Carb Talk
A Sticky Situation
About 6 months after I bought my 1999 KTM
300EXC, the throttle started sticking.  Imagine
this...riding through a tight trail, trying to use
just the right amount of precision and throttle
control, and suddenly the bike accelerates like
a bat out of hell and won't stop, even though
the throttle is off.  This happened after I had
removed the carburetor for cleaning and was
brought on by rough terrain, such as a series
of whoops.  With a quick full-on twist of the
throttle, the engine would usually return to its
normal rpm range, but that was only if I was
quick enough to pull in the clutch when the
engine began racing.  A couple of times I
crashed, but fortunately was never injured.

Over the last two years I have heard numerous
stories of guys with sticky throttle problems,
mostly on various KTM-related message
boards.  The threads are very predictable and
go something like this:

First guy: My throttle sticks.
Second guy: You've got dirt in your carb.  
Check your air filter and the air boot/carb
junction.
Third guy: Get a new throttle cable.
Fourth guy: Take apart the throttle tube and
clean it.
Fifth guy: Check the routing of the throttle
cable...it might be hung up on something.
First guy again: Did all that, still sticks.  
Broke 10 bones because of it last weekend.
Sixth guy (that's usually me): Listen
carefully....

Here's the real deal.  The Keihn PWK carb
spring has a little plastic piece on the end that
fits inside the slide.  There is only ONE WAY
that plastic piece seats itself in the slide.  Look
carefully and you'll see a small notch on the
plastic piece that has to be matched up with a
groove in the slide.  If you don't match the
notch with the groove, the spring won't be
seated far enough down in the slide.  The
spring will have a bow-shape, as if the spring is
too long and is compressing too much.  If you
put it back together like that, the spring being
out of shape will rub against the inside of the
carb as the slide moves up and down.  
Eventually it will cause the slide to stick and the
engine will rev to the moon, resulting in a scary
situation.  This explains why I could crank open
the throttle and get it to un-stick, and also
explains why it started happening after I had
taken the carb apart (incorrect assembly of the
spring).

So, if you're having a sticky throttle, CHECK
THIS FIRST!!  Stuck throttles can cause serious
injury.
I Gotta Use Race Gas?!?!?
The first rides on my 2002 300MXC were ripe
with pinging at the lower end of the throttle
range, which I assumed was caused by a lean
pilot jet. After my experiences with properly
jetting the '99 carb (came jetted extremely rich
from the factory), I was quite surprised that the
stock settings would be lean. I switched from a
42 to a 45 pilot, which lessened the pinging
somewhat but didn't cure it. Then I moved up
to a 48 pilot, and it still pinged. The mid- to
upper-throttle range jetting was spot-on, so I
did a little investigative research on the
discussion boards.

The PWK carb was redesigned slightly on the
'02 bike, with a 6.5 slide (the '99 had a 6.0
slide). The 6.5 slide was a good compromise
between the stock 6.0 slide on the '99 and the
7.0 slide to which many guys had switched in
an attempt to solve jetting issues. For whatever
reason, however, some people (like me) had
lots of pinging with the stock setup on the '02.
The spark plug gave me no reason to believe
that the 42 pilot was too lean, so I figured it
was time to try some race gas. Like Osama in
the desert, the pinging was not found since and
the engine ran flawlessly. But buying race gas
and mixing it 50/50 with 93-octane pump gas
was a pain the @ss.

Later in 2003, I stumbled across a thread on

KTMtalk.com
about a guy named Clay Wolfe
who had identified KTM's lack of quality control
in the engine manufacturing process.
Apparently back then, KTM had trouble meeting
its manufacturing tolerances, which may have
caused the problems I was having with
excessive engine knock. Clay's small machine
shop, C&M Machine, did (among other things)
engine work that is primarily "cleaning up" the
internals of the cylinder. A few different cylinder
packages are offered, from basic head work to
a full-on total cylinder overhaul.

I decided to start with a head work to see if the
squish band was the culprit. For $40, it was
money well spent. I tested the bike with pump
gas, and there was no evidence of knocking
with the same jetting I had been using with
race gas. It should be noted, however, that I
did go with a slightly thicker base gasket, which
may have also helped with the knocking. Either
way, race gas was part of my past.