Yamaha YZ250 2021 - Examples of Carburetor Setting Depending on Symptom; Road Condition and Examples of Carburetor Setting

ENGINE
8-6
EAM30601
ROAD CONDITION AND EXAMPLES OF CARBURETOR SETTING
TIP
Optimum pilot air screw setting can be obtained by adding the ex-factory number of the same screw
back-out turns to any required value provided in the chart. For example, if the ex-factory number is “1”,
add “1” to the value chosen in the chart.
EAM30602
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOM
General condition Sandy condition
Under 10 C
(50 F)
(Winter)
15–25 C
(59–77 F)
(Spring,
Autumn)
Over 25 C
(77 F)
(Summer)
Under 10 C
(50 F)
(Winter)
15–25 C
(59–77 F)
(Spring,
Autumn)
Over 25 C
(77 F)
(Summer)
Main jet #178 #178 #178 #182 #180 #178
Jet needle
N3CW-3 N3EW-2 N3EJ-2 N3CW-3 N3CW-3 N3CW-3
N3EW-3 N3CW-3 N3EW-2 N3CW-4 N3EW-3 N3EW-3
Pilot jet #50 #50 #50 #52 #52 #52
Pilot air screw-1/40000+1/4
Power jet #50 #50 #50 #50 #50 #50
Symptom Setting Checking
• At full throttle
Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug

Lean mixture
Increase main jet calibration no.
(Gradually)
• Discoloration of spark plug 
If tan color, it is in good condition.
• If cannot be corrected:
Clogged float valve seat
Clogged fuel hose
Clogged fuel cock
• At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug

Rich mixture
Decrease main jet calibration no.
(Gradually)
*In case of racing slight enrichment of
mixture reduces engine trouble.
• Discoloration of spark plug 
If tan color, it is in good condition.
• If cannot be corrected:
Clogged air filter
Fuel overflow from carburetor