How to Adjust the Carburetor on a 2000 Yamaha Big Bear
by Chris GillilandThe Yamaha Big Bear debuted in 1996 as a 400 cc utility-type all-terrain vehicle that quickly became popular among game hunters. This fact prompted Yamaha to produce its first camouflaged ATV in 2000 with the Buckmaster Edition Big Bear. The Big Bear's engine was fed a by a single carburetor. The carburetor could be adjusted to alter the engine's idle speed as well as the amount of air mixed into the carburetor's pilot fuel system, which fed the engine from idle to a quarter-throttle. While the carburetor can be adjusted with a single screwdriver, an inductive tachometer ensures the engine is idling exactly where it needs to be.
Step 1
Park the ATV in a well-ventilated area and engage the parking brake. Crouch to the left of the ATV, near the left front fender. Reach around the fender to the engine cylinder head to locate the spark plug cable. Clamp an inductive tachometer over the spark plug cable.
Step 2
Crouch to the left of the ATV, near the left floorboard, and look into the engine bay to locate the carburetor. Turn the pilot air screw -- located on the bottom of the carburetor and directly below the engine intake flange -- clockwise with a flat-head screwdriver until it is lightly seated. Count and record the amount of turns needed to seat the screw. Ideally, the screw should take 2 1/2 turns to seat. Turn the screw counterclockwise 2 1/2 turns to reset the pilot air mixture.
Step 3
Start the engine and let it idle in place for three to five minutes to warm the engine to operating temperature. Do not stop the engine.
Step 4
Turn on the inductive tachometer. Ideally, the tachometer should indicate an engine idle speed between 1,450 and 1,550 rpm.
Step 5
Look for the black idle adjustment knob attached to a cable on the left side of the carburetor. Turn the adjustment knob clockwise to increase the idle speed, or counterclockwise to decrease the idle speed, as needed to bring the idle speed to between 1,450 and 1,550 rpm.
Step 6
Turn the pilot air screw counterclockwise a half-turn at a time, while watching the tachometer. Stop when the idle speed begins to increase, then turn the screw clockwise a quarter-turn. If the idle speed does not increase after more than two full turns, reset the screw to its 2 1/2 turn position; repeat this step and turn the screw clockwise instead of counterclockwise.
Step 7
Remove the inductive tachometer and release the parking brake. Mount the ATV and accelerate to a moderate speed from a standstill. The ATV should accelerate smoothly without hesitation. Stop and turn the pilot air screw clockwise a quarter turn, then repeat the test ride. Turn the screw another quarter turn if the hesitation does not improve.
Step 8
Park the ATV and stop the engine once the ATV smoothly accelerates from a complete standstill.
References
- Yamaha YFM400NM Service Manual; Yamaha Motor Corporation
- Yamaha-Motor.com: Epic Yamaha ATVs
Tips
- Take the time to locate the spark plug cable, idle adjustment knob and pilot air screw before warming your Big Bear's engine to avoid burning yourself on heated engine and exhaust parts.
Things You'll Need
- Inductive tachometer
- Flat-head screwdriver
Warnings
- Never adjust your Big Bear's idle without first warming the engine. Any adjustments made to a cold engine may cause the idle speed to increase beyond the specified 1,450 to 1,550 rpm range; this can make it difficult to shift gears and cause the engine to race at idle.
- Do not turn the pilot air screw counterclockwise more than 4 1/2 full turns from its seated position. Turning the screw past this point can allow the screw to vibrate loose and completely throw off the carburetor's air-and-fuel mixture.
Writer Bio
An avid motorcyclist, Chris Gilliland has immersed himself into the two-wheeled world while balancing work life and raising three daughters. When he is not managing the parts department of a local, multi-line motorcycle dealership, Gilliland can often be found riding, writing or working on his motorcycle blog, Wingman's Garage.
