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I found a drawing just now on manualslib.com that shows a spacer going above the e clip, as i look at the spacer i can see it is shaped to mate against the spring, i put the carb back together the way i took it apart but i am thinking i found the problem, its about an 1/8 inch spacer and if its wrong it has been holding my jet needle way to high making it run way to rich , i hope this is the problem, i am so sick of this one ATV holding me up on so many other things, lol.
Here's the specs on the OEM carb:
143080

Here's the breakdown on the carb
143081

There is a thick plastic spacer under the needle e-clip and a spring seat on top of the e-clip

If it was a Mikuni BST-34 or 40 carb, then the assembly is as pictured - if it is a Chinese counterfeit, it's anybody's guess as to what is right. Only the carb manufacturer knows and they are many times defective. For my money, I would find one or two Mikuni carbs and get a quality Shindy or All Balls carb rebuild kit as a starting point.

I see you do not understand the operation of the CV carb.

As the velocity of air passing through the venturi increases, the slide rises - if you are holding the engine speed steady and the slide has risen 1/4 inch and you 'snap' the throttle open, the slide drops - this is in an attempt to keep the velocity of the air passing through the venturi constant - as the engine speed increases (and thus the velocity of air) the slide will rise - at idle the slide is usually only slightly 'bouncing' or is bottomed out and stationary - as you increase engine speed, the slide will rise and drop with the 'pulse' of air passing through the venturi as the intake valve opens and closes.

Being a mechanic is one thing, being a tuner is another thing and being able to solve a puzzle is extraordinary.

You have made excellent progress, not it's just a matter of getting it tuned - don't waste time on a POS Chinese carb, but don't get me wrong - there are quality Chinese carbs to be had - I have had success with KF Standard of Japan, Kun Fu and on small two strokes Sun, but it's easier to tune with Mikuni and Keihin because you can get parts - with the Chinese counterfeit carbs, they might look the same, but Mikuni and Keihin parts usually will not fit - the main jet for a Mikuni may not have the same threads as the Chinese carb and the main jet in the Chinese carb does not usually have a size stamped into it - this is because the Chinese size the jet to the flow characteristics of each individual carb - their quality control is not good enough for every carb to be jetted the same as the next one - thus 10 (or more) of the same brand, same size and same model of Chinese carb may all have different sized jets and since the drilled air passages are not precise, the settings of the slide needle and fuel screw are all different - there is no published factory setting - the setting is the setting that gets the carb from the assembly line to the box that it is sold in. When working with Chinese carbs, they must be calibrated for the engine they are installed on and that is the job of the mechanic installing the part and tuning the engine.
 

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2020 sportsman 850
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Here's the specs on the OEM carb:
View attachment 143080
Here's the breakdown on the carb
View attachment 143081
There is a thick plastic spacer under the needle e-clip and a spring seat on top of the e-clip

If it was a Mikuni BST-34 or 40 carb, then the assembly is as pictured - if it is a Chinese counterfeit, it's anybody's guess as to what is right. Only the carb manufacturer knows and they are many times defective. For my money, I would find one or two Mikuni carbs and get a quality Shindy or All Balls carb rebuild kit as a starting point.

I see you do not understand the operation of the CV carb.

As the velocity of air passing through the venturi increases, the slide rises - if you are holding the engine speed steady and the slide has risen 1/4 inch and you 'snap' the throttle open, the slide drops - this is in an attempt to keep the velocity of the air passing through the venturi constant - as the engine speed increases (and thus the velocity of air) the slide will rise - at idle the slide is usually only slightly 'bouncing' or is bottomed out and stationary - as you increase engine speed, the slide will rise and drop with the 'pulse' of air passing through the venturi as the intake valve opens and closes.

Being a mechanic is one thing, being a tuner is another thing and being able to solve a puzzle is extraordinary.

You have made excellent progress, not it's just a matter of getting it tuned - don't waste time on a POS Chinese carb, but don't get me wrong - there are quality Chinese carbs to be had - I have had success with KF Standard of Japan, Kun Fu and on small two strokes Sun, but it's easier to tune with Mikuni and Keihin because you can get parts - with the Chinese counterfeit carbs, they might look the same, but Mikuni and Keihin parts usually will not fit - the main jet for a Mikuni may not have the same threads as the Chinese carb and the main jet in the Chinese carb does not usually have a size stamped into it - this is because the Chinese size the jet to the flow characteristics of each individual carb - their quality control is not good enough for every carb to be jetted the same as the next one - thus 10 (or more) of the same brand, same size and same model of Chinese carb may all have different sized jets and since the drilled air passages are not precise, the settings of the slide needle and fuel screw are all different - there is no published factory setting - the setting is the setting that gets the carb from the assembly line to the box that it is sold in. When working with Chinese carbs, they must be calibrated for the engine they are installed on and that is the job of the mechanic installing the part and tuning the engine.
Yep, listen to latebird. He probably knows more than all of us put together . If anyone can help he you can. I can say for sure he’s correct about the Chinese carbs . I order a pair for a older brute force back in the mid 2000s and both came from same place same names and both had totally different jets even different needles. Like they are just thrown together from a pile of parts with no qc
 

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Discussion Starter #23
You are running it with the airbox and stock "snorkel" installed, right?

If not, it will NOT open the throttle. Even with the snorkel missing, those CV carbs won't open properly.

