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What is the ideal Pre-Mix Ratio of fuel and oil?

IN February 1978, Gordon Jennings published an article in CYCLE magazine illustrating the results of using different pre-mix ratios of Premium Unleaded Fuel 98 RON and Castrol 40R oil (based on castor-oil or bean oil) in the engine of a SUZUKI PE250 motorcycle. The results indicated, even with many other factors taken into consideration, that changing from a 30:1 ratio to a 15:1 pre-mix ratio (that is, doubling the oil content) reduced the gum varnish on the piston crown, reduced bore scuffing and increased the power output by 15.7%.

Before deciding on the best pre-mix ratio to use, for example 33 : 1 (3%) or 50 : 1 (2%), it should be remembered that most modern semi-synthetic 2-Stroke oils have many qualities and additives that allow higher pre-mix ratios while providing excellent lubrication and freedom from gum varnish.

It is therefore recommended that the VeloSoleX S 3800 is run at 50 : 1 (2%) pre-mix ratio if using a Semi-Synthetic oil such as Castrol Scoot-R 2-Stroke Oil or a 100% Fully-Synthetic oil such as Silkolene Pro 2. This ratio represents one fuel cap full of oil to one tank of fuel BUT:

  1. Fuel and oil must be mixed thoroughly BEFORE pouring into the fuel tank otherwise erratic running of the engine will occur.
  2. Some 2-Stroke experts recommend a 33 : 1 (3%) ratio and 100% Fully-Synthetic Ester-based oil, particularly if Unleaded Fuel is used.
  3. Using a 16.6 : 1 (6%) ratio for the first 50 miles (80 km) and then 33 : 1 (3%) for the next 250 miles (400 km) for a NEW VeloSoleX S 3800 will greatly reduce the risk of engine overheating or piston seizure.

What can I do about the Front Brake vibrating while braking?

ON some models of VeloSoleX S 3800, a Caliper type of front brake is used which is sometimes ASSEMBLED INCORRECTLY causing vibration while braking. Dismantle and reassemble correctly using a slight smear of WD40 Oil on the sliding surfaces.

WD40 Oil can also be used on chrome wheels and spokes to prevent corrosion but if a Caliper type of front brake is fitted you must clean the rims afterwards with Iso-Propyl Alcohol if the brakes are to work properly.

What can I do about the Handlebar Controls loosening?

ON the VeloSoleX S 3800 the black plastic Handlebar Controls sometimes loosen with engine vibration. This problem can be solved as follows:

  1. Loosen one small screw and remove each Handlebar Control.
  2. Clean handlebar chrome with Iso-Propyl Alcohol.
  3. Wind Self-Amalgamating Rubber Tape over the ends of the handlebar.
  4. Leave about a day for the tape to vulcanize.
  5. Refit and tighten Handlebar Controls.

An alternative solution suggested by Bill of Minnesota, US is to replace the original screw thread with an ISO 6 mm x 1.0 Heli-Coil.

What can I do about the Fuel Pipes fracturing?

ON the VeloSoleX S 3800 the Fuel Pipes are made from Aluminium which can easily fracture if the Fuel Pipe Unions are over-tightened. To improve reliability, replace the Aluminium Fuel Pipes with Copper Fuel Pipes as follows:

  1. Unscrew 10 mm Bolt on the left side of the engine and remove Crash Bar from Front Fork assembly.
  2. Prise off two black Rubber Clips and remove black round plastic Magneto Rotor Cover.
  3. Unscrew 13 mm Nut, disconnect Electric Wires and remove the black plastic Headlight Assembly.
  4. Unscrew two Fuel Pipe Unions at the ends of the Fuel Tank-To-Fuel-Pump fuel pipe.
  5. Unscrew two Fuel Pipe Unions at the ends of the Fuel-Pump-To-Carburettor fuel pipe.
  6. Unscrew three 10 mm Bolts and remove the Fuel Tank and Fuel Pipes.
  7. Cut the fuel pipes in half with a small hacksaw and remove the Fuel Pipe Unions.
  8. Enlarge the Fuel Pipe Unions with a 3/16" diameter drill.
  9. Fit two Fuel Pipe Unions to a 230 mm length of 3/16" (outer diameter) copper fuel pipe and swage ends.
  10. Fit two Fuel Pipe Unions to a 260 mm length of 3/16" (outer diameter) copper fuel pipe and swage ends.
  11. Bend and fit the 230 mm pipe between Fuel Tank and Fuel Pump.
  12. Bend the fit the 260 mm pipe between Fuel Pump and Carburettor.
  13. Reassemble all parts but DO NOT OVERTIGHTEN the Fuel Pipe Unions.

What can I do about loose Magneto Coil and Lighting Coil bolts?

