THE Decompression Valve on the VeloSoleX S 3800 is a small valve in the Cylinder Head opened by operating a lever on the handlebar to release engine compression and thus stop the engine when desired. To ensure that the Decompression Valve has a completely air-tight seal when closed:

1. Remove the Cylinder Head and Decompression Valve
2. Put a VERY SMALL amount of fine Valve Grinding Paste at several points around the edge of the Valve Seat
3. Insert the Decompression Valve so that it sits on the Valve Grinding Paste
4. Rotate the Decompression Valve using a Flat-Blade Screwdriver between the palms of your hands, lifting the Decompression Valve every now and then and replacing it about a ¼ turn from where it was and do it again
5. Repeat this for about 30 seconds.
6. Remove the Decompression Valve and check the Valve Seat and Valve Face
7. Repeat steps 3 - 6 until the Valve Seat and Valve Face appear to have a continous, smooth ring about 1½ mm wide
8. When satisfied, clean all parts thoroughly with Carburettor Cleaner Spray
9. Fit the Decompression Valve to the Cylinder Head
10. Fully tighten the Decompression Valve nut and then unscrew it 1½ turns
11. Reassemble the Cylinder Head

It must be remembered that the Decompression Valve must make a very good seal because the Peak Combustion Pressure can easily reach 500 PSI in the Combustion Chamber after combustion has occurred.

THE Cylinder Head on the VeloSoleX S 3800 is an Offset-Plug Offset-Hemispherical type with a wide Squish Band designed to remove heat from the Piston Crown near the Exhaust.

To check Liner Top Edge Protrusion, remove the Cylinder Head but leave the Head Gasket in place. Using Feeler Gauges and a Steel Rule, check that the Liner Top Edge is 0.7 mm higher than the 0.8 mm thick Head Gasket. If not, try a new Head Gasket or remove Cylinder and rub the top of the Cylinder Liner on a sheet of 600 Grade Wet & Dry Abrasive Paper taped to a perfectly flat kitchen surface top (test for flatness using a Steel Rule).

Now check that the Cylinder Head Squish Step (shown below) is 1.5 mm in height (see S 3800 Data section). If not, rub the Mating Surface of the Cylinder Head on a sheet of Abrasive Paper as detailed above. Reassemble engine and with the Spark Plug removed rotate the engine by hand a few times by turning the front wheel making sure that the engine turns freely.