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Model Specifications

The PX125 dates back to 1978 and has had various modifications and improvements since then. The major changes are:

1982/83 models use a derivative of the P200 gearbox with lower primary drives, and have electronic ignition and autolube as standard (in the UK).

1985/86 EFL models introduced the latest gearbox. Ratios are the same as T5 but they still use a 6-spring clutch.

The PX150 was only available in the UK (officially) until 1982, when the learner laws changed from 250 to 125cc.

In 1999 models a disc was introduced.

Most performance improvements to these machines are based on improving power output and raising the gearing to make best use of the improved power. Modifications to obtain the best reliability are desirable and mostly obtained by using components from the latest machines.

Cylinder

The standard cylinder bore is 52.5 for the 125 and 57.8 for the 150. Capacity increases can be made by fitting a PX150 cylinder and head (125 only!), a big bore standard style VE cylinder at 61mm (166cc), a Malossi 61mm (166cc) kit, Polini 63.0mm (177cc) kit or one of the cheaper DR/Olympia 63.0mm (177cc) kits.

All the above are cast-iron cylinders. The only aluminium Nicosil cylinder is from Pinasco at 63mm (177cc), but this does not produce the power of the Malossi or Polini cylinders, especially when the crankcase inlet modifications recommended by the manufacturer are carried out.

Our personal preference is to use a Malossi or Polini cylinder. The Malossi, despite its smaller capacity, has a better porting layout and cylinder head and gives better power when inlet modifications and larger carbs are used.

The cheaper DR/Olympia type 177 cylinders are useful when a large increase in punch is required but not necessarily a large increase in top speed. They are not that far behind in performance if they are just bolted on, but the more work that is carried out to the motor the greater becomes the advantage of using a Malossi or Polini cylinder.

Carb

When larger cylinders are used a larger carb can be of benefit. For those wanting to keep the autolube system the 24mm autolube carb from the PX200E is useable. A 28 or 30mm Dell'Orto carb and manifold will give more power on the standard rotary valve, or for ultimate performance a reed valve should be used. A reed valve will improve power by 10% to 15% compared to the large carb rotary valve. Also, as the reed valve needs the sealing pad removed it will give a larger feed area into the crankcase, which helps the high rpm power on tuned motors.

Gearing

The early PX125/150 points models use a gearbox based on the 125/150 Super. It uses a different diameter cluster (Christmas Tree) shaft and this would require changes to the cases to fit later gearboxes in (it is possible!).

The best gearbox is the T5 style and late P gearboxes (1986 onwards), which are extremely reliable. A small increase in gearing can be obtained by using the Cosa 200 21-tooth clutch. This fits with the later cluster arrangements and raises gearing by 5% (use 7 or 8 spring clutch).

The PX has a 6-spring clutch that has a tendency to explode at high RPM. The cure is to use the 7-spring clutch as fitted to the T5, Cosa 200 and P200 with the correct drive gear, and modify the clutch cover and crankcase to obtain clearance for it (the clutch is a larger diameter). The latest 8-spring 4-plate clutch as fitted after 1997/98 to all models can be used instead of the 7-spring clutch.

The gear options are: on models after 1983 use 21-tooth from Cosa in 7 or 8 spring clutch, use 23-tooth from P200 with P200 cluster or drive gear from P200 cluster on 125 cluster (7 or 8 spring clutch). If the 7-spring clutch is used there are straight cut gear kits that will fit. These are 23/64 tooth from Polini and Malossi that give ratios similar to standard P200E (23/65). There is also a 24/63 from Malossi which gives approximately 6% higher gearing than P2.

For models before 1983 using the 150 Super style small cluster shaft, a straight cut 23/64 gear kit is available to fit the 6-spring original clutch, but it is better to use the 7-spring 200 clutch as above.

Light Flywheel

Our modified PK flywheel unit is considerably lighter than standard. It gives better acceleration and more importantly increases reliability by reducing the strain on the crankshaft, clutch and gearbox when gear changes are made at high RPM. This is an electronic ignition flywheel - early PX's with points can be converted to electronic if required.

It is possible to modify the Polini and Malossi cylinders to improve top end power when a reed valve and an expansion chamber are used.

Exhaust

Basic exhausts such as Sito Plus, LeoVinci and Pitone exhausts will work well if you are after a modest improvement in power on a standard cylinder or a basic tuning job.

The ultimate exhausts are the purpose-built expansion chambers. It is possible with our Mk4 exhaust to obtain power figures above 16bhp with bolted-on parts and inlet modifications, and 18+bhp with reed and porting work. Depending on the gearing used this is sufficient power for 85-90mph.

When modifying crankshafts for inlet timing for rotary valve engines it is important to remember that both the crankcase and crankshaft can be modified to obtain the necessary increase but it is important not to go too far if good bottom end power is important.

Summary

A step-by-step summary runs as follows:

1. Use a better exhaust - depending on what other modifications are going to be done.

2. Fit a larger cylinder.

3. Modify crankcases and inlet timing to suit the cylinder being used.

4. Raise overall gearing.

5. Fit a larger carb and/or reed valve.

6. Fit a light flywheel.

Obviously, if the motor is apart to raise the gearing or to modify the inlet it is worthwhile doing both operations at the same time and also taking the opportunity to make sure that the gearbox and clutch are in sound condition.

Nicely modified engines using expansion chambers and reed valves can give 17+bhp (without extra cylinder modification) and 80+mph top speed.

A 166 or 177 kit bolted on with a Pitone or Sito Plus exhaust will give more than a standard P200 at around 10bhp, and will give around 13bhp if the crankcases are done.

© Taffspeed Racing - September 1999