Where did you get the new carb? I'll buy your old one and pay the shipping.

 

Where did you get the new carb? I'll buy your old one and pay the shipping.

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I bought mine from ebay but then I realized, it was shipped by Niche. They have a bunch of new OEM carbs in stock for $190.

I'm gonna try to rebuild the old carb, its physically damaged. The enricher plunger was stuck/corroded in and so I clipped off the wire, hammered the plunger all the way in and put a set screw in to seal it. It'll still work but doesn't have the enricher option anymore.

Also, I think I may have figured out the issue im having. I believe the rubber boot that mounts the carb to the engine is to blame. It's old and stiff and the rubber seal that mates to the engine is worn out. It looks like air has been making its way past the seal. This might explain why it'll idle fine and respond up to half throttle but bog down at wide open but if I take off the airbox/filter, it'll run better at WOT. I ordered a new rubber boot.

 

Niche does not have OEM Mikuni BST carbs for $190 - what they sell are Chinese counterfeit Mikuni copies - the OEM carb was Polaris part number 3131200 and the Mikuni was sold with a cover over the operator linkage.

The carb Niche is selling is a copy of 3131463 that was fit to the 2003 Magnum 500 HDS - both the 3131200 and 3131463 parts are obsolete and NLA. The sold for over $500 new and dealer cost was right around $400 - there is no Genuine BST 34 carburetor for $190 when used (and needing rebuilt) BST 34 carbs) sell for $150 if you can find one.

Amazon has that carb for $222 - EBAY has it for $195 and it is designed for the 2003 Magnum 500.

Now, any carb will work as long as it fits and you jet it correctly - jet it with the air box on and if it runs better with the air box off, it's jetted too rich.

 

If this carb is a counterfeit, i'll be honest, its the finest counterfeit I've ever seen. I've dealt with smaller engine carbs and the "Chikuni" replicas are subpar, rough work that's obvious. This supposed counterfeit BST 34 is one hell of a reproduction. It's also marked "Mukuni" and "Japan" on the body, something I've never seen before on clones but I wouldn't put anything past China.

To be honest, I thought the fit and finish was better on the supposed replica than the original right down to the jets having the same exact markings in the same format and text as my original parts that I got from my local dealer in Polaris packaging.

If these counterfeit's are this good at a 1/4 of the price and completely indistinguishable, I'll take three please. I just don't see how Niche is able to get away with selling counterfeits in the US branded and marked as a Mikuni Japan. I work in the medical field and I deal with a lot of parts that have to go through regulatory bodies and before they even reach the suppliers or US warehouses, they are all screened by commerce division. I'm not saying you're wrong, I'm just saying I would have a hard time believing these are counterfeit. They may not be a BST 34 and you're probably right, they were designed for the Magnum 500 but I've poured over the fine details between the two and I can't find a single difference so its all the same to me.

As far as the original BST carb, the enricher plunger was stuck half way open. With the might of thor, I wasn't able to pull it out so I clipped off the mangled wire, tapped it all the way in with a punch and managed to find a threaded plug with an allan socket on it. I'd like to rebuild the original to be honest but I would need all the parts including the fiber/plastic bracket that holds the slide and emulsion tube. I tried to heat up the area where the pluger sits from the inside of the bellow bowl and it deformed the plastic part. Any idea on how much that rebuild kit would cost with the plastic part?

 

Here are some images. The first set is of the supposed counterfeit. Only thing I can't decipher are the laser markings. 50L08D and L241.

Here are some pics of the original that I pulled out. You can see the plug I installed to cap off the enricher. Also the damage to the plastic bracket that holds the slide when I heated up the housing to try and release the plunger for the enricher. It can be replaced I'm assuming.

 

Is the fuel screw on the Niche carb capped as the OEM carb was or is?

It's a shame you buggered the by pass starter valve - I remove those that are stuck easily by boiling in aluminum cleaning solvent in a sonic tank - normally they fall out from the heat and sound wave assault on the gummed and corroded part.

I just rebuilt a BST34 for a forum member - the plastic jet block is $65 the starter valve plunger is $15.

 

We live and move on, not gonna cry over spilled milk. I'd like to rebuild it but honestly not too concerned. If I could find a Nibbi with a pulse fitting, I'd swap out all together. I'm not by any means trying to preserve the original state. I'll pull the plug and see how bad it is inside, maybe there is hope for it yet. 🤷

I've got a three liter ultrsonic cleaner from vevor. Is there something available commercially that I can cook mine in?

 

BTW - counterfeit $100 bills have all the markings of real $100 bills, but they are still not worth anything.

 

Also wondering if mikuni has a remote air/fuel adjuster for this carb? Any chance you've got a part number?

 

The BST does not have an air/fuel screw - it has a fuel screw to minutely adjust the amount of fuel mixed with the air as it enters the air stream throttled by the butterfly throttle plate. Once set it does not need to be adjusted again for a very long time - probably only after the carburetor is rebuilt by a competent carburetor mechanic.

To adjust the fuel screw correctly: the initial setting is sufficient to get the engine started, running and idling, but is far from it's final setting.

Ride the vehicle for about 15 minutes to warm it up to operating temperature - set a window fan in front of the vehicle for air movement across the engine and carburetor simulating riding at moderate speed and connect an accurate tachometer - start the engine and set the desired idle speed - using the special fuel screw adjustment screw driver, turn the fuel screw in by 1/8 turn increments pausing 3 to 5 seconds between adjustments - when the engine drops idle speed by 50 rpm, turn the fuel screw out one full turn - that is the final setting of the idle fuel adjustment screw until the next carburetor service.

 

Good news, I got the plunger out using my screw trick. No damage to the orifice. Looks like I'll be rebuilding this carb after all. Gonna do some research to see what I can soak it in inside the sonic cleaner.

 

This it the only thing I buy from Amazon - it's what I use in my sonic cleaner to clean carbs - it mixes 10% with water (I use distilled) and is good for about 30 to 50 carb cleanings, a gallon last me over a year and I clean carbs almost daily, but I also clean cylinders, heads, small engine parts, fuel pumps and anything made from white metal.

Sonic cleaner detergent

 

What is the method for determining if the engine is running lean, right or rich? After putting the new intake manifold on, it's running really good but I'm not sure how to tell if it's lean or not.

 

With the OEM CV carburetor if it's lean at full throttle and top speed it will kind of taper off on speed and engine power and perhaps surge like it wants to do more but can't and sometimes opening the bypass starter valve (choke) will temporarily give it some extra speed - too rich and it runs good up to a certain speed and then falls on it's face and blubbers like the choke is on. To tell for sure, it need to have a sensor installed in the exhaust and check the exhaust gas oxygen content. Get some leaded 90 to 92 octane leaded gas from an airport and run the engine about 1/2 mile at full throttle. Without closing the throttle, cut the ignition , shift to neutral to prevent engine braking and coast to a stop. Pull the spark plug out and check the color of the porcelain insulator around the center electrode. Tan to dark brown is perfect - clean white is too lean and grey to black is rich. Grey is OK, but black is wasting fuel and losing power.

BTW - the carb holder is not a manifold - a manifold has more than one opening for connection of more than one carburetor or cylinder. A manifold block is used in air conditioning recharging devices.