Hi, I'm Jeff Slavens with Slavens racing. This video is about pilot circuit on two-stroke
carburetors. Today's subject is a 2010 KTM 300, but this
applies to all two-stroke carburetors. So, what is the pilot circuit? On two start carburetors there's usually a
brass ear screw on the left side of the carburetor sometimes it can be on the right, it's usually
on the left. And that brass air screw is connected to the
pilot jet. This is what a pilot jet looks like and it
comes up to the bottom of the carburetor. If you remove this float bolt here, this float
bolt drain plug, there you'll see the hex headed main jet and right next to it in a
cavity is the pilot jet, you remove that with a flathead screwdriver. That connects directly to this hole back here
and this screw opens and closes that hole. This is called the pilot air screw. This is the pilot jet, if you have an idle
issue this is the idle screw which controls the location of the slide. So, how far up or down the slide is, so if
your idols too high or too fast or too slow, well then you would adjust this accordingly. And you should have that adjusted to where
the idles fairly slow for this test. So, from there what does the pilot circuit
do? It controls the lower rpms of the bike from
about idle just a little way above idle, quarter we'll say 1/8 throttle. From there the slide covers the next little
section then the needle the needle covers about three-quarters of the power band, then
the main jet covers three-quarters to wide open throttle. This just covers low RPM and the reason I'm
covering this today is because every year in the fall when the temperatures drop I get
a lot of emails and phone calls that people complaining that their bike is bogging at
low rpm and a pilot circuit is the problem. When the weather gets cool the air gets denser
and that makes the jetting leaner. So, at that point, you have to compensate
for that. So, how would you know if you have a problem
with that? First, you have to warm up your bike. So, get on your helmet, your gloves, your
boots, goggles, and run it up and down in your riding area on your on a dirt road or
whatever and get your bike warmed up. I do not recommend starting a two-stroke up
and letting it idle to warm it up, that just loads up the power valve the pipe and the
muffler full of goo. So, it's best to get on all your gear, start
it up, take off as you're accelerating a little in first or second gear, reach down and shut
the choke off and continue on. Then ride it easily first until it warms up
some, then you need to run it through the gears and rev it out and blow it out real
well before you do this test. Once you've done that come back, park it,
put it into neutral let it idle and grab a handful of throttle as quick as you can. If it hesitates or bogs then you have an issue. So, now I'm going to start the bike scare
the heck out of the cat here. This bike has been sitting quite a bit because
it's wintertime in Colorado I have been riding very much, so it might start a little hard
because the battery's low. But, start I’ll start it up and let you
know what sounds like when they're lean. I've backed out the air screw on this bike
for this test. That's what they sound like when they are
too lean, a lean condition on the pilot jet can blow can blow up an engine just as easily
as a lean main jet or lean needle position. That could quite often cause a problem when
you're going down a long downhill or if you've been running down a straightaway and then
you chop the throttle. Right when you chop the throttle, it’ll
go instantly onto the pilot circuit and at that time is easy to lock up the motor. So, if you've got a lean condition on the
pilot circuit you need to attend to it immediately. You can turn that pilot air screw in all the
way if necessary typical settings are from about 3/4 from 3/4 of a turn out to about
2 and a half turns out if it's in that range that's fine. If it's beyond that range then you need to
go to if it's you have to run it a long ways in say less than 3/4 of a turn and you would
need to go to a bigger pilot jet. If you need to run it farther than two and
a half turns out then you need to go to a smaller pilot jet. Those are all the clues and methods for testing
the pilot circuitry. Thank you.