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2-Stroke Carburetor Set-up Technique
(By Fabio Dougie)


2-Stroke Carburetor Set-up


On a stock 2-stroke carb there are three main ways to adjust the carb settings/mixture.

To do this well you have to understand what parts of the carb do what but the general gist of it is that 3 adjustments cover 3 parts of the rev range.

- From idle to about 1/4 throttle you are mainly looking at adjustments to the idle mixture screw
- From 1/4 throttle to about 3/4 throttle the needle position comes into play.
- From 3/4 to Wide Open Throttle (WOT) the main jet size is the one to look at.

So here's the way to set it up.........


1) Main jet
Tackle the Main Jet first - do some Plug Chops. This means to run the scooter at WOT for a few blocks and then cut the engine dead, remove the spark plug and have a look at its condition. If its black or/and overly oily then its too rich, if its pale and whiteish then its too lean, you're looking for a medium brownish colour (creamy coffee colour) but if its a tad rich (slightly darker brown) then thats ok too because running too lean will risk seizing your engine.



Here's a link with more detailed instructions...

http://www.kawtriple.com/mraxl/carb/plugchop.htm




When you see the results of your plug chop then its time to swap out the main jet for one either bigger or smaller, go bigger if you want the carb to be richer, smaller if you want it to be leaner. The main jet can be found in the float bowl at the bottom of the carb. Then when you've swapped the main jet try plug chopping again and assess the colour/condition again. You'll have to do this main jet swapping on a trial and error basis until the colour of the plug is correct when its inspected. The higher the number of jet, the richer it is. It is best when setting the main jet to start rich, and come down one step at a time to get it correct.




You'll need to undo 2 screws to get at the carb (one fastening the carb to the inlet manifold, the other fastening it to the airfilter box though this screw may be replaced by a cable tie on some scoots), then turn it up side down and remove another 2 screws holding on the carb bowl. A splash of petrol will likely come out but its not too much, just put some paper towels down over the engine first, and also make sure that the engine isn't red hot.

You will see 3 brass jets. The Main jet is indicated in the photo.




2) Idle Mixture
When you're happy with the main jet then go onto the idle mixture setting, you need to be able to set this so that the engine idles correctly for a long period without stalling and also without the engine revs rising and dropping. Roughly speaking if your scooter revs up and down on its own it will be weak, if the scooter ticks over unevenly and lumpy it will be too rich. It's easy to diagnose that the idle mix is wrong when you quickly twist the throttle from idle to full. If the engine bogs or labours for a second before it revs, then wind the idle screw in half a turn and try again, if this makes it worse wind it back and go past where it was originally. Pause here and then 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.



To start off..... I'd look to see if the slow idle screw is set correctly as per the manual - its usually around 1.5 full turns from fully closed on many scooters but check your manual to find what's right for your carb, and starting to diagnose the air/fuel mixture from this idle setting will cure many problems. All you need to do is to locate the slow idle screw and turn it fully closed (whilst at the same time counting how many full turns so that you can turn it back if you want to) and then open it 1.5 full turns and then try that.


3) Needle Clip
The 3rd setting is the needle clip setting....As you twist the throttle, the carb lets in more fuel by moving the needle in/out of the bore of the main jet, 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. The clip is mounted in the throttle slide and it's basically a kind of stopper.



Moving the clip up or down a notch can affect the richness of the mixture over the 1/4 to 3/4 throttle range. You could tell while running, if your scooter feels as if it has a flat spot on and around mid range, or it does not start to perform until the throttle is fully open, it will be too weak. If too rich the scooter will splutter at this position, but it will probably clear as you go onto full throttle. The needle must be dropped if the engine struggles to rev at all, if there's a strong smell of fuel or the engine pops or backfires.


And finally......
The larger thumb-wheel type of knob on the carb is the idle speed setting and this is where you can set the revs at which the engine ticks over. If its racing turn it down a touch, if its stalling a bit then screw it in a bit to raise the revs slightly.

On your first outing with a new carb set up its best to start of at a slower pace, and gradually increase speed once you are happy that everything is running fine. If the scooter feels wrong in any throttle position, stop and adjust your settings until it becomes correct. Keep an eye on your spark plug colour until you are totally happy it is the correct colour and all is fine.

Some other thoughts - if the carb is running too rich, the engine tone will deepen when you go to full throttle, it will possibly bog down too. If the running lean, you'll hear the engine tone soften up although still revving it may sound very flat and can also just die out too

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Last Updated Sat Oct 06, 2018 10:05 am
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