Carburetor Adjustments & Tuning Tips
Adjustments for Competition 127
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 CRF. If
larger jetting changes are necessary, check for air
leaks, blocked or restricted exhaust or fuel
systems, or a dirty air cleaner.
Condition
long straights
uphill sections
sand
mud
high humidity
raining
hotter than 113 °F (45 °C)
Main Jet Adjustment
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One 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.
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 • Go richer on the main jet, by one
number, when: the track has a very
long straightaway or uphill section,
a high percentage of sand, or the
track is muddy.
• Go leaner on the main jet, by one
number, when: it is very humid or
raining, or it is hotter than 113
degrees F.
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 CRF. 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.
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
• 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 151 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.
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