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, does
(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.
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