Motorcycle Mechanics Project 60 mph MB5!

Part 4

So far when tuning our MB50, we have always taken the lesser of the options open to us, aiming for a series of gradual improvements rather than going for maximum power right from the start. For example, where it was possible to raise the exhaust port by 7mm, we only lifted it 5mm. Similarly, we were pretty conservative with alterations to the transfer and inlet ports.

But now there is no going back! And of course, weve been criticised quite heavily by a couple of organisations and not a few individuals as well. But at no time have we told people that they should alter their engines and we have been at pains to point out the legal complications involved in so doing.

Anyway from the readers letters we have recieved, it's obvious that the thing you are interested in is the performance so here goes....

You may remember that we bored the standard carburettor out from 14mm to 17mm and then altered the inlet manifold to match. just too late for this final part of the story, we found out that the Kawasaki AR80 has an 18mm carb. Fitting this would be a worthwhile experiment for anyone who has the time and the inclination.

we have raised the exhaust port by a further 2mm, giving a total increase of 7mm over the standard height. we've also cleaned up the rest of the port, carving away some of the metal to give a smoother flow of burnt gases from the combustion chamber. The tops of the transfer ports have now been raised by a further 1mm, giving a total height increase over standard of 2.5mm.

Early on in this series, we achieved a very significant gain in performance by cutting away the extended primary pipe in the exhaust header pipe (Mechanics Feb 3rd -16th 1982). Since then, we have tried out some aftermarket expansion boxes. true some of the chambers we tried made the bike sound more like a racer, but that was all they did. They did nothing for the performance. In the end, we decided that the modified standard system gave the best engine performance in terms of flexibility and exhaust outlet noise.

We did make one mistake though. a photogragh in the march 17th - 30th issue of Mechanics showed that we were using Hermetite engine lacquer to paint the exhaust. we were indebted to Hermetite for pointing out that this piant only has a service temperature of 150 degrees centigrade-too low for the heat of the pipe it was used on. We should have used Hermetite Pot Black which is now available to withstand temperatures of up to 450 degrees centigrade.

One item we have changed is the cylinder head. Our MB now sports a service exchange head from Haywood Power. This has given us a modest increase in compression ratio and, used in conjunction with the modifications we have already listed, it seems to be a worthwhile investment at £9 including postage and packing plus your old head.

The main effect of increasing the secondary compression ratio is the increased mep (mean effective pressure) of the engine which is perhaps more useful than an increase in absolute power at peak revs. It's not much use on a road bike to have a 65mph top speed if the power band has to be cut to a ridiculously narrow limits to achieve it. The thing would be almost unrideable in traffic.

Our final maximum speed came out at 50.5mph with the rev counter well over the 10,000rpm mark. we have already fitted a 14 tooth engine sprocket (two more teeth than standard) but it seems that the machine could pull even taller gearing quite happily. Although it would not be quite so easy to ride as it is now.

We reckon that the state of tune we have now reached with our MB is just about right for road use. There is undoubtedly more power available from the engine but only at the cost of flexibility.

One big criticism that was levelled at Mechanics was that the standard frame and chassis parts would not be able to cope with the increased power. Well, we're happy to be able to say that they were wrong. The frame and suspension work as well if not better at 50mph than they did at 30mph. The same is true of the brakes. If anything, the MB in restricted form was overbraked, now it is just right.

Well that's about it. If you want to be the next owner of our MB50, cut out the coupon below and see the next issue of Mechanics.

No don't cut out the coupon! Don't forget this is an extract from a 1982 magazine article!

Seriously does anybody know what became of this unique MB5?

If you have any details email me via the contacts page

This a letter taken from the same issue, does anybody have any details of the Dutch de-restriction kit? What items were included in this kit?

email me if you have any details.