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

Improving Air Flow

THE VeloSoleX S 3800  has an Air Filter Housing over the carburettor. It was designed with two small air inlet holes in the Air Filter Housing Base to keep Induction Noise to an absolute minimum. At high RPM the mixture tends to be a little rich due to the air flow being restricted by the two small air inlet holes. So to improve the air flow at speeds above 4000 RPM, the Air Filter Housing must be modified:

Air Filter Cover Bolt

1. Unscrew a 9 mm Bolt & Washer and remove the black Air Filter Housing Cover.

Air Filter Base Washer

2. Remove the aluminum Air Filter Housing Base together with the black Rubber Washer underneath.

Foam Filter

3. Remove the original Foam Air Filter by prising off the metal Filter Retaining Ring.

Air Filter Inlet Holes

4. Drill out the two 5.7 mm Inlet Holes in the Air Filter Housing Base using an 8 mm drill. Clean all components with Carburettor Cleaner Spray. Squeeze engine oil lightly into the foam air filter with your fingertips and reassemble all components OR if you wish to fit an S&B Air Filter continue with Step 5.

Main Jet
Main Jet Close-Up

5. Unscrew and remove the brass Main Jet with a 9 mm ring spanner. With the new Air Filter Housing in place the mixture will be too weak particularly at high RPM and the Main Jet hole will require enlargening (from 0.48 mm to 0.5 mm equivalent to a 31 Main Jet) using a 0.5 mm drill held in a miniature drill chuck available from a model shop or Maplins Electronics who stock a Microbox Metric Set (20 drills from 0.3 mm to 1.6 mm, Stock Code: GU87U). The drill must be turned VERY CAREFULLY forwards and backwards by hand, half a turn at a time, otherwise the drill WILL break off in the jet. After drilling clean thoroughly with Carburettor Cleaner Spray making sure that the hole is perfectly clear. Re-fit Main Jet.

Air Filter Base Collars

6. Using a small hacksaw, cut the 2 internal Air-Inlet Collars so that only 5 mm remains (one shown with arrow). Smooth any sharp edges with a file.

Air Filter Base Slot Top
Air Filter Base Slot Side

7. Using a small hacksaw, cut a 17 mm wide slot (shown) across the width of the Air Filter Housing Base so that both Air-Inlet Collars and part of the round Air Filter Foam Support is cut through. Smooth any sharp edges with a file.

Air Filter Base M6 Bolt

8. Drill a 2 mm diameter hole (shown by top arrow) in the centre of an M6 x 30 mm bolt, about 4 mm from the thread end. Drill a 6 mm hole through the Air Filter Housing Base about 45 mm from the end. Fit the M6 bolt to the base using an M6 locknut and washer (shown by bottom arrow). Hammer in a 64 mm long piece of 2 mm diameter aluminum coat hanger wire into the bolt end using a small hammer and bend wire downwards slightly on both sides. Cover the two original Air-Inlet Collar holes underneath the base with two small pieces of electrical insulating tape.

Air Filter Base JB-Weld

9. Cover the base with JB-Weld Epoxy Resin (shown by arrow) and leave to harden. After the mixture has fully hardened, remove the two pieces of electrical insulating tape.

10. Now, either drill out the air inlet holes in the Air Filter Housing Base using an 8 mm drill and jump to Step 12 OR for a even better airflow (but also increased Induction Noise) continue with Step 11.

Air Filter Cover Front
Air Filter Cover Side

11. Remove the sticker from the Air Filter Housing Cover. Cut a circumferential slot 95 mm long by 17 mm wide in the Air Filter Housing Cover, 15 mm up from the base. Smooth the corners of the slot with a file. Glue a 105 mm long by 25 mm wide rectangular section of black metal gauze (pierced with approximately 1.5 mm diameter holes) inside the Air Filter Housing Cover using High-Temperature Epoxy Resin. Use wire to hold the gauze in place until the High-Temperature Epoxy Resin has hardened.

S&B Air Filter

12. Cut a piece of Filter Material (75 mm wide by 35 mm high by 17 mm thick) from a new S&B Air Filter. Oil it with S&B Air Filter Oil and fit it into the slot in the Air Filter Housing Base so that the coat hanger wire cross restrains it against the centre of the housing. Reassemble all components.


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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.