WillBrown Internet - Ref:WP10*CT-Main-120517q15_cookies -  Updated 27/5/12 © 2005-11.

Having now had the opportunity to look very carefully at the Bultaco we are improving, it seems the main things that are compromising engine performance are the very poor OE induction and exhaust systems. There is also a lot of potential for making alterations to help improve handling, and rear wheel grip, which we will outline here.

Engine:

The stock air box is not particularly well designed, which means power delivery is not as smooth and linear as it would be with a more modern  type box. Another consideration is that OE air filtration is not that good, which can lead to rapid engine wear on machines used in extreme conditions.

Standard “clubfoot” exhaust back box, is heavy, restrictive, and uses a combination of passive and reactive internal elements, which is not well suited for use in serious competition conditions. The intermediate silencer is also heavy, and does very little to help engine performance.

In relation to the induction, we will be fitting an accurately set up OKO carb which is far superior to the poorly made and inefficient OE carburetion. This will work in combination with a very special large capacity alloy air box, that uses a modern top fitting element, and a quality silicon rubber carb/air box connection hose.

Initially we will be using a heavily modified stock exhaust back box, but if there is enough interest we will be looking at making tooling to allow manufacture of large capacity lightweight alloy back boxes. The intermediate silencer, will be replaced by a compact and light weight alloy expansion chamber, that will provide increased power and torque, across a wide range of engine speeds.

Clutch action on the Bultaco is very heavy, and one of the main reasons for this is due to friction between the clutch operating spindle, and the flywheel cover, which it fits into. Extending the clutch operating arm improves the action slightly, but obviously does nothing to reduce the amount of friction between the spindle and flywheel cover. The solution to this problem is to modify the operating spindle, and flywheel cover, to accept a needle roller bearing, which can be easily lubricated, which greatly reduces friction and improves clutch action substantially.

Bultaco development project

Please feel free to contact us for further information on anything mentioned above, or for general help and advice on anything to do with your twinshock or P65 trials machines. Email us at classictrial@gmail.com or phone Chris on 02380 600 850 or alternatively 07761 971 756.

Updated 29/11/2009

Bultaco clutch lightener

Click to enlarge

Chassis:

In terms of chassis upgrades for Bultaco the old 1970s alteration made to the swinging arm pivot position is still something that seems quite popular, but its worth bearing in mind that the primary reason for this modification is to increase rear wheel grip, and that the tyres available in the 1970s, are not really comparable to modern day tyres. The point here being that changes which worked well in the 1970s, may not be the ideal way to go more than 30 years later!

We will be altering the steering angle on our development bike, relocating the footrest mounting position, and removing the very poor cable operated rear brake, and fitting a direct acting brake pedal on the left hand side of the bike. Rear wheel traction will be improved by careful attention to suspension geometry, which will be optimised using our Tony Foale chassis set up software program.

Braking on the Bultaco can be very poor, due the chrome plated friction surface's on the wheel hubs, which means brakes are pretty much non existent in wet conditions, and if the chrome is starting to become detached from the wheel hubs, is very poor at all times! It is quite possible to repair the chrome plated hubs using liners, and we will be outlining how this work is done should it be required on our machine.  

A common error made by many people who make changes to the steering angle of a bike, is to retain the stock “tiller” (bars mounting behind the steering axis) top yoke, which is something that makes for very strange handling, and negates much of what is gained by using a steeper steering angle. Our bike uses a modified OE top yoke, which allows fitment of forward mounted fat-bars, which provides a cost effective method of getting rid of the stock “tiller” handlebar position.

Cosmetic appearance of our bike will be updated, and we may well be investigating the possibility of an all new tank/seat unit, which will allow removal of the rather dated OE tank, seat, and side-panels.