No, you can "mess" with it. There are two screws, so to speak, that will effect your idle. One is an idle speed screw, the other is a fuel / air ratio valve, sometimes referred to as a slow speed mixing screw or slow speed needle valve.
The idle screw can be turned in or out to adjust your RPM's without turning in so much as to cause the gears to grind or slam into gear when you shift. That can happen if you raise the RPM's too high. If you do, just back off on the idle screw is all. A little turn makes a big difference on the RPM's. Too slow, turn it in. Too fast, turn it out.
* DO NOT RUN THE MOTOR WITHOUT HAVING IT ON MUFFS OR IN A TUB OF WATER!!!!
The slow speed mixture screw goes into the side of the carburator. It usually has a spring that you can see around it for tension so it does not vibrate in or out. (On some motors, the spring is inside of the carb body & you cannot see it) Turn this screw (valve) IN all the way, but once you feel resistance, stop turning it in, You do not want to turn it in real tight so you do not damage the point on the valve (screw). COUNT how many turns you turned it in so you can always put it back where you started from if you choose to.
Once you get the screw in all the way so it is just seated on the valve seat (inside the carb), turn this screw out 1 & 1/4 turns. Start the engine and let it warm up. Put it into forward gear to adjust the slow speed screw (slow speed needle valve). Now, turn that screw COUNTERCLOCKWISE 1/8 turn at a time, giving the motor a few seconds to re-act to that adjustment. Keep turning it out 1/8 turn at a time until it begins to stall at which point you will turn it back the other way that last 1/8 turn. *Give the motor time to re-act to every 1/8 turn.
Now, turn it CLOCKWISE 1/8 turn at a time, again giving the motor time to re-act to the adjustment, then keep turning it 1/8 turn until it begins to stall, then turn it back that last 1/8 turn. Between the two points where the motor began to stall is your happy medium "honey spot". Then adjust the idle speed screw to change your RPM's that will let the motor idle at a good idle speed so you can shift gears with no grinding or banging of the gears. (around 800-900 RPM's or so).
NOTE: You can remove the fuel / air mixture screw (slow speed adjustment valve) all the way out if you want to, and spray some carb cleaner with the nozzle on the can into the hole where that screws goes. This will get rid of some crud in there that may be plugging things up. Spray it with carb cleaner, then some air from a compressor (not too much pressure) or canned air that you would use on your keyboard sold at Staples or Office Max). I use the canned air as it is drier than the compressed air from your compressor, but either one will work. Then put the mixture screw back in, starting at 1 & 1/4 turns out from being fully seated. Watch a few videos first to get comfortable with things.
