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Here is an inexpensive reed valve for the Grubee engine: http://www.arrowmotorizedcycles.com/shop-now---power-reed-block-kits.html But the problem is that it creates more compressible crank space between the piston and reed valve and so decreases the crankcase compression ratio (CCR). This makes the pressure of the intake charge less when the transfer ports are opened and so enters slowly and with less inertia to pull more charge after it. To offset that you would need to occupy unused empty space in the crankcase area with JBWeld. Below is a picture showing that space. Here is what I did: Measure the inner diameter of the hollow circular area of the side of the crank (the fat weighty part). Place the crankcase halves on the floor so that their interior side is facing up. Cut a strip of aluminum (from a cheap bread pan) and make a circle of it of a diameter slightly less than that of the inner crank circle that was measured. Oil it so JBWeld won't stick too well to it. Place it around the crankcase area that is needed to be filled in. Mix up some JBWeld and fill in that area and let it set and dry for a day. Pull off the aluminum. Check to see if the crank touches any of the JBWeld and file/cut/dremel those spots down. Put it back together. (Get a crankcase gasket before starting and maybe also crank bearings and seals from pistonbikes.com. Get two of the skinny left seal since that seal wears out faster with increased crankcase pressure). Luckily it is easy to replace in the future by removing the magneto and just prying it out with a jewelers screwdriver. Grease everything and just push the new one into place with your fingers.