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.
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 had been redesigned slightly on the '02 bike, and has a 6.5 slide (the '99 has
a 6.0 slide). The 6.5 slide is 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) get 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 has not
been found since and the engine runs flawlessly. But buying race gas and mixing it 50/50
with 93-octane pump gas is a pain the @ss.
Update January 2004
No More Race Gas!!!
Awhile back 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
KTM has trouble meeting its manufacturing tolerances, which can cause the problems I was
having with excessive engine knock. Clay's small machine shop, C&M Machine, does 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 appears to be part of my past.
