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Adjustments for Altitude & Temperature
All jetting recommendations are based on
standard jetting with an unmodified engine. The
following conditions can affect the fuel mixture.
Adjust to
Condition
Mixture will be
Richen
Cold temperature
Lean
Lean
Warm temperature
Rich
Richen
Dry air
Lean
Lean
High humidity
Rich
Lean
High altitude
Rich
Special Tuning Conditions
There are some unique atmospheric conditions or
situations that may require additional
adjustments.
It should not be necessary to go more than one
jet size richer or leaner to fine tune your CR. If
larger jetting changes are necessary, check for air
leaks, blocked or restricted exhaust or fuel
systems, or a dirty air cleaner.
Condition
Main Jet Adjustment
long straights
next larger
uphill sections
sand
mud
high humidity
next smaller
raining
hotter than 113 °F (45 °C)
Once you've adjusted the carburetor for
temperature and altitude, it shouldn't need major
readjustment unless race conditions change
drastically. However, there are some unique
atmospheric conditions or race day situations that
may require additional adjustments. They are as
follows.
Component affected
To prevent engine damage, always adjust the
main jet before adjusting the jet needle. Always
choose the next richer jet if there is any doubt so
there is a margin for safety.
Main jet
(jet needle stage)
Main Jet • Go richer on the main jet, by one
• Go leaner on the main jet, by one
After using the chart, and making any
adjustments for special conditions, it shouldn't be
necessary to go more than one jet size richer or
leaner to fine tune your CR. If larger jetting
changes are necessary, check for worn crankshaft
seals, air leaks, blocked or restricted exhaust or
fuel systems, or a dirty air cleaner.
Carburetor Adjustments & Tuning Tips
number, when: the track has a very
long straightaway or uphill section,
a high percentage of sand, or the
track is muddy.
number, when: it is very humid or
raining, or it is hotter than 113
degrees F.
Pre-Adjustment Checks
Before adjusting carburetor settings, check the
following:
• air cleaner
• air leaks
• float level
• clogged carburetor jets
• spark plug fouling (improper heat range or
other cause)
• freshness of fuel and fuel/oil ratio
• owner modifications (such as exhaust system,
holes in the air box, etc.)
• ignition timing
• compression
If the above check out, then, adjust the carburetor
for your specific racing conditions. Engine
response and appearance of the firing end of a
spark plug are highly indicative of the engine
condition.
Refer to the list on page 131 for the optional
carburetor parts.
To prevent engine damage, always adjust the
main jet before adjusting the jet needle. If there
is any doubt — always choose the next richer jet
so there is a margin for safety.
Adjustments for Competition
105
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