Dead Sled Wrenchers

Clean a Mikuni Roundslide Carb

So You've got a newer sled, its got a Mikuni roundslide carb and the sled won't run. Maybe it'll load up and backfire through the carb, maybe the motor runs but when you give it throttle it cuts out. Whatever the reason you need to clean that carb. If you've read the article on the Tillotson rebuild you might wonder why we're cleaning the Mikuni but rebuilding the Tillotson. Well the Mikuni is a much easier carb to fix and probably you aren't going to need any new parts. I say probably, the one exception is the bowl gasket, if this is your first time cleaning this carb plan on replacing the bowl gasket. If you've replaced the gasket in the last couple years you're probably okay.

Disassembly: Okay to get started locate the carb on your sled, you'll probably have to take the airbox out of the way to get to it. Be careful about the airbox its pretty important. Set the airbox aside someplace safe, we'll come back to it. Now look at the carb. The throttle cable enters through the top, the choke is usually on the right side (front is front, like toward the engine) and is threaded into the carb. Lets do the choke first because its usually the hardest to get at. Take your 12mm wrench and unscrew it. It helps if its a shorty wrench and if the jaws are thin, be a bigshot, get a dollar store wrench and modify it into a "choke removal tool". Once the choke is out of the way unscrew the top of the carb, this'll release the slide, theres a lot of fine threads there, keep turning... It may take a little wiggling to get the slide out, sometimes it helps to pull the throttle or reach a finger down the carb throat and help it along.

All thats left holding the carb now is 1 screw on the band around the mounting flange, loosen it alot but don't take it out, then grab the carb and wiggle it around and out of the flange. Be a little careful to keep the carb upright and level or you'll get a hand full of gas. Put the carb upside down in your clean stainless bowl.

Fig 1. Mikuni Roundslide carb      Fig 2. Exterior fuel bowl

List of tools/parts:

1.Service Tool Kit for Mikuni VM carbs:
Yes you could assemble the tools yourself but the kit is cheap and the tools are good.  I think mine came from Parts Unlimited.
The kit includes: 
Screwdriver, this may seem like a simple thing but its a GOOD screwdriver, it fits the screws on the carb correctly and is less likely to strip them out. Its got flathead and phillips ends.
Nut driver, fits the seat for the needle valve, I haven't had to use it yet.
Jet wrench, handy little wrench
Dogbone wrench, least useful tool in the kit. Throw it in the trunk of the sled for emergency use

2. 12mm wrench:
Because the dogbone wrench is hard to use

3. Non-rebound hammer or rubber mallet: Whaddya mean you don't have a non-rebounding hammer? This is a good time to get one.

4. Penetrant:
Penetrating oil is one of the most important tools in the toolbox. Stuck fasteners are common on old sleds and need to be removed gently. I like PB Blaster because its available at most autoparts stores. WD-40 is a LOUSY penetrant, WD means "Water Displacing" which is what its for, not for being a penetrant...

5. Cleaning Bowl:
This is a bowl to clean the carb in. I use a small stainless steel mixing bowl. Its important to use a metal or chemical proof plastic bowl because the cleaning chemicals can eat cheaper plastic bowls.

6. Carb cleaner:
Cheapstuff, from the dollar store.

7. Rubber gloves:
Carb cleaner is hard on the hands.

8. Shop Vac:
The mice have probably built a nest in the airbox, DO NOT use your wife's vac. This is a good excuse to get a shop vac.

9. Bowl gasket:
If you've never cleaned this carb before you'll probably need it.  This has to be sized correctly for YOUR carb.

