check the air filter and exhaust ports. if clean then you need to adjust the high and low adjustments on the carb. (if you have them). you can google this. below is a copy and paste from a snowblower article which may be helpful.
Three external adjustments are possible on the carburetor; the high-speed needle, low-speed needle and idle stop screw. Ride the snowmobile for a short distance to fully warm-up the engine. Remove the top access panel and adjust the carburetor in the following manner:
Adjusting Low-Speed Needle
1. Shut off the engine.
2. Carefully turn the low-speed needle clockwise until lightly seated. Do not force.
3. Back needle out I turn (Walbro) or 1-3/4 turns (Bendix). Back needle out 1-1/8 turns on 800 Snowmobile.
4. Start engine.
5. On Walbro carburetors, turn low-speed needle clockwise or counterclockwise until smoothest running and highest rpm is obtained.
NOTE: On machines equipped with Walbro carburetors, keep the low-speed needle adjusted as lean (clockwise) as possible, while still providing smooth idling and unfaltering acceleration. This will reduce spark plug fouling.
6. On Bendix carburetors, turn low-speed needle clockwise slowly until engine falters (looses rpm). Then back needle out 1/4 turn. Readjust idle stop screw as necessary.
Adjusting Idle Stop Screw
The idle stop screw is adjusted only when a change in idle speed is required. Set the idle speed at 2200 to 2600 rpm. Generally, if the idle stop screw is adjusted, the low-speed needle will have to be readjusted.
Adjusting High-Speed Needle
1. Shut off the engine.
2. Carefully turn the high-speed needle clockwise until lightly seated. Do not force.
3. Back needle out (counterclockwise) 3/4 turn (Walbro) and 1-1/2 turns (Bendix). Back needle out 1-1/8 turns on 800 Snowmobile.
4. Start engine and take a highspeed trial run
5. Stop, turn high-speed needle 1/8 turn or less clockwise, and make another trial run.
6. Continue this procedure, 1/8 turn or less at a time, clockwise or counterclockwise, until optimum performance is obtained.
7. Turn needle 1/8 turn counterclockwise for final adjustment.
IMPORTANT: Too lean (clockwise) a high speed needle setting can seriously damage your engine. On the 800 Snowmobile DO NOT turn the high-speed needle (clockwise) LESS than I full turn open.
NOTE: High altitude (distance above sea level) requires a leaner carburetor setting. Use the following rule for leanest possible high-speed needle setting.
Above 5000 feet altitude (1524 meters) = 5/8 turn open (300), 1-turn open (400, 600 and 800)
kevin k., March 2008