THE Magneto Coil and Lighting Coil on the VeloSoleX S 3800 are sometimes secured by bolts held by only 3 or 4 screw threads. These bolts therefore tend to unscrew with the vibration of the engine. The securing of the coils can be improved as follows:

  1. Unscrew and discard original bolts.
  2. Tap holes with an ISO Thread Cutter.
  3. Fit Allen Bolts with a drop or two of Nut Locking Liquid.

What can I do about a loose Headlight?

THE Headlight on the VeloSoleX S 3800 is sometimes a loose fit in its black plastic housing. To make the headlight more secure, squeeze a 5 mm beading of Clear Silicone along the top edge of the Headlight and allow to dry for a few hours before reassembling.

Make sure that the Headlight is not pushed too far in otherwise the front bulb contact will short electrically against the top left bolt of the Fuel Pump.

What can I do about an intermittent Headlight beam?

THE Light Switch on the VeloSoleX S 3800 often becomes unreliable with engine vibration as it consists solely of a contact pressing a screw! It can be replaced with a modern Rocker Light Switch (available from most electronic shops) as follows:

  1. Remove the old Light Switch.
  2. Drill a larger hole and mount the new Rocker Light Switch in the position shown below.
  3. Connect the RED AC Supply wire from the Lighting Coil to the Input of the switch.
  4. Connect the Headlight / Rear Lamp circuit to the Output of the switch.

NOTE: An illuminated Rocker Light Switch has an extra terminal on one side which must be connected to an Earth connection (eg: Engine Body) for illumination in the "ON" position.

Rocker Light Switch

What is the effect of shortening or lengthening the Piston?

THE Piston on the VeloSoleX S 3800 is already quite a lightweight affair and there is little real advantage to be gained in shortening the piston by removing some of the Piston Skirt. In fact doing so will increase the "rocking" (or "piston slap") and general wear within the cylinder. A shorter piston also requires a smaller gap between piston and cylinder to reduce this effect. Too short a Con-Rod also increases cylinder wear. The reason for reducing the weight of the Piston or Con-Rod is to try to relieve the crank of as much inertial load as possible, thus giving increased Acceleration (lighter components) but this does NOT necessarily mean increased Top Speed.

If your engine has the Long Piston shown below (49.45 mm long [measured at the edge] for cylinders made before March 1986 such as the TUM 80 [V1], TUM 80A [V2] or TUM 80A CM 6,5 [V3]), a slight increase in Torque (but not top speed) can be obtained by filing some metal from the Piston Skirt nearest the Inlet Port in the shape of a Crescent 20 mm wide and 2.5 mm high. No more should be removed otherwise the Exhaust Port may be partially open beneath the piston at TDC, allowing the old exhaust gases to enter the crankcase and pollute the fresh mixture. You can undo this modification by replacing the piston with a new one OR by turning the piston 180 around by removing the Crank Pin (Big-End) Bolt and turning the Con-Rod by half a turn before replacing Crank Pin (Big-End) Bolt. To stop the flywheel turning when loosening or tightening the Crank Pin (Big-End) Bolt, fit a Flywheel Clamp.

After you have modified the Piston Skirt, drill out the Inlet Port hole in the side of the cylinder using a 9 mm drill.

Long Piston


If your engine has the Short Piston shown below (46.98 mm long [measured at the edge] for cylinders made after March 1986 such as the TUM M 80A CM 6,5 [V4, M = MBK version]), there is no benefit to be gained by filing the Piston Skirt. Also the Inlet Port hole in the side of the cylinder for that engine will already be 9 mm in diameter.

Short Piston


A slight increase in Torque can be obtained if the round Transfer Holes in the piston are filed square (remembering to leave a slight radius in each corner). This increase in Torque is due to a more efficient transfer of the mixture from the crankcase.


With regard to Bore/Stroke Ratio:

  1. "Stroking an engine" = "increasing the Stroke"
  2. "Boring an engine" = "increasing the Bore"
  3. A "Square" engine means "stroke" = "bore"
  4. An "Over-square" engine means "bore" > "stroke"
  5. An "Under-square" engine means "bore" < "stroke"

The VeloSoleX S 3800 is very slightly "Under-square", a design better for low-RPM torque, but usually not for high-RPM horsepower. "Under-square" motors were much more common in the pre-war period than they are today.


With regard to Rod/Stroke Ratio on the VeloSoleX S 3800:

This high Rod/Stroke Ratio (ie: long Con-Rod) means LESS WEAR to cylinder walls, pistons and piston rings as the combustion forces acting in the downward motion are increased while those toward the cylinder wall are reduced. Also the Piston Dwell Time is longer, meaning that piston remains at the top and bottom of the stroke for a longer time. This allows for better flow of combustion and exhaust gases since the piston accelerates slower in the transition between the "up" and "down" strokes. Intake gases have a longer time to enter the cylinder while exhaust gases are given more time to escape. This means more torque over a wider RPM range. On the other hand, even though the piston accelerates slower in transition, the Maximum Piston Velocity is greater than for a low Rod/Stroke Ratio resulting in some increased component strain.

