Tech Torque: Building a Lambretta or Vespa Street Racer

While the essentials are a compulsory part of building a street racer, the extras are what make it stand out from the crowd…

Part 6 of our tech guide to building a Lambretta and Vespa Street Racer. This month: Rev counters, gauges, and finishing touches

Over the last five editions, we have gone through the important steps of building a true street racer, and the equipment and engineering required to make your project a successful one. This, the last part of this series, covers the things that finish it all off, including the fine details that make the difference between a street racer which is good to one which is exceptional. Some touches are to a certain extent essential items, whereas others are done purely for cosmetic purposes.

Rev counters

The majority of street racers tend to have a rev counter fitted and, in all honesty, any tuned two-stroke should have one. As both the Vespa and Lambretta fall into this category, then the same principle applies to both. The reason for needing one is simple. in that it allows you to know where exactly your engine is in relation to the power it is producing. Virtually every Vespa and Lambretta owner these days has their scooter put on a dyno. The problem is that many just look at the pretty little graph and see what is the highest BHP it has created. That’s fine, but what is more important is where the power band starts and where it finishes. Basically, what is commonly known as the ‘power spread’ or, in easier terminology, usable power’.

As an example, imagine an engine revs up to 9000 rpm. Looking at its dyno graph there will be a steep incline as the revs build up. The graph will then plateau out before tailing off towards its maximum revs. The period where it plateaus is where the engine is running at its most efficient. Just for argument's sake, let’s say between 6000 to 8000 rpm, which is known as the power spread. The power before and after

is far less, therefore if you keep the engine in the usable power area through each gear, then you will be accelerating as fast as is possible.

By using a graph and a rev counter you can translate this to your throttle. Without the rev counter, you have no way of knowing exactly where the revs are. What is the point of keeping the throttle open until it can't rev out anymore with the power fading, when you should have

changed up a gear at least 1000 rpm earlier? That is why a rev counter is so important on a tuned two-stroke engine, not because it looks good, but because it tells you how to run the engine at its most efficient. Now you know what the rev counter is for, which one should you fit? There are many types available and all work in different ways. Make sure the one you are fitting works with the ignition system you are using.

Above left: Pulse type rev counters measure each spark by a wire cable tied to the HT lead. They are compact and have their own power supply. Above right: The Scitsu rev counter is far easier to read, certainly under fast acceleration. It only has around 15 hours run time between each charge, which is its downfall for general use.

On this RB250 graph, it's easy to plot the power spread from around 6500 to 9200rpm. This gives over 2700 rpm of usable power, which can be controlled accurately via the throttle in conjuction with the rev counter.