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FUEL’S GOLD

Getting the fuelling right is essential on any tuned bike. But howcan you sort it? We check out the options – and the problems with the latest bikes.

FETTLED FUELLING

Thar be some injectors under yonder assembly! Yarg!

Power Commander and a pipe. They go together like pie and pint, vodka and coke, wheelie and stoppie. For almost the entire 21st century so far, it’s been the basic level of bike tuning. The pipe – a full exhaust system ideally, a slip-on can if your other half has restricted the bike spending this month – will save a load of weight, make you look and sound like an utter MotoGP hero, and add on plenty of ponies.

And the Power Commander? Well, it’s there to sort the fuelling, or smooth out the flat spots, or add an extra power band. Or something. Isn’t it? And didn’t your mate say something about having his bike ‘flashed’ instead of fitting a Power Commander? Is that the best thing now?

It’s fair to say that some folk are a little confused about what an ECU flash, a Power Commander, or other fuel injection tuning kit, is all about. Luckily, we’re here to help. Even more luckily, we’ve actually gone and asked some grown-ups all about it…

What’s it all about, then?

So – the basics. Bike engines need petrol and air, plus a spark and some compression to make them go vroom. The lovely folks at Honda, Yamaha, Triumph, Ducati et al, make it their life’s work to provide us with gorgeously-fuelled motos, purring away under us, consuming the unleaded and emitting fumes that are less noxious than a sparrow’s fart. All’s well with the world, and all you need to do is keep putting in the Optimax, twisting the throttle, and perfecting your wheelies on the ring road.

But the minute you alter anything on the engine – the aforementioned pipe, say, or maybe a flash race air filter, different camshafts, inlet trumpets, altered airbox – the factory fuelling won’t be spot-on any more. So you’ll need to work out some way of changing how much fuel is fed into the engine, and at what times.

In the past that was dead easy. Carburetted bikes used small brass ‘jets’ – accuratelymade orifices which let a precise amount of petrol flow through them. As you opened the throttle more jets were opened, and a needle moved out of a large jet, allowing more fuel to flow as the engine hit full throttle. Change these jets for ones with physically larger holes, and you let more fuel in. Smaller holes let less in. A differently-shaped needle will alter how the rate of fuel changes through the rev range. Simple.

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