Here's what Scott Summers and Fred Bramblett (Scott's mechanic)
do to their XR600R's to make them more competitive in closed
course competition. Some of these tips can be applied to other
model XR's as well.
| 1. |
To increase intake airflow, Scott removes the airbox snorkel and uses
an aftermarket air filter. Scott recommends not removing the backfire screen
from the air filter cage because it changes the intake air velocity and
causes the bike to have poor throttle response at low rpm. |
| 2. |
To increase exhaust flow, Scott replaces the stock muffler with an aftermarket
unit with an open end-cap for closed course competition. Scott advises
against using an open end-cap for trail riding, as it is too loud for use
on public land. He also recommends sealing the muffler to the header junction
with high temp sealant. Scott uses Hondaline Hondabond HT Sealant. |
| 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 weak. |
| 4. |
With the above modifications, Scott uses the following jetting: |
|
|
Stock |
Modified |
| Main: |
152 |
155 |
| Pilot: |
62 |
68 |
| Airscrew: |
2½ 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 his race bike run cooler and extend the clutch life, Scott mounts
an XR250R oil cooler to the steering head and removes the headlight for
more airflow. |
| 6. |
For better cooling Scott sometimes wraps the header pipes with exhaust
tape made by Thermo-Tech to keep heat from being transferred back into
the cylinder and head by airflow. |
| 7. |
On non-muddy terrain, Scott trims 4"-5" off the back of the
front fender for better cylinder cooling. |
| 8. |
Scott recommends against the use of plate-type skid plates. He feels
they restrict air flow and trap engine heat. |
| 9. |
Scott recommends good pump gas with an octane rating of at least 91,
and higher if you have a heavily modified high compression engine. Scott
runs a stock engine because the increased compression braking throws his
timing off. |
| 10. |
Scott recommends keeping your valves and decompression system adjusted
often and properly. This is extremely important for any four-stroke dirt
bike. |
