10 Go-Fast Tricks
One strong thumper pilot.
Here's what Scott Summers and his mechanic, Fred Bramblett, do to his
XR600Rs to make them more competitive. These ten tips can be applied to
any production thumper, but the jetting specs are for recent-model XR600s
1. To increase airflow, remove the airbox lid
and use an aftermarket air filter. Do not remove the backfire screen from
the cage. Removing the screen changes the intake velocity and causes the
bike to have poor throttle response at low rpm.
2. To increase exhaust flow, Scott
replaces the stock muffler with a White Bros. Megalloy system, with an
open end-cap for closed-course competition. Scott says, "Please don't use
the open end-cap for trail riding, as it is way too loud to use on public
land." He also recommends sealing the muffler to header junction with high-temp
sealant - they use Hondaline Hondabond H.T.
3. Check the header pipe where it mounts
to the exhaust manifold. Sometimes the welds are overdone, and the extra
material can restrict airflow. Grind away the protruding material to increase
flow, but don't overdo it to the point of making the area weaker.(br>
4. With these mods, Scott uses the
Jet Stock Mod
Main 152 155
Pilot 62 68
Airscrew 2.5 turns out
Scott says, "Most four-strokes come jetted on the lean side and must be
richened when you increase intake and exhaust flow."
5. To make the race bike run cooler
and extend clutch life, they mount an XR250R oil cooler to the steering
head and remove the headlight for more airflow.
6. For better cooling they sometimes
wrap the header pipes with exhaust wrap made by Thermo-Tech to keep heat
from being transfered back into the cylinder and head by airflow.
7. If a course or trail isn't going
to be too muddy, they trim 4"-5" off the back of the front fender to allow
more cooling air to reach the cylinder.
8. Scott recommends against the use
of plate-type skidplates. You got it, he feels they restrict air flow and
trap engine heat.
9. Run good pump gas with an octane
rating of 91 unless you have a modified high compression engine. Scott
runs a stock engine because "the increased compression braking throws off
10. Keep your valves and valve decompression
systems adjusted properly! This is extremely important for any thumper.
Last Updated Feb 7, 1996