carb jets

[-Ali-]

ok when people change their exhaust on 50cc from standard some of the companies supply lighter rollers and clutch springs yet i was led to believe that when you did this you also increased the size of the jet. does them not supplying it with the pipe mean it isn't needed or that you just need to spend more money?. also above the transmission cover there is a black box is that where the filter is? what does it filter?

[divuk83]

The black box above the transmission cover is the air box which contains the air filter.
Normally its best to put a bigger main jet in when changing to a sports pipe as they often run leaner. You can sometimes get away without changing the jetting, but I would be careful and keep and eye on the colour of the spark plug.

Dave

[bish777]

you can also raise the needle in the carb to richen the mixture, and try turning out (anti clockwise) your pilot screw a half or quarter turn.
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GSXR1100J

[TheShaggyDA]

Dunno what carb you have, but I thought loosening the pilot screw would have made it leaner, being an air bleed. You'll want to check before doing that.

A basic way of ensuring the mix is right (for idle, at least) is by adjusting your idle screw so that you have a fast, even tickover, then adjusting the pilot screw until you get maximum revs, then reduce the idle a bit, and readjust the pilot again. Once you are getting the best revs for that, you can reduce your idle back down to normal again.

A quick way to see if your main jet isn't big enough is to ride somewhere (safe) at full bore, then put the choke on. As this enriches the mixture, if you get an increase in speed, then your main jet isn't big enough.

The way to establish it certainly though, is a plug chop. Since I'm guessing you are on a scooter and therefore don't have a clutch, this may not be easy, but you can give it a try. Again, find somewhere safe for this, ride full bore for about 30 seconds, then cut the ignition (pull in the clutch too, if you have one), throttle off and coast to a stop. Now you check the colour of the spark plug's insulator. If it is white, then it's had all its deposits burnt off, and is running lean. If it is black and sooty, it's running rich. If it is a grey or chocolatey tan colour, it's just right. Since full bore running is affected by the main jet, this is what you have to upgrade.

For checking the needle position, you do the same as above, but at half throttle. If it is running lean, you want to raise the needle (moving the clip down a notch), if rich, lower the needle. If it's ok, leave it alone
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