In years gone by nearly all competition two strokes featured a decompressor in their specification. To fit one to standard D7 - D14 engines the cylinder heads need some machining modifications to the head casting. Modern decompressor are normally 14mm x 1,25p by 5/8" reach, i.e. standard long reach sparking plug dimensions. Therefore what is required is another plughole fitted into the head. The obvious place is to place it central in respect of the combustion chamber. Unfortunately mere is insufficient metal on the top of the head to accommodate a 5/8" reach plug. By moving the centre line of the hole forward a little the head can be opened up to accept an insert of ¾"x 26 tpi. This insert has a flange thickness to accommodate the 5/8 reach plug. This insert is made a close fit in the new tapped hole and a dowel is fitted through the flange to prevent any chance of it unscrewing even during a " hot" plug change.  Moving the plug to the central position offers benefits in better combustion characteristics especially when used in conjunction with a squish band in the cylinder head.

With the plug relocated to the central position, the decompressor can now be fitted to the original sparkplug hole. Of course, you can decide to retain the original locations should conditions dictate this. Should a head just require a central plug for a road use the old hole can be plugged with a stainless steel insert.

The usefulness of the device in trials is when descending slippy banks. Using brakes in these conditions often ends up with the rear wheel locking, stalling the engine and you finishing up in an untidy heap at the bottom of the hill and five marks on your score sheet. Skilful use of the decompressor allows the engine to keep turning, even in a higher gear and still achieve some degree of engine braking. Releasing the control lever and opening the throttle the engine will instantly pickup, as it is not flooded with unburnt two-stroke mixture that is drawn into the even on small throttle openings. Many times a smart pull on the device's lever has allowed a stalled engine to be restarted in slippery conditions when operating the machines clutch would only postpone the inevitable stop.

The photographs enclosed show the completed modification fitted to a road head with a blanking plug.

On later B175 central plug heads a decompressor can still be fitted but it will be located in a vertical position to the rear of the plug.

Peter Savage