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Discussion Starter #1
I'm just wondering what all is recommended for a carb rebuild, as the new carb I bought online still isn't quite right. I see some kits online with a couple things in them, but they don't seem to have as many parts as I think they should. I could be completely wrong as I've never rebuilt one of these before, but what do you guys consider necessary for a carb rebuild? I've got the original and plan to soak it in some solvent and get it nice and clean, then rebuild it. It's pretty bad, but I'll concoct something and get it as clean as possible before trying anything. It worked fine before the cam went, so I'm thinking with a rebuild and a few tweaks it should be good to go.

Thanks guys
 

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I don't buy a rebuild Kit unless I need multiple parts - if I need a float needle or seat, then I buy a kit that includes the needle and seat because the entire kit is cheaper than just the needle/seat at dealer price. I install the entire kit once purchased and either save the old parts as spares or simply toss the old parts separating the metals into recycle containers. When I buy a kit, I prefer Shindy, All Balls and K&L in that order, but have resorted to other brands due to availability. The only kits I truly abhor are Honda kits - Honda only includes the rubber parts, but sometimes certain rubber parts are only available in the OEM kit. i.e. - I just rebuilt a TRX450R carb - it had been previously worked on by a clown - parts were missing and nothing was adjusted properly. I got a Shindy kit and a Honda kit - the Shindy kit had all the jets, float needle, slide needle and needle jet, float bowl gasket, a drain screw o-ring, pilot screw o-ring and pilot screw. What the Shindy kit did not have that the Honda kit did have was the rubber plug for the choke feed well and the top cover gasket. The TRX450 carb has a fuel metering jet for the enrichening circuit in the side of the fuel well - the well is capped with a mushroom shaped rubber plug - if the plug is left out the amount of fuel available to the enrichener is unmetered - opening the enrichener for just one kick floods the engine. So, the rubber plug is required, but it never wears out and seldom needs replaced unless it is lost by a clown.

90% of all carbs I rebuild only need a few new o-rings and a bowl gasket. Float needles and bowl gaskets can be obtained from K&L Supply and other parts suppliers - the myriad of o-rings that need replacement I buy in bulk from an industrial supply where I can select the o-ring material. Example; the float needle seat is sealed to the carb body by an o-ring - the o-ring that comes with a needle seat or the o-ring in a rebuild kit are made of Buna rubber - Buna is a good fuel resistant rubber, but when exposed to alcohol it will first swell and then later shrink. Once shrunk, it will not re-swell, therefore is loses it's seal and fuel bypasses the seat to overfill the bowl and causes flooded crankcases, dripping from the bowl overflow, sometimes dripping from the float bowl vents and drained fuel tanks. I buy Viton o-rings for the needle seats - Viton is more chemical resistant than Buna and lasts much longer especially when exposed to alcohol. The o-ring is 7.1 mm ID x 10.3 mm OD x 1.6 mm material diameter. This is not universal - some smaller and larger carbs use different sizes of o-rings, but most 250 to 700 cc carbureted machines with Mikuni VM and BS carbs will use this size of o-ring.
 

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Yep, as latebird said, you don't always need a whole kit but sometimes its more cost effective to just get the whole thing than to buy individual parts. It all depends on what you need.
 

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Discussion Starter #5
Awesome thanks guys! Good to know about the o-ring size and material, as well as the preferred brands of carb kits you guys use. I'm not seeing much for Magnum carb kits, mainly just for Sportsmans which is par for the course. I'm looking at a Freedom County carb kit for a 99-00 Sportsman, it has good reviews and is a decent price. What do you guys think? I can always run to Polaris if the jets are different or something, but I think it should work for the most part eh? There were kits that covered more years, and also one that listed an EFI model as a fit, but I'll stick with the same year at least.

 

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LOL, not sure what vendor you was looking at but there is no such thing as a CARB kit for a EFI machine since they don't have a carb. :unsure:
 

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Discussion Starter #7
Haha when I read it I'm like ya I kind of question everything about this kit and vendor. I'll keep scrolling lol
 

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Interesting - I just searched the carb for the 99 Magnum (part 3130958) and it was only used on the 99 Magnum 500 - no other models, but it looks like the only difference between the Magnum and the Sportsman is jetting - they both used BST34 Mikuni carbs. Here's the spec. of each.
136916

136917
 

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Discussion Starter #10
Thank you very much for the specs latebird, I've got them saved. I'm now thinking that maybe I can soak all the parts and pieces and reuse them from my old carb? Or get the main jet and jet needle from Polaris and put those in as well as some new pieces from a rebuild kit. I'm thinking maybe I should fiddle with the carb I have on it now just to see if I can fix the problem.

The problem I'm having is that it will idle fine, but right off idle it will start to rev and then hit a flat spot in the carb and bog down. I can work the throttle and get past it, and it will run alright up to about half throttle or so and then sputter and bog down and want to backfire lightly. If I turn the choke on it helps slightly, but I still can't get any real power out of it. Like I'm pretty sure I can't get past 40 Km/h, (speedo doesn't work). I'm thinking I'll spray the intake boot with carb cleaner to see if the engine revs up and check for air leaks. Then maybe adjust the e-clip to see if that helps, I should be able to do that without disconnecting the throttle and choke cables hopefully.