While it is true that piston dwell at TDC is longer with a longer Con-Rod and hence a higher combustion pressure exerted for longer on the piston crown, it should be remembered that maximum torque applied to the crank is when the rod to crank angle is at 90. When a shorter rod is used, this 90 angle is reached sooner in the engine cycle and therefore at a higher combustion pressure due to the smaller combustion volume at that point. So, for low-speed maximum torque a shorter rod is better, while for high-speed maximum power a longer rod is better (ignoring the increased rod mass for a longer rod).

In the end, it must be remembered that the VeloSoleX engine was never designed, regarding component tolerances and general wear, to be a high-revving "sports" engine and many VeloSoleX tuners have found the engine life to be reduced considerably by excessively changing the existing engine parameters.

What can I do to improve the lubrication of the Pedal Axle?

THE Pedal Axle on the VeloSoleX S 3800 is simply a shaft rotating in the frame. To make sure that it is well-lubricated without necessitating disassembly, drill and tap a hole into the frame tube below the axle centre, clean thoroughly and screw in a Zerk Grease Fitting. Use a shot from a grease gun every few months:

ZERK Grease Fitting

Source: John Ingram, Canada (Solex Owners of America Yahoo! group)

What can I do about broken or worn rivets on the Seat Hinge?

THE Seat Hinge Rivets turn and may wear with time on the VeloSoleX S 3800 but they may be easily replaced with a Clevis Pin of the appropriate size, Washer, Spring & Clip. Cut Clevis Pin to fit, if necessary:

Clevis Pin 1 Clevis Pin 2
Clevis Pin 3 Clevis Pin 4

Source: John Ingram, Canada (Solex Owners of America Yahoo! group)

How do I replace an Old Fuel Pump Diaphragm?

THE Fuel Pump Diaphragm on the VeloSoleX S 3800 is easily replaced:

  1. Unscrew and pull out both Fuel Pipe Unions from the Fuel Pump
  2. Remove the 4 bolts from the Fuel Pump
  3. Lift off the Fuel Pump carefully
  4. Clean all parts with Carburettor Cleaner Spray
  5. Remove the old Fuel Pump Diaphragm and replace with a new one
  6. Reassemble all components, making sure that the White Plastic Diaphragm Seat is fitted to the engine casing with the Concave Inner Surface facing the Fuel Pump, that the hole in the engine casing lines up with the hole in the White Plastic Diaphragm Seat and that the 2 longer bolts secure the top of the Fuel Pump

Symptoms of an old, dry or hard Fuel Pump Diaphragm include lack of power when the throttle is fully open but the power returning when the throttle is partially closed.

The Fuel Pump works as follows:

  1. When the piston ascends a vacuum is created in the crankcase which is transmitted to the back of the Fuel Pump Diaphragm via a small, internal air channel
  2. The Fuel Pump Diaphragm flexes towards the engine and fuel from the tank is pulled into the centre channel of the Fuel Pump via the lower glass ball which is lifted off its plastic valve seat
  3. At the same time the upper glass ball is pulled down onto its valve seat keeping it closed
  4. When the piston descends a pressure is created in the crankcase which is again transmitted to the back of the Fuel Pump Diaphragm
  5. This time the Fuel Pump Diaphragm flexes away from the engine and fuel from the centre channel of the Fuel Pump is pushed towards the outlet (and on to the Carburettor) via the upper glass ball which is lifted off its valve seat
  6. At the same time the lower glass ball is pushed down onto its valve seat keeping it closed
  7. This cycle then repeats with each revolution of the engine
  8. Fuel which cannot be used by the carburettor is returned to the fuel tank via the Fuel Return Pipe
  9. A working VeloSoleX should have fuel constantly flowing in the Fuel Return Pipe

Do not attempt to remove the two glass balls unless:

  1. You are a brain surgeon
  2. You want them to fly into some tall grass
  3. Your name ends with BALLS

NOTE: If you do manage to lose one, you will find an similarly-sized ball at the end of a Waterman Pen Ink Cartridge.

How do I replace a worn Rotor Key?

