Reed valve problems

by Had Robinson
updated November 12, 2020


Two stroke engines do not have intake or exhaust valves as in four stroke engines.  When the piston moves up, a vacuum is created in the crankcase which is connected to the carburetor.  Air/fuel is then 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 and its perfect operation is critical to the performance of the engine.

The reed valve consists of the aluminum body and the actual petals which open and close against the valve body thousands of times a minute (see the last photo on this page for a detailed view of a typical reed valve).

As reed petals age, they loose their straightness, may break off, become chipped, and bend more easily.  As a result they will not do their job.  The result is that the engine may not idle or start well nor reach full output.  The engine may also "spit" at the carburetor and drip fuel excessively.  If the petals break off, the engine may not run or start, depending on how much of the petals is missing.  Polini, for example, recommends that pilots replace the petals every (100) hours.

One ingenious mechanic 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 carburetor, and open the throttle all the way.  Now get someone to spin the motor, either with the starter, or by rope pull, while you listen to the carburetor throat. Use a piece of clear plastic tubing for a stethoscope and put it down into the throat. On a good reed valve, 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.

A little more difficult way to check the valve requires removing the carburetor and look into the reed valve body with a flashlight.  It will be pretty obvious if you have a problem.

On all paramotors, a leaky reed valve body (not leaky petals) will lean out the fuel mixture, may cause overheating, and/or may result in stall at full throttle.  Be certain your reed valve body does NOT leak.  Here is a video of leaking reed valve body.  A gas leak detection fluid was applied to the area where the valve contacted the engine.  Even though the screws which fastened the valve body to the engine had been tightened correctly, the valve body face and the crankcase face were not quite flat.  The variation was greater than a dry gasket could seal.  The fix was easy: Apply RTV to both sides of the gasket (see instruction #5 below).

Most paramotor engines e.g. the Polini Thor have a small piece of tubing between a fitting mounted directly on the crankcase and the fuel pump on the carburetor.  These engines will not experience fuel pump failure if the reed valve body leaks.

ENGINES WITH AN INTERNAL PULSE PORT e.g. Top 80, newer Moster 185's  Some engines have a reed valve body with a special passageway or port that goes from the crankcase to the carburetor and then to the fuel pump diaphragm.  This port transmits the pulses from the crankcase that operate the fuel pump.  The port must not be obstructed with sealant or debris and must be air tight and not leak.  If the reed valve body is loose and the port is leaking, the fuel pump will not work well.  I have found loose reed valve bodies on both brand new and used Top 80 engines.  A new engine may run fine for a number of hours and then the screws that attach the reed valve body to the crankcase begin to loosen (because they were not torqued properly at the factory nor was threadlock applied to keep them from loosening).  As things loosen, the fuel pump does not work as well and the pilot may notice "hiccups", especially near or at full throttle.  When the problem gets severe, the engine will quit when the throttle is fully opened.  Nonetheless, the engine may run OK at idle and at lower RPM because there is enough suction present in the carburetor throat to overcome a weak or failed fuel pump.  It is important to note that there are other things that can also cause fuel starvation.  The pump cannot work unless the strong pressure impulses that exist in the crankcase get adequately transmitted to the pump.  Such things as sealant in the reed valve body passageway, a defective or incorrectly oriented gasket, or a leak can prevent the pulses from reaching the pump.  There have been defective carburetor/reed valve body gaskets that have the hole for the pump port either misaligned or too small to adequately transmit the pulses needed at the pump.

This carb - reed valve gasket was so misaligned that it completely covered the pulse port on the reed valve body.  The actual port is just below the hole in the gasket.  There is enough vacuum in the carburetor jet circuits to suck some fuel from the tank but not enough to run the engine off idle.  Typically, the carburetor will have enough fuel in it to run at WOT for a second or less.  If the pilot immediately goes back to idle, the carburetor may supply just enough fuel to run.

misaligned carb - reed valve body gasket

Test, examination, and reinstallation

The reed valve in the photos below is from a Top 80.  Other paramotors may be a little different.  Always check the instruction manual for your engine before and during assembly.  Generally, reed valve petals should be replaced every 100-150 hours.

1. Remove the carburetor in order to access the two reed valve screws (other reed valve bodies may have 4 screws).    Remove the (2) screws and pull out the reed valve body.  Be careful not to tear up the gasket.  If you do it correctly, you can reuse the gasket between the reed valve body and the crankcase.  If you do not have a Top 80, some of these procedures may be somewhat different.

2. ENGINES WITH AN INTERNAL PULSE PORT   Check the small port that goes from the crankcase side of the reed valve to where the carburetor mounts.  It must be free of any obstructions, such as sealant.  I recommend that pilots carefully increase the port size to 3.5mm, including the hole in the gasket, using a #27 (3.5mm) bit if it has not already been done.  This will increase the passageway's area by about 25% and help improve fuel pump performance.  It is also possible that in certain conditions an even greater diameter port will help.  This would include the presence of high viscosity oils and/or much colder temperatures.  Vittorazi specifically recommends that the internal pulse port passageways in the carburetor be reamed oversize.

3. Carefully examine the reed valve.  If the petals are missing (.), 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.  The 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. 

Below, this reed valve had one of its four petals missing (broken off).  The broken petal went through the engine and likely damaged it.  (photo courtesy of Tom Bird)

broken reed valve petal from a Polini Thor 130

4. Check the closing force of each petal (optional).  If you replace the petals, you will know that the closing force is correct and any more testing is unnecessary.

If the petals have a weak closing force, they will not act efficiently and not close quickly.  This will allow the fuel/air mixture to come back into the carburetor rather than stay in the crankcase.

To check the closing force, 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. or more.  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 for a variety of engines.  Polini petals and parts are available from Polini Parts

The Reed Valve on the Top 80

The red arrow points to the measurable gap between the reed petal and the valve body.  The curved steel plates prevent the petals from opening too far.

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 fully seal and are as flat as possible.  There is some play in the exact position of the petals which is why you may need to move them around a bit to get them as flat as possible.  If the petals will not seal the opening, you may need to replace the entire reed valve.

5. Reattach the reed valve to the crankcase.  Be CERTAIN to clean the threaded holes in the crankcase and the reed valve body screws with brake cleaner and blow dry.  Apply blue threadlock to the screws and torque to the correct specification for your engine (Top 80 should 2.5 Nm/22 in lb).

6. Reattach the carburetor, etc.