Originally Posted by
MiddleAgedBiker
Before diving in to tune the carbs, you first need to understand what each circuit in the carb does, and which screws control which circuit. Then you need to know what kind of carb is on your bike, CV or slide? When you read instructionals on the web, you must also know, what carbs are they meant for, CV or slide?
You also need an understanding of what circuits come into play at what throttle openings, you need this to do proper troubleshooting.
E.g. idle mixture ciruit controls 0 - 1/4 throttle opening, pilot jet roughly the same, needle profile perhaps 1/4-3/4 opening, main jet 3/4 - WOT, etc.
And most importantly, you must record the factory setting (or at least the setting on the bike when everything ran properly) before you fiddle with anything.
E.g. how many turns out was the fuel-mixture screw, originally?
Now to answer your question, assuming your bike has a CV carb, you'll need an accurate tach with graduations down to 50rpms to do this correctly. You need to hook this tach up to your bike. If you don't have this tach, try to do it by ear.
Set the idling screw so your bike idles at what your factory manual recommends. Most bikes are set at around 1,000rpms. Turn the bike off, then carefully turn the idle mixture screw all the way in (clockwise) till it seats and will turn no more. Count the number of turns and write it down. Most carbs come from the factory 1.5 turns out. You'll need a stubby flathead screwdriver, plus heat-resistant gloves to do this properly, or you might get burnt.
Unscrew the mixture screw back out to the original setting, let's assume it's 1.5 turns out. Now start the bike. Write down the RPM registering on your tach. Next slowly turn the screw in, 1/4 turns at a time (record this somewhere, you will forget) until your RPMs start to drop off about 50rpms. Record the number of turns you've turned it in.
Now turn it back out to the original setting (1.5 turns out). Now do the same thing as before, but turn the screw out, until the RPM drops 50RPM. Then record the number of turns out from factory setting, and write this down.
The correct setting will be the two extreme settings divided by two. E.g. if RPMs dropped at 1/2 turns out, and 2-1/2 turns out, (0.5 + 2.5) / 2 = 1.5 turns out. This would be your ideal fuel mixture setting.
If you don't have an accurate tach and don't want to buy one, you can try listening for when the idling speed drop happens. However, from personal experience, it's very hard to catch this by ear, because the change is so slight and it happens so progressively.
This takes time, and it will heat your engine up, so you need some kind of fan or something to blow your radiators when you're doing this.
Additionally, if you've not done any modifications to your bike (exhaust + air cleaner, engine porting, etc), just set the thing to the factory recommended setting. Honestly, the potential to muck this up is far greater than any kind of practical improvements you're going to feel.
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