump will lean out the fuel mixture resulting in engine overheating and irreversible topend damage.  The fix for this in the Top 80 takes 15 minutes, if that.  Be certain that the reed valve body bolts are torqued to 2.5 Nm and that they BLUE threadlock on them so they will not loosen.

Test & Examination

These instructions apply to the Top 80.  Other paramotors may be slightly different.  Always check the instruction manual for your engine before and during assembly.

To examine and test the reed valve, it must be removed from the engine.  To access the (2) reed valve screws, the carburetor must be removed.  If you do not have a Top 80, some of these procedures may be somewhat different.

If the petals are curved away from the valve body, cracked, chipped, or deformed, they must be replaced.  An easy way to see if the petals are closing symmetrically is to put a small led flashlight into the valve body and look around the outside edges of the valve body for light leaking out.  The light seen, if any, should be uniform around the part of the reed that opens.  There should be no gaps.  Your reed cage may not be perfectly flat which will cause a small gap (<0.5mm) between the reed petal and the cage that is not uniform around the edge of the petal.  This is not a cause of great concern

If the petals look OK, they will still need to be checked for their closing force.  To check this condition, you will need a letter scale that can be held in the hand and some dental floss.  Carefully lift a petal with a toothpick and thread a piece of floss under the lip of the petal, through the center slot, through the bolt hole, and tie it around the long carburetor bolt.  Make a loop in the other end of the floss.  Using the scale, measure the force needed to lift the reed petals 2mm (see arrow on 2nd photo).  The value should be about 2.5 oz.  If it is less than 2 oz. I would replace the petals.  Be sure to check both petals of the reed valve.

Even if the petals opening force is correct, the petals may still have to be replaced if they are more than a few years old.  New petals are a good investment that will keep your paramotor running well.  Miniplane-USA stocks the correct petals.

The Reed Valve on the Top 80

The red arrow points to the measurable gap between the reed petal and the valve body.

The Reed Valve on the Top 80

When replacing the petals be sure to use BLUE threadlock on the screws and make certain the new petals are as flat as possible.  If the petals will not seal the opening, you may need to replace the entire reed valve.

When reattaching the reed valve to the crankcase, be CERTAIN to clean the threaded hole and screws with brake cleaner.  Apply BLUE threadlock to the screws and torque to 2.5 Nm (Top 80) or to what the owner's manual specifies for your engine.

If you do not do this correctly, the valve body will leak and the fuel pump (Top 80) will not work properly.  You will notice this problem on new and old engines: going above 1/4 throttle, the engine will suddenly die (for lack of full).  This happens because the pump must rely on the strong pressure impulses that exist in the crankcase.  If these pulses are weak or non-existent due to an obstruction or leak, the pump will not work.  The reed valve body has a special port that goes from the crankcase to the carburetor and then to the fuel pump diaphragm.  The port must not be obstructed with sealant or debris and must be air tight and not leak.  On all paramotors, a leaky reed valve body will lean out the fuel mixture and may cause overheating.