On Niche Industries website, it says this carb replaces the Magnum carb (3130958) as well as about a dozen other P/N's. It is most likely not jetted correctly, maybe I'll just get the correct main jet and jet needle for the Magnum and put them in to see? I'm really not sure lol I didn't realize this carb was setup specificaly for this year/model.
 

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Carbs are set up for the flow characteristics of the carb. The Chinese carbs are copies of patented designs, but the machining is not as precise. Therefore, they look the same and bolt up the same, but they don't necessarily react the same to changes in air flow and it the flow of air that is responsible for picking up the fuel (siphons it out of the float bowl) that mixes with the air to support combustion.

The first thing to do is to use fuel the carb is designed for. The Chinese and Japanese do not have alcohol in their gasoline. Alcohol is harder to ignite and burns cooler. It takes almost twice as much alcohol mixed with the same amount of air to provide acceptable power from an IC engine. Mixing alcohol with gasoline means you need to adjust the fuel/air mixture as the alcohol displaces gasoline leaning the mixture. Normally a leaner mixture means increased combustion temperature, but alcohol burns more slowly and cooler than gasoline, so the increased temp of the lean gasoline to air mixture is offset by the cooler burn of the alcohol mixture. Using the fuel the carb is designed for may eliminate bogging at certain throttle settings as the fuel to air ratio varies.

The BST carb was engineered by Mikuni and is designed for economy - not performance. If you observe the operation of the carb closely; the slide rises in relation to the vacuum the engine creates on the engine side of the butterfly valve. The vacuum is drawn on a small port and is internally ducted to the top chamber where the reduced pressure attempts to lift the slide valve. The slide valve operation is counteracted by a spring and the weight of the slide. In theory, the slide position is dependent on engine speed, but on a single cylinder engine, the vacuum is in pulses, so at lower speeds, the slide bounces. As the slide rises it allows more air into the engine and the needle in the slide controls the amount of fuel that can mix with that air. As engine speed increases and the vacuum becomes more constant, the slide does not 'bounce' as noticeably and positions itself at a relatively constant position.

Now if the butterfly valve is opened suddenly from a constant setting, the vacuum drops and so does the slide. The slide reduces the air flow into the cylinder and likewise the amount of fuel being mixed with the air. Each intake stroke causes the slide to rise, but during the compression and exhaust cycles, the slide descends. This is the action of the Constant Velocity carburetor. Keeping the velocity constant increases the efficiency of the carb while providing acceptable acceleration performance.

Getting rid of a bog (lean condition) is difficult to engineer out of a carburetor. It is a matter of compromises. It has to be too rich at some throttle openings and too lean at other settings, but as long as the lean areas are not so lean as to cause engine damage, then the compromise is acceptable.

To get rid of a bog just off idle, it has to be too rich at idle. This is accomplished by either adjusting the fuel screw to a richer setting, raising the slide needle to enrich the entire RPM range or changing the slide needle taper which may enrich the low speed while maintaining the mixture of the original needle in the mid range. But a bog from mid-range to full throttle may require changing the speed of the rise and fall of the slide. If the slide rises too high for certain throttle settings, it may provide an over rich mixture at mid-range while going lean momentarily before the slide falls sufficiently to increase the air velocity necessary to pull fuel through the needle jet. The reaction of the slide may benefit from a stronger or longer spring.

The Mikuni VM and HSR tuning manuals are 20 pages, but I have never seen a CV tuning manual. You have to use the info in the other manuals and apply the techniques to the CV carb.

Try changing fuel first - sometimes it is a simple as switching brands. Switch from Phillips 66 to Casey's or vice versa and see how it performs. Switch from alcohol blended to pure gas or vice versa. Or tear into the carb and start changing calibrations, but only change one thing at a time in case you need to change it back.
 

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Discussion Starter #12
Very interesting latebird, thank you for sharing all that! I'm thinking that I'm gonna get the old one soaking and eventually rebuild it, buying the main jet and jet needle from Polaris. When I get some time however, I think I'm going to adjust the e clip on the new carb to hopefully try and fix it or at least get some more power from it. Stay tuned, as nothing goes smoothly with this quad LOL
 

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Hey guys I'm back with an update! Hope everyone is staying safe against this virus. I made some time to play around with the carb, and here's what I did:

I cleaned the main jet from the old carb, as well as the jet needle, and swapped them into my Niche carb. It got rid of the hesitation off idle pretty well entirely, and is smooth all the way till WOT. I still have a little backfire/pop at WOT, but no where near as bad as before. That still means we're lean, correct? How can I richen up the top end?

I also think I cooked the head gasket with our lean condition, as I have coolant disappearing and it had overheated on me once before. Once I get the carb tuned decently I'll make sure its the headgasket and then repair that; but in the mean time the mechanical seal is leaking quite a bit of oil in the snow now so I might give the dealer a shout to see what that would cost to replace.
 

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Discussion Starter #14
I think I'm going to jump to a 160 main jet, as I read a post by latebird on an old thread about carb operations, and how the WOT circuit is mostly controlled by the main jet. I'll grab one from the dealer next time I'm in town, and we'll see where she goes.
 
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