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The carb on a DRZ 125 or KLX 125 comes pre-set from the factory with a
very lean tuning. At first it may run well but once you get some time on
it with a little build up and some dirt the lean setting gets even
worse. To correct this you need to thoroughly clean the carb ,
specifically the fuel screw. Remove the carb from the intake and airbox
rubbers and take off the slide cap. Drain the carb from all fuel. The
fuel screw is in a recessed tube located on the bottom of the carb on
the front side towards the cylinder. It may ahve a brass plug covering
the screw. This is done at the factory to prevent tampering for
emissions purposes. The plug can be removed by taking a very small drill
bit 1/8" or so and very carefully drilling a hole in it. Put a small
sheet metal screw in the hole and pull with pliers and it should come
out. Make a note of the position of the screw and slowly turn it
clockwise in to see what the setting is. Do not apply heavy pressure
when you feel it getting tight stop. It is a soft brass screw with a
fine point an is easy to damage. Typically the factory settings range
from 1 1/2 to 2 turns out. This is way too lean for the bike to idle and
run well or even start without the choke. back the screw out and remove
the spring and O ring. Using carb cleaner with the hose attached insert
it into the hole you just removed the crew from and spray it out. You
should have a good stream of spray into the center bore of the carb.
Re-insert the O ring and then the spring and fuel screw. turn it in
until you feel it making contact and then note the position and back it
out 3 1/2 turns. Put the carb back on and it should be able to start and
run without the choke. Some bikes will still need a bit more tuning.
The older carbs had a slide needle that had 3 to 5 positions on the
needle that could be adjusted with the movable clip. Since this does not
have that option you could go to a Yamaha dealer and buy the needle
from a TTR 125 and install it as it is the same carb. This will cost you
around $15.00. Or you could go to a hardware store and buy some 3/0
stainless washers and remove the needle and put 2 or 3 under it to shim
the needle up just like the clip on an adjustable needle. You may spend
$1.00 with this option. The bike should run better than new.
Turn the air screw and throttle screw all the way inward. IMPORTANT > do not tighten these two adjusters down. Only screw these in until they LIGHTLY seat. Now turn each adjuster one and one half turns outward. The carb is now set back to factory specs assuming no one has changed out the main jet or played with the float level.. Please rate this solution. Thanks!
i'm not too familiar with dirtbikes but my experience with street bikes it has been on the underside of the carbs and if stock it is typically covered by a brass plug. this plug is just there to satisfy the epa you need to drill it out but be careful the screw is directly underneath the plug so as soon as you break thru pull back then use a screw to extract the plug. hope this is the answer you are looking for.