This is a Mikuni BSR36 carburetor found on the 2003 Suzuki LTZ400.

Spring has come and many off-roaders will be starting their ATVs up for a great summer of riding. Well, most will anyways. Others will sit in the garage with thoughts of riding only to be let down by a faulty fuel-delivery system.

Fact is that the stale fuel in the carburetor, along with the condensation that has dropped into the bottom of the fuel tank, will keep many grounded until serviced properly. Gasoline does indeed have a shelf life contrary to popular belief, and when it goes bad it leaves behind a residue or varnish on almost everything it is in contact with. This varnish coats small openings in the carburetor, and that’s when it becomes a problem.

Another view of a dirty old carburetor before its bath.

For this segment of our ATV tech we want to show you what you can do to fix this problem after it occurs, and we will also give a tip or two on how to prevent it next time around. We have a used Mikuni BSR36 OEM carb off of a 2003 LTZ400 that will be our demo for the article. These four-stroke carburetors are fairly simple and can be disassembled with common hand tools. There is, however, a few extra tools that will be needed, and most of them can be found around any hardware store, but we will get into that in just a second.

The internal parts of our Mikuni BSR36 noted in red makes an easy map for reassembly.

Our first priority is to lay out a very clean and visible workspace. This is an important step because many of the internal parts in all carburetors are small and easily lost to the naked eye. Laying a clean, light-colored towel down will help prevent parts from rolling or bouncing around if dropped. Most of all, be patient!

Removing the float bowl off of the bottom of our carb we find several key parts. The first item you’ll notice is the off white set of floats. The float system is held in place by a small Phillips head screw that can strip easily, so be sure the screwdriver you use fits snugly into the screw. Removing the screw will allow the floats, float hinge pin and needle to be removed from the carburetor. This will then open up some room to look around.

After removing the floats and jets we expose the needle seat and a little more crud.

The main needle seat can be removed also by removing yet another small Phillips head screw just under the area where the floats sit on the carb. This seat has a rubber o-ring seal and can be removed by using a set of thin, long needlenose pliers to gently pull it out of its hole. No matter how tempting it is, be sure to grab the sides of the seat and not the inside where the needle rides because damage can occur really easy in the brass guide there. There is a small filter on the bottom of the seat and this catches debris that slips past the filter in the tank. Feel free to clean this up as well, but be careful here also because the filter mesh is super thin and can tear easily.

The needle and seat are in great need of cleaning but be careful not to harm them either.

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This area is now ready for a blast from the brake parts cleaner.

You will notice in the center of the carburetor a main jet, start-up jet and a primary jet, which will all be removed by using a flathead screwdriver. The primary jet will require a smaller flathead screwdriver, and be sure to use one that is as wide as the slot in the jet or you will break away the lip on either side – and that’s no fun. Once these jets are removed, this would be a good stopping point for the less-experienced mechanic. The majority of the varnished parts that can cause problems with the starting of your ATV should now be in a safe place on your workstation.

The float bowl needs a good cleaning as well and possibly a new o-ring.

The first few items I can suggest for cleaning the jets in a carburetor would be a small safety pin; an aerosol can of brake parts cleaner and for the rough stuff maybe a chemical dip. Most mechanics will advise to not use any kind of probe or prick-type device such as a safety pin simply because you can damage the jet opening. This should be a last resort if a new pilot or starter jet is not available, and the one in question has major buildup in the port. The holes in the jets are relatively small.

This is our parts that will need to be meticulously cleaned for the carb to perform better.

The primary and starter/enrichening jets will have the smallest passages so this is where a safety pin will come in handy. Holding the primary jet with the slotted end toward you you’ll look to see if the hole is clogged. Holding the jet up to a light will give you a clearer view. Typically, the primary jet in this carburetor is a 22.5, so the hole should be small but open. If not, take the safety pin and, with the pointed end, very gently press the pin into the hole. There shouldn’t be any concrete in the jet so don’t go in there ramming the pin around, as you can damage the jet or create a larger hole which will create another set of problems. We have a special set of jet cleaning pins for our carb and these can be purchased at your local dealer, but the small safety pin can probably be found in your wife’s make-up drawer in the bathroom.

Looking through the port on our Starter/Enrichening  Jet.

After you have a visible path in each of the jets, you can move on to the brake parts cleaner. Brake parts cleaner dries really fast, has a lower probability of melting plastic parts and it simply works better in most cases. It is more expensive but you get what you pay for.

Be sure to wear safety glasses and rubber gloves when using these aerosol cleaners, because it really isn’t something you want in your eye or on your skin. And use these or any aerosol cleaners outside in a well-ventilated area away from sparks, as they are flammable!

Our needle valve was right on the verge of needing to be replaced.

Start by mounting the straw that comes with the brake parts cleaner can into the nozzle and position the straw over the end of the jet. Spray through the jets several times with the cleaner until you get a good, clear hole through the jet. Clean each jet thoroughly and set these aside for the moment. The next order of business is to inspect the needle and seat. Looking closely at the needle, make sure the rubber tip is in good condition and that it doesn’t have major pitting or deformities that would keep it from seating properly. Using the brake parts cleaner, adjust those safety glasses tightly against your head, put on your set of chemical-resistant gloves and spray into the holes where each jet seats to be sure there isn’t any other varnish or debris in the carb. Clean the float bowl of any small sediment or funk and you’re ready for reassembly.

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This little part is what the main jet rides in and it needed a good bath as well.

OK, this is where we can reassemble the carburetor in the reverse order of which it came apart. It helps to add just a very light touch of grease to the o-ring on the needle seat when reinstalling to help it get in place easier. Be sure the starter or enrichening jet, as well as the main jet, get back into their proper places. These jets are easily identified by the main jet having the larger hole in the center and the starter jet having the smaller of the two. When mounting the float bowl back into place be sure the rubber o-ring is in its seat so you don’t have any unexpected leaks. These o-rings are cheap so, if possible, just replace the part. Also, the screws that hold the bowl on do not have to be torqued into place! The o-ring seals the bowl and if you over tighten the screws you’ll take a chance of stripping the Phillips head screw. I typically replace the Phillips head screws with a hex/allen head screw just to make it a bit easier when I have to go back in later on.

Inside cleaned and ready for reassembly.

This would be the most basic way to clean up a varnished carburetor, and most home mechanics can finish this in under a couple hours. There are debatable methods to keep the varnish from forming in the bowels of your ATV’s carburetor. Some say to fill the tank with gas so it cannot condensate and add a fuel stabilizer to the tank to preserve the fuel. This does indeed work, however, if the ATV sits for months at a time I would worry that all the fuel would be less that acceptable to run through the ATV after a long period of time. Personally, I turn the fuel petcock off while the ATV is running and let the carb run dry. Then I drain any leftover fuel from the tank. There is also a small Phillips head screw on the bottom of the float bowl from which you can drain any leftover fuel. Either way you do it, the most important thing is to run the gas out of the carburetor before putting the ATV away for any length of time. This will prevent most of the varnishing that occurs during storage.

Our jets are shiny clean and ready to go back in the Carburetor.

Reinstalling the screws in the float bowl doesn’t require a jack hammer. Just snug them up and if they are stripped then install a hex/allen head screw in their place.

As I stated in the beginning, this is for a generally skilled mechanic and can be completed with common shop tools. Do not attempt this if you have no patience or mechanical ability. I hope this helps you get back on the trail fast, and if you have any questions feel free to send them in as we will try to get them answered promptly.
RS

The completed carb is ready after a good spray of cleaner to get the dirt off the out side too.