10. Carb Mounting flange

11. Fuel filter

12. New fuel line

13. Fuel pickup strainer

Fig 3. Fuel bowl removed  
Fig 4. Slide bore             Fig 5. Interior fuel bowl


With the carb upside down in the bowl take your phillips screw driver and break loose the 4 screws that hold the bowl onto the carb, don't pull them out yet, just break 'em loose. If one of them wants to strip tap your screwdriver into the screw with your hammer and try again. If it still wants to strip flip the carb rightside up squirt some PB Blaster into the screw hole and let the whole thing sit for an hour, then try again. If it still wants to strip get some small vicegrip pliers, grab the screw head and work it free that way. Try not to break the screw if you can help it. Don't try to heat the carb its got gas in it remember!

Once you've loosened the all the screws take them all out. Next take your hammer or mallet and tap the bowl in the direction it seats. Then tap all around it. Just tap here, not trying to break it free, just loosen things up. Now grab the bowl with your hand and see if it'll come off. Jiggle it a bit if you have to, varnish may have developed and glued the bowl to some of the internal bits. If that doesn't work tap some more, wiggle some more. Take a break and walk away, then try again, you'll get it. When the bowl comes off you're likely to get a splash of gas, be prepared, do this over your bowl or some other tub that'll handle having gas in it. 

Now lets ponder what we've got. Look at the carb bowl first, inside there are two black things about the diameter of a quarter but alot thicker. They ride on metal posts which are supposed to be topped with plastic stoppers but alot of times the stoppers have been lost. These are floats and serve to monitor the amount of gas in the bowl. They've got metal bits sticking horizontally out of the bottom, make sure you always put them back in the same way.... At the very bottom of the carb is usually a threaded plug which is the drain. Some machines like my TX-L have a length of fuel line. The idea here is that crud and water will settle down into the length of fuel line and not get drawn into the jet. However your carb is setup spend some time cleaning the bowl. If the gasket tore when you took the bowl off its quite possible some of it stuck to the bowl, clean it off with a razor blade but be sure not to scratch the mating surfaces. Spray the bowl with your carb cleaner real good and set it aside.

The carb body itself is more intimidating because theres more bits. Right in the center is a brass bolt with a hole through the center. Thats the jet, use your jet wrench to remove it. The number should be stamped on the face near the hole, its small and not a deep stamping and might be tough to read. Spray it good with carb cleaner. Can you see through the hole? Try blowing on it, can you see now? You might have to take some very small wire to poke through the jet if its clogged. They make a tool for this which is alot easier but I've had good luck just spraying the jet.  Its been my experience that the highspeed jet is usually not the one that gets clogged. Anyway hose it down good with carb cleaner and put it aside.

Just forward (toward where the engine would be) of the highspeed jet is another tube up into the carb. The lowspeed jet lives up there. Its been my experience that this is usually where the problem is so spray carb cleaner in here until you can see it spraying out good into the carb body. Then spray some more, get this good and clean. Maybe swab inside with a toothpick or a jet cleaning tool. You should be able to hold the top of the carb toward a light and see right through both jets.

Finally just behind the high speed jet is where the needle valve lives. Its controlled by an arm, if the carb is upright the needle should fall out of its hole against the arm. If it doesn't spray it with carb cleaner and it should free up. If it doesn't you'll have to drive the pin that holds the arm out. Pull the spring that holds the needle in and use your nut driver to pull the seat out. Boil/clean/whatever the needle and the seat until the needle doesn't stick anymore.

Cleaning: Spray the carb all down and scrub it real good with a toothbrush, get it really clean inside and out. You'll thank yourself later. Look down the throat of the carb and you'll find some ports, usually three with the middle one plugged. Spray a bunch of carb cleaner in each port until it runs out someplace else.

