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Air Filter
Compression Ratio
Crankcase Stuffing
Cylinder Head
Electronic Ignition
Engine Ports
Exhaust Silencer
Expansion Chamber
Magneto Ignition
Spark Plug
Transmission Roller


THE VeloSoleX S 3800  performance can be significantly improved by tuning its engine.

Before tuning make sure that the bicycle is fully serviced and running well. This means checking that the Tyres are in good condition and pumped to the correct pressure, the Brakes working correctly, the Engine decoked if necessary, the Exhaust Silencer replaced if old, the Spark Plug in perfect condition and gapped correctly, the Engine and Head Gaskets replaced if necessary, all Nuts & Bolts tightened to the correct torque, the Carburettor clean inside and out, the Contact Breaker and Condenser in perfect condition, the Ignition Timing adjusted correctly and the Fuel Tank Filter, Carburettor Fuel Filter and Carburettor Air Filter replaced if necessary.

With regard to Engine Ports, "Two-Stroke Tuner's Handbook" by Gordon Jennings (1973) correctly points out that mistakes can EASILY be made while tuning, but are COSTLY to rectify! A lot of the data in the Gordon Jennings book was derived from empirical research on 2-Stroke engines with the recommended TIME AREAS based on Scatter Plot Data trend-lines measured on a large number of 2-Stroke engines - the VeloSoleX S 3800 engine not being one of them!

Also some points need to be clearly understood BEFORE attempting to modifying any ports:

  • When using the VeloSoleX S 3800 on long downhill runs, do not keep the throttle closed all the time but open it every now and then. Otherwise the engine will have a lack of lubrication, overheat and run slowly or even seize completely.
  • It must be remembered that the VeloSoleX S 3800 is a motorized Bicycle and that the Engine was not designed to exceed 4000 RPM.
  • The VeloSoleX S 3800 engine has small ports which are not easy to map onto graph paper in the standard way and some Rules Of Thumb are required. Even if the theory suggests that the TIME AREA or ANGLE AREA requires changing by filing away material there is little metal available around the port in the cylinder to do much with.
  • The VeloSoleX S 3800 engine is Piston-Ported which means that the piston is used to open and close the port holes. Although simple, this does result in undesirable SYMMETRICAL port timing either side of TDC (Top Dead Centre), normally resolved by fitting a One-Way Reed Valve in the Inlet Port. Unfortunately, there is not enough room in the Inlet Port to fit a Reed Valve, so other port improvement techniques must be found.
  • Port Height changes of 1 mm can result in SIGNIFICANT changes to engine performance.
  • Port changes are INTER-DEPENDENT on others.


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Copyright 2008 BRIAN'S VELOSOLEX. All rights reserved. This is a personal website containing information for and by VeloSoleX enthusiasts. It is not intended to replace any official information and whilst every effort is made to ensure the accuracy of all published information no guarantee is offered nor liability accepted for any loss or damage that may arise from any errors or omissions.