Disassembly and Cleaning of a Mikuni Round slide Carburetor

Introduction

Your carburetors are the critical link in the performance and reliability of your snowmobile. When your carburetors are clean and adjusted properly, your sled will be a rocket and at the same time won't leave you stranded. Keeping your carburetors clean is not difficult once you are familiar with their inner workings.

Carburetor Function

The carburetor is the device that meters and mixes the air and fuel before it enters the cylinder for burning. Not just any air-to-fuel ratio will work, it must be close to optimal for a snappy and responsive engine. There are 3 main adjustments on a carburetor for air-to-fuel ratio. These are the pilot system (pilot air jet and pilot jet) for idle to 1/4 throttle. The jet needle system for 1/4 to 3/4 throttle. And the main jet system for 3/4 to full throttle. All of these systems rely on an orifice of a certain diameter in order to meter fuel or air through them. You will see them up close in photos further on.

When the throttle slide is mostly closed, the jet needle is pushed down into the needle sleeve and it lets by very little fuel. At the end of the sleeve on the bottom is the main jet, so it is effectively shut off. Air and fuel are entering the motor via the vacuum created by the shut slide. The pressure in the carburetor body is lower than the pressure in the float bowl due to the venturi effect, and the fuel is pushed up through the pilot jet where it mixes with air and enters the intake tract on the motor to be burned. The pilot jet is always metering fuel, no matter the throttle position.

The jet needle is a tapered, needle-like device that rises and lowers in the needle sleeve with the slide valve. The needle is fat near the top, and thinner near the bottom. When the throttle is mostly shut, the fat section is in the sleeve and it allows little fuel by. When the throttle is opened, the needle rises and the space between the sleeve and the needle increases due to the taper, allowing more fuel by for the motor to consume. Also at this time the slide valve is rising, allowing more air, so the air-to-fuel ratio is relatively constant, and hopefully near optimum.

When the jet needle pulls all the way out of the sleeve (nearly full throttle), the only thing metering fuel now is the main jet. This is the final stage of fuel metering, and is also the most critical in keeping clean and adjusted properly. When a motor is at full throttle the demands on the mixture being safe are very important. With a lean (higher than optimum air-to-fuel ratio) condition, the motor's aluminum piston will reach its melting point, and begin to disintegrate. With a rich condition (low than optimum air-to-fuel ratio) the motor is quite safe from burndown, but also will run sluggish and be down on power.

Carburetors usually stay clean as long as they are taken care of properly, but many times they are forgotten about. Sludge in the bowls, clogged main jets, dirty pilot jets, and dirty sleeves are common in ignored carburetors. These are very dangerous conditions because they tend to lean out the mixture (plugging an orifice allows less fuel through), bringing the motor dangerously close to burndown.

What follows is a general overview of disassembling and cleaning a Mikuni round-slide type carburetor. These are common on 1990 to 1998 Polaris snowmobiles. While the exact carburetors may differ slightly, they will all have the same basic parts and construction. So let's start!

This image is the property and Copyright of Polaris Motor Co.

Step 1 - Overview of engine Compartment	1. Shut off the fuel valve. It will be located between the fuel tank and the fuel pump, often on the right side of the engine compartment when sitting on the seat. The carburetors are located between the motor and the airbox.
Step 2 - Airbox removed	2. Next, you will need to remove the airbox. There are two bolts, one on each side that need to be removed.
	3. Disconnect all lines going to the carburetors (except float bowl drain line). There should be 2 vent lines per carburetor and a fuel line.
Step 4 - Slide and jet needle	4. Unscrew and remove the cap. Now, pull up and the slide and jet needle should come out.
Step 5 - Choke plunger assembly	5. Unscrew and remove the choke plunger and cable. You may now clean the choke plunger and spring assembly, taking care not to lose any pieces!
	6. Loosen the clamp on the rubber flange to the carburetor. This should be a Philips headed screw on the clamp.
	7. You may now pull the carburetor off.
	8. Tip carburetor upside down and shake gently. The gasoline still in the float bowls will drain out the vents. Do this over a bucket to catch the gasoline.
	9. Take carburetor to a comfortable working area.

Step 1 - Cleaning pilot jet	1. Carefully stick a fine wire through the pilot jet from the bottom until it emerges from the top. Now twist around to clean. Spray carburetor cleaner in the hole while twisting the wire to get it completely clean. Remove the wire and set aside.
	2. Spray through the pilot air jet hole. Using carburetor cleaner, clean out the pilot air jet hole.
	3. Spray and wipe off the pilot air jet and the associated spring.
	4. Using the same technique as for the pilot jet, clean the main jet.
	5. Spray out the needle sleeve with carburetor cleaner.
	6. Inspect float bowl. If there is any slime, goo, or crud in it spray a liberal amount of carburetor cleaner in it, then scrub it until it is clean with a toothbrush. Then do a final rinse with more carburetor cleaner. Also clean the float bowl seal.
	7. Clean the sealing area on the carburetor body that the float bowl seals up to.

	1. Screw back in main jet and carrier. Not too tight, just snug.
	2. Screw back in the pilot air jet with spring. Screw it all the way in until it becomes snug, then unscrew it the amount recorded in disassembly step 4.
	3. Line float bowl up and screw on. Torque bolts in a X pattern until they are snug, do not over tighten. X pattern means alternating bolts so that the bowl is tightened evenly over the seal area.
	4. Bring carburetor back out to the snowmobile.
	5. Insert the carburetor back into the carburetor flange on the motor. Be sure the carburetor is perfectly vertical!
	6. Tighten clamp on the rubber flange to the carburetor. This should be a Philips headed screw on the clamp.
	7. Screw back in the choke plunger and cable. Tighten it down snugly. Make sure the plunger stays attached to the cable correctly.
	8. Slide in the slide valve assembly. It will only go in one direction. The slot on the side of the carburetor should be oriented toward the clutch side of the sled.
	9. Connect all lines going to the carburetors. There should be 2 vent lines per carburetor and a fuel line.
	10. Replace the airbox. There are two bolts, one on each side that will hold it on.
	11. Turn on the fuel valve. It will be located between the fuel tank and the fuel pump. The sled will take 20-30 pulls with full choke to fill the carburetors back up with fuel before the motor will start up.
	12. Enjoy your sled!

Philips head screwdriver	toothbrush
large pliers	shop cloths
hex driver	very fine wire
	carburetor cleaner spray

Exploded view of carburetor parts	1. Remove the 4 screws holding the float bowl to the carburetor body.
	2. Pull gently with a rocking motion on the float bowl, to loosen it without damaging the gasket between the two parts. Set bowl aside for cleaning.
	3. Using hex driver, remove main jet and the ring that the jet holds on. Set aside for cleaning.
	4. Using a flathead screwdriver, screw the pilot air jet in until it is fully closed, while counting the number of turns. Now write this number down, and you can remove the pilot air jet by fully unscrewing it. You will need to make sure the air jet ends up the exact same amount of turns out as it was originally when you put it back together. Set this aside for cleaning also.