Now everything should be good and clean, take a good long look at that bowl gasket. Is it broken? Is it old and hard? Is it coming apart? If that gasket is even slightly questionable replace it. Take your existing gasket to your local sled shop and compare it to the ones in the book. Get an extra, you might break one or something, they're not real expensive. 
Right, lets examine that mounting flange. If this flange fails either the carb will fall off the motor or it'll leak air and burn a piston. The outside of the flange will usually be weather checked, thats okay, feel around inside it, are there any cracks inside? Cracks inside are bad and indicate a flange that wants replacing. If theres any doubt replace it.
While we're looking lets check out the choke plunger. Sometimes the beanie on the end of the cable strips off the cable. In which case replace the whole assembly. I've found that the handles on the new choke cables are made of really cheap plastic. Keep your old handle and pin, move them to the new assembly.
  A note on gasket goop:
I never use any gasket goop in a carb, if somethings warped badly enough that it won't fit tight without some gasket goop the carb is too badly beat to be rebuilt, replace it. Used carbs can be found on the web pretty easily.
Oh the airbox, we were going to get back to the airbox... Take a look inside, see how many mouse or chipmunk nests are in there. Take your shop vac and suck all that crud out. Turn the box up and shake it, shoot some carb cleaner in there and swab around with a paper towel until its clean. On the TX-L I had to spend alot of time cleaning the airbox because ants (ants, of all things!) had made a home in the seat and had dragged that seat foam into the airbox. The foam was getting dragged into the carb and keeping it from starting. My Blizzard 5500MX had airbox right full of leaves and fluff that plugged the carbs. I didn't know how to clean carbs then so I ended having to sell the sled. If I'd cleaned the airbox before I started running the sled it wouldn't have been a problem.

Reassembly is the reverse of the above.... Heh, thats a good one huh?
Okay if you had to take the needle and seat out put the seat back in with your nut driver, put the needle in, put the spring on, put the arm back in place and drive the pin back in. Put the high speed jet back in, but your new bowl gasket on (hint hint) and the bowl next to it. Put the 4 screws in but down run them in real tight just snug them up. Once all 4 are snug then one at a time tighten them good. Not too tight you don't want to break something, too loose and it'll leak. I tighten until the tendons pop out of my wrist just a little. Its easier to get right if you've got a new gasket. (Get the hint yet?)
Right, thats sorted, back to the sled. Put in your new carb flange, its held on with 2 allen nuts. With the screw on the band loose push the carb into the flange. The flange has a ridge in it that matches a detent around the carb body, match 'em up good. You'll know its right when you can turn the carb in the flange and it doesn't tend to unscrew. Then tighten up the screw so the band is tight. Remember the flange failing? The same stuff will happen if the band is loose so get it TIGHT but don't rip the flange...
Put the slide back into the top of the carb, it only goes one way, screw the top on. While you're screwing the top down put your finger down the carb throat to make sure the slide is seated, if it won't go down all the way try to wiggle the slide back and forth so it'll go all the way in. If you can't get it to seat pull it apart and try again.
Last step put the choke plunger into its hole, again this can take some wiggling. Tighten the nut down so it won't leak. Hook your fuel lines up. You did get new fuel lines didn't you? C'mon now, they're like $0.10 a foot, you only need like 2 feet. A new fuel filter is a real good idea while you're at it.

Once its all hooked up pull it over, it'll take a couple pulls to get gas to fill the carb bowl, you might want to put a little premix down the carb throat to make it run for a second to help pull gas. At this point things should be running good. If they aren't either theres something wrong with your carb or you didn't get it clean enough, pull it off and clean it again...

After you've run the engine a bit to prove you did a good job shut 'er down and put the airbox back on. You can't run without the airbox without rejetting the carb. If you do you'll get to much air through the carb and the engine will run lean and burn itself up. The carbs connect to the airbox with little rubber boots, sometimes the boots want to fall into the box and can be troublesome. Spend the time to get the boot on so it seals good, remember too much air, leans out the mix and burns down the engine.

Anyway, you'll notice that we haven't changed any carb settings here, I assume through all of this that the sled ran before and the carb is just gummed up with last year's gas. If the sled ran before it sat the steps above should make it run again. Theres no promises of course, there could be other issues...

So good luck, have fun and keep the shiny side up.

What do you think about this article? Let me know!