THE Rotor Key inside the rotor centre on a VeloSoleX S 3800 is normally cast with the body. Sometimes the Rotor Key can become worn or sheared off. If this happens a new Rotor Key will need to be fitted as follows:

  1. Remove the Magneto Rotor using a Magneto Rotor Extractor
  2. Remove any remaining part of the old Rotor Key by filing it away with a suitable rat-tail file
  3. Obtain a new Rotor Key (3 mm wide and 27 mm long, with the last 4 mm tapering off as shown below)
  4. Cut a 3 mm wide slot in the rotor centre where the Rotor Key will fit
  5. The Rotor Key must lie in the slot in such a way, that it protrudes into the rotor centre by 1 mm nearest the outside of the rotor, increasing linearly to 1.5 mm nearest the inside of the rotor (shown below)
  6. The centre line of the Rotor Key may be found to be in line with the centre of two 7 mm diameter casting holes on the inside edge of the rotor (shown below), as well as the leading edge of one of the magnetic poles on the inside edge of the rotor


  1. The short distance between the 23.5 BTDC Timing Mark on the rotor (this mark is lined up with the "Rupture" mark on the crankcase when adjusting the ignition timing) and TDC (not usually marked) is 29.5 mm (measured around the circumference)
  2. The short distance between TDC and the centre line (shown below in cyan) is 5 mm (measured around the circumference)
  3. The rotor external diameter is 143.5 mm, with 1 of rotation equivalent to 1.2523 mm (measured around the circumference)
  4. The rotor internal diameter is 110.8 mm and there should be a gap of 0.2 mm between the rotor and coil armatures
  5. The rotor cam diameter is 27 mm (round) and it is offset from the centre by 0.5 mm
  6. The rotor centre hole diameter tapers from 14.5 mm to 11.5 mm
  7. The Rotor Key position is at 176 BTDC


How do I test the Condenser?

THE Condenser on the VeloSoleX S 3800 is quite often unreliable, poorly made, susceptible to humidity and heat and tends to fail at the most inconvenient time!

The Capacitance of a Condenser can be measured but it is no guarantee that it will work properly in the hot engine environment while out riding one's VeloSoleX S 3800. For those who want to check the Capacitance of a Non-Polarised High Voltage Dielectric Condenser, one can use one of the following instruments:

  1. Digital Capacitance Meter
  2. Digital Multimeter with Capacitance Range
  3. Analogue Ohmmeter (sensitive type)
  4. Analogue Multimeter with 3 or more Resistance Ranges

If using a Digital meter, the Capacitance of the Condenser should be in the range 0.18 uF - 0.25 uF.

If using an Analogue meter, one can still do a basic test of Capacitance and also check that other faults do not exist by following the steps below:

  1. Set the meter to the most sensitive range (eg: if the meter has x1, x10 and x 1000 Resistance Ranges then use the x1000 Resistance Range)
  2. Join the RED and BLACK meter leads together and turn the Zero Adjustment Knob until the meter needle points to 0 ohms on the meter scale (NB: If not possible then put some new batteries in the meter)
  3. Warm the engine up if possible
  4. Disconnect all wires to the Condenser centre terminal
  5. Connect the RED meter lead to the Condenser centre terminal and the BLACK meter lead to the Condenser body
  6. Now connect the BLACK meter lead to the Condenser centre terminal and the RED meter lead to the Condenser body
  7. Repeat 4 & 5 several times
  8. Check that each time you connect the meter leads, the meter needle kicks over approximately 10% of the meter scale for a fraction of a second then returns immediately to maximum resistance (far left of meter scale)
  9. If the meter needle simply stays near 0 ohms (far right of meter scale) with the leads connected, the Condenser has Short-Circuited and is FAULTY (this is a rare fault)
  10. If the meter needle does not return immediately to maximum resistance with the leads connected but stays somewhere in the middle of the meter scale, the Condenser is Leaking Electric Current and is FAULTY (the more the needle is towards the right, the more the Condenser is leaking electric current)
  11. If the meter needle simply stays at maximum resistance (far left of meter scale) all the time, the Condenser has Open-Circuited and is FAULTY (this is a common fault)
  12. If necessary, compare readings by testing a known good condenser (or any good 0.22 uF non-polarised capacitor)

Note: Do not touch the metal prods on the end of the meter leads with your fingers when testing the Condenser, otherwise you will get false readings.

The most typical Condenser faults include the Intermittent Open-Circuit Fault due to a loose internal connection caused by engine vibration and the Electric Current Leakage Fault due to the ingress of moisture. Both faults can lead tosome strange engine starting or running symptoms particularly when the engine is hot! The best way to deal with a possibly faulty Condenser is to replace it with a known good one.

I recommend replacing the Condenser with a 0.22 uF 1000V DC Metallised Polypropylene Film Non-Polarised Capacitor with a minimum temperature rating of 105C. This high-quality capacitor is available from most good electronic component stores and has the following features:

  1. High Voltage Rating of 1000V DC (500V AC) able to handle the Contact Breaker peak voltages of 300V to 400V DC
  2. Very Low ESR (Effective Series Resistance) and so introduces minimal self-heating
  3. Extremely Low Di-electric Loss over a wide frequency range and so has excellent pulse handling
  4. Self-Healing Recovery Ability by vapourising the metallised film at the point where a short-circuiting puncture occurred


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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.