how to check float level on mikuni bst 34 carb

SturgisATV · #1 (**permalink**) 01-14-2012, 01:01 PM

The patient is a 1996 Polaris Magnum 425 4x4. An acquaintance of mine asked me to look at it in his garage, as it used a whole tank of gas in only 4 miles of use (over probably 6 months). Crankcase and oil FULL of gas. Oil change needed now.

So I stripped the carb down, and soaked it in Chem-Dip, minus any rubber or other parts that shouldn't go in Chem-Dip. Then it went into the ultrasonic cleaner for an hour. Re-used the original float and all other parts, as this guy didn't want to spend any $ on it if just a cleaning would work. Noticed that the float was set really high. Not even sure if the float was stopping the gas ever. So I lowered the float level to the point where it looks like it should shut off the gas at the right level. I have successfully rebuilt probably 25 of these carbs, but the float level is always a guess.

Changed the oil.

So we put the carb back on, and it wouldn't start. I noticed that there wasn't a lot of gas in the tank and asked if he had the selector on reserve. He said yes. Still wouldn't start. As you may know, the selector valve has a long plastic extension on it, and we had it off with the tank. Assuming that we were getting gas from the tank, I ended up pulling off the bowl, and setting the float back to its original position. Still wouldn't start, and gas was running out the overflow. I then noticed the fuel selector valve was in "on" instead of "reserve". Dammit, take the carb bowl back off, and re-set the float where I thought it should be. Then it started right up, and when we shut it off, there was no gas coming from the overflow and everything looked fine. Put it all back together. End of story.

Got a call today. He thinks theres a gassy smell in the crankcase again, and maybe a gas spot under it from the overflow again.

That tells me that the float needle isn't shutting off the gas.

Here's what I need:

Does anyone know EXACTLY what level the float should be set at, and HOW do I measure it?

I'll just pick up a new needle and seat assy for it on Tuesday at the dealership since I need it right away. Some tech help would be great.

Mikuni's website really sucks, as they seem to offer no assistance at all for ATV'ers. I wish somebody else made a replacement, new carb for these older ATV's.

Russ C · #2 (**permalink**) 01-14-2012, 05:51 PM

with carb upside down on a level table, from bowl surface to highest point on the float ,13mm +or - 1 mm this is out of polaris manual

bmorton · #3 (**permalink**) 01-14-2012, 05:53 PM

level with the carb body. you want the metal part of the float right when it closes not when the little spring loaded thing bottoms but just when the needle bottoms in the seat the metal piece should be level with the carb body, that is what polaris wants.

brassmagnet · #4 (**permalink**) 01-14-2012, 06:00 PM

I saw a parts manual for a 98' 425 Magnum and it indicated that it used a Mikuni BST34 carb. I'm not sure if its the same thing for a 96'...

My 02' Sportsman 600 also uses a BST34 and I'm assuming the float height setting would be the same (.51-.55").

I'm a newbie at this, but if its the same as mine you want to place the carb on a level surface such that its output it facing up. This way the weight of the float will be supported on its pivot point and not compressing the float spring. With the carb positioned such that you are looking into the side of the float you want to measure from the mating surface of the carb housing (directly next to the side of the float) to the top of the float. Do this for both sides to make sure they are even.