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How can I improve the performance of my scooter via the carburettor?

January 19th, 2010

This little machine is probably more complicated than the rest of your engine, if you really want to get technical, but it doesn’t have to be the cause of so much hair pulling.

The simplest feature of your average carburettor is the idle speed control which usually comprises of a big knob situated on the side of most standard carbs. Most of you will know what this is as the idle speed sometimes needs a bit of adjustment on any bike. Put simply, the idle screw lifts what’s called the ‘throttle slide’ slightly in the carb in pretty much the same way that the throttle does but in very small increments. Engine RPM is raised the more the screw is wound in and lowered as the screw is wound out. This doesn’t really cause any problems, and if the idle speed is set incorrectly it’s easy to rectify. The idle fuel mix is where things start to get a little complicated.

It’s relatively easy to deduce that the idle mix is wrong when you quickly twist the throttle from idle to full. If the engine bogs out at all or labours for a second before it revs, then something’s wrong. In which case wind the idle screw in half a turn and try again, if this makes it worse wind it back and past where it was originally. Set and do another half turn. One direction will make the bike run better at initial pull off so keep winding the idle screw that way until all seems right. Remember that as you twist the throttle, the carb lets in more air. And inside the carb the needle lifts as you twist the throttle, this needle is adjustable and can be lifted or dropped by moving a clip up or down at the top of the needle shaft. This clip is mounted in the throttle slide and it’s basically a kind of stopper. If engine revs start to rise or the engine struggles to sit at a constant RPM when it’s ticking over and at a standstill, then it’s running lean part throttle. This means that the needle needs to be re-positioned. This is a case of moving the aforementioned clip down a notch. Similarly, the needle must be dropped if the engine struggles to rev at all, there’s a strong smell of fuel or the engine pops or backfires.

The ‘main jet’ meters full throttle only so it will only affect the engine when it’s running flat out. This affects only the throttle position and the running speed of the engine. If the carb is running too rich, the engine tone will deepen when you go to full throttle. Spark plugs will blacken quickly, the engine will return very poor fuel economy and appear as though it’s going faster if you ease off the throttle very slightly. So in terms of jetting, go down in size one at a time until this annoying little habit goes away. If the engine is running lean, you’ll hear the engine tone soften up although it still revs, it may sound very flat and can also just die out if it’s running really lean. The best thing to do is make it run richer by installing a bigger main jet than you really need and work down from there.

This is a VERY simple guide intended to help you understand the carb as a unit, not a definitive guide to carbs by any means, it usually takes a thick book to do that and there’s plenty of literature about covering this subject in a lot more detail. But with a bit of luck, I may have got those brain cells of yours firing a bit!

Words: Graham Gaymer

This article was originally published in the print version of PETROL ISSUE.04