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Discussion Starter #1 (Edited)
As many others have written about, I had accelerator 'lag', a 'bog down' condition when trying to accelerate, as well as the constant 'dripping' through the bowl overflow (apparently a common problem for the overly complex Mikuni carb). This combined with poor/rough idle and irregular 'steady state' RPM led me to the logical conclusion, a carburetor rebuild was in order (carb rebuild kit –Amazon $30). While I was in there, I decided to take care of some additional fuel related issues:

1-A fitting leak at the base of the fuel tank
-Carefully adjusted/tightened, OK
2-Leaking fuel line(s)
-Replaced all (1/4” ID) + Clamps (auto parts $8)
3-Fuel valve that never shut off the fuel
-Replaced ($20 new eBay)
4-Old, original fuel filter had never been changed
-Replaced new (auto parts $7)
5-A slight 'varnish' smell to the old gas.
-Drained tank (used gas in old lawn mower)

JUST MY TWO CENTS: Other threads I have read here state that they have ‘Never put a Rebuild Kit in’, just thoroughly cleaned and reassembled.
Good practice dictates that you should replace the gaskets, seals, compression washers, ‘O’ rings; anything that is held in compression, tension or shear or can be distorted by disassembly. Although materials are superior now to those of many years ago, nothing lasts forever, particularly when exposed to heat, vibration, fuels and cleaners. With respect to internal parts (Jets, Needles, ‘E’ clips, springs, etc.), the general rule is; If the carb is known to you (You purchased the ATV new or rebuilt/replaced these parts previously), then, in general, it’s OK not to replace them (unless you notice changes in the parts from previous rebuilds), INSPECT. If the carb is UNKNOWN to you (You purchased it USED, EVEN from a dealer), REPLACE the PARTS with new ones from the rebuild kit! It is NOT worth the risk that someone previously, used a screwdriver to scrape buildup from a jet damaging it, accidentally bent a jet needle while cleaning, stretched out a spring or ‘E’ clip, stripped an adjuster screw. You (and the engine) will pay for someone else’s mistakes sometime in the future (usually at the worst possible time).

Carb Rebuild Lessons Learned/Suggestions:

1-Be VERY careful during the disassembly of the ‘Diaphragm/Piston’ assembly, those things cost as much as a new carb! CARFULLY remove the diaphragm from the carb body and NOTE the location of the rubber TAB for reassembly.

2-Float adjustment seems to be a topic of great debate!? This carb isn’t mixing hypergolic rocket fuel! It’s not gravity fed. It’s not a generator, on a concrete slab, running at exactly 2137 RPM for months at a time. No need to hang upside down, with a float gage between your teeth, balancing the carb on the end of a pin, wearing a eye patch to aide in concentration while updating Facebook; Simply stand the carb on a flat surface, GENTLY move the floats up and down to make sure the hinge is not binding, place a gage on the flat edge of the carb body (bowl mating surface) adjacent to the float and EYEBALL the marking (1/2” for mine) parallel across the float end of the metal attachment to the floats. <CARFULY bend the small metal tab over the needle valve to adjust float height.> That’s it! (see diagram) The object of the floats are, and have always been, simply to regulate the inflow of fuel, based on usage, keeping the primary jet covered while allowing air over the top of the bowl to prevent excess fuel from entering the carb throat, et al. (The first time you hit a bump in the road, the Floats are all over the place anyway.)
---TIP: When the floats are off and clean, in a quiet area, gently hold the float next to your ear and give it a quick shake. If you hear something splashing around, you’ve got a defective float! There is fuel inside. This will make the float sink lower in the bowel, causing incorrect fuel level. You can repair them but it’s not worth the risk. With plastic; Leak once, leak again! Replace.

3-Jet needle adjustment- Be mindful during reassembly, TYPICALLY, the ‘E’ clip will be placed in the MIDDLE groove (#3 on mine). Under normal circumstances, placing the ‘E’ clip one groove higher or one groove lower will cause the carb to run LEAN or RICH. I have attached a simple chart to help. (The end with the grooves is ‘UP’) Move the ‘E’ clip UP, carb runs LEAN. Move the ‘E’ clip DOWN, the carb runs RICH.

4-Assembel the ‘Diaphragm/Piston’ with the ‘Diaphragm’ TAB lined up with the notch in the carb body. Also, make sure the piston EASILY moves up and down, NO binding.

5-When assembling the jet needle, DOUBLE check to make sure; it is not binding, can ride easily up and down WITHIN the piston. And the spring is squarely on the small nylon washer (looks like a tiny hat) over the end of the jet needle (hasn’t slipped off).

6-Gently assemble the vacuum dome over the top of the ‘Diaphragm/Piston’ assembly, paying attention to locate the end of the piston spring over the pin in the vacuum dome top. Evenly work the top down and loosely assemble the retaining screws. Visually confirm that the ‘Diaphragm’ is even across the top of the carb body and does not have any wrinkles or gaps. Cross torque the screws, in steps, to specs.

7-Carb fuel ‘Enrichment’ assembly. This one gets me really p*%#@d off! Some idiot at Mikuni thought; ‘I’ll make the carb cheaper to build, we’ll save a yen or two’ So they put in this KLUGE! Doesn’t mean much you say? Those of us who have had FUEL PUMP issues beg to differ!!!!!!!!! <see my Fuel Pump thread -- http://www.polarisatvforums.com/forums/atv-repair-maintenance/45763-2001-sportsman-400-fuel-pump-repair-engine-dies-fuel-issues.html> Anyway…..When I disassembled the brass plunger, I found DIRT ground into the barrel and plunger! (Yet another example of poor quality and design) Obviously it wasn’t functioning properly, STICKING, causing vacuum and fuel problems. After cleaning, during reassembly, I filled the rubber cable/adjuster cap with grease to prevent dirt from entering the barrel and slid it tight against the carb body.

Hope this helps.


96 Posts
fantastic write up... i just got done about 8 weeks ago doing carb work.. overflow bowl screw was broke and it wouldnt start to save its life and when it did it wouldnt idle 12 dollar fix also replaced the bowl screws with much stronger none stripped screws bowl drain screw that and 1/2 a can of carb cleaner and it ran like a champion
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