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someone had removed the snorkel from the top of the air box, i cant see that having much effect but if you say so i can try to close that hole some to get some more vacuum and see what happens, i ran it with the air box and old air filter on and i ran it totally open with no air box at all , just open carb, it has no effect at all. I guess i can try looking at the intake manifold boot to see if it is ok, I haven's looked at the plug since yesterday when i got it runni9ng a little better by putting that piece in the right spot above the jet needle , in fact in haste i actually broke my brand new spark plug when the plug tool apparently slipped off , so im running and old shitty spark plug now, that i can see works but not exactly right. Before i changed the jet needle configuration it was way to rich, it made lots of carbon really fast. I can't imagine i have a vacuum leak at the intake boot.
 

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Discussion Starter #24
Here's the specs on the OEM carb:
View attachment 143080
Here's the breakdown on the carb
View attachment 143081
There is a thick plastic spacer under the needle e-clip and a spring seat on top of the e-clip

If it was a Mikuni BST-34 or 40 carb, then the assembly is as pictured - if it is a Chinese counterfeit, it's anybody's guess as to what is right. Only the carb manufacturer knows and they are many times defective. For my money, I would find one or two Mikuni carbs and get a quality Shindy or All Balls carb rebuild kit as a starting point.

I see you do not understand the operation of the CV carb.

As the velocity of air passing through the venturi increases, the slide rises - if you are holding the engine speed steady and the slide has risen 1/4 inch and you 'snap' the throttle open, the slide drops - this is in an attempt to keep the velocity of the air passing through the venturi constant - as the engine speed increases (and thus the velocity of air) the slide will rise - at idle the slide is usually only slightly 'bouncing' or is bottomed out and stationary - as you increase engine speed, the slide will rise and drop with the 'pulse' of air passing through the venturi as the intake valve opens and closes.

Being a mechanic is one thing, being a tuner is another thing and being able to solve a puzzle is extraordinary.

You have made excellent progress, not it's just a matter of getting it tuned - don't waste time on a POS Chinese carb, but don't get me wrong - there are quality Chinese carbs to be had - I have had success with KF Standard of Japan, Kun Fu and on small two strokes Sun, but it's easier to tune with Mikuni and Keihin because you can get parts - with the Chinese counterfeit carbs, they might look the same, but Mikuni and Keihin parts usually will not fit - the main jet for a Mikuni may not have the same threads as the Chinese carb and the main jet in the Chinese carb does not usually have a size stamped into it - this is because the Chinese size the jet to the flow characteristics of each individual carb - their quality control is not good enough for every carb to be jetted the same as the next one - thus 10 (or more) of the same brand, same size and same model of Chinese carb may all have different sized jets and since the drilled air passages are not precise, the settings of the slide needle and fuel screw are all different - there is no published factory setting - the setting is the setting that gets the carb from the assembly line to the box that it is sold in. When working with Chinese carbs, they must be calibrated for the engine they are installed on and that is the job of the mechanic installing the part and tuning the engine.
Thanks for the help, i do understand the function of the CV carb though, i have not read your entire response but i will read it. What i am saying is the slide does not raise when i hold the throttle in a fixed position, the engine does not rev to increase the air flow to make it raise up, if that makes sense, i have a little trouble explaining this. Already i have found a major issue with the way i assembled the carb initially, i had piece number 1 in the wrong spot, i put it back together the way it came apart and it was wrong. So i put that back in place on top of the E clip and it runs better but still no power, very sluggish and just will not rev right, the slide does open a little now but half way at best and very slowly , it has the power of a 125 or maybe a 200 , lol. I have not checked the plug since yesterday but want to add that i broke my good spark plug and am using an old shitty plug that i can see doesn't work right, it appears to have a cracked insulator and often sparks to the side and never from the tip of the electrode like its supposed to . So i know i need a new plug again, any suggestions on the best plug? I had champion but most suggest NGK always for ATV's and bikes. Once i get another plug i can look at it and see if it is lean or rich, as you can imagine the plug was completely carboned before i moved piece number one to above the e clip instead of below it. Like i said before when i got this it was all screwed up, missing the air screw spring , missing the float needle clip, the new rebuild kit i got on ebay might be the whole problem right now since the float needles would not seal for one, so who really knows if the jets are even close to spec, my next plan is to completely get rid of everything in the rebuild kit and clean and re use the old parts, well i will keep the new bowl gasket of course it actually works, lol. None of the new jets had any markings for size and the old ones are not visible . And one more question , how do i know which mikuni carb i have as far as 34 to 40 mm? And if i want to upgrade to a 40 mm what year can i search that will interchange ? For real this ATV is supposed to be for my girlfriend so it doesn't have to be the fastest in the world but i still want it to run at least half way decent for her, its so sluggish and weak it makes it almost dangerous on the other end of the spectrum, lol. I have a Clymers manual and it is hard to navigate and incomplete on a few things i have found already, and it was false advertised as having color pictures, the pictures are so low quality i had to just keep guessing on the cam decompression spring until i got it right, lol. And i have heard of people deleting the decompression function of the cam, but this ATV will not turn over without it, you can't even pull with the rope without it, it stops the starter dead without it, so i don't know how some people are getting by without it.
 

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Yes, you have to have the airbox lid AND "snorkel" on and just removing the snorkel alone messes up the vacuum needed.

I'm not sure exactly why, because the CV carb used on the Z400/KFX400/DVX400 responds just fine to no airbox lid, a pod filter & rejetting. But these on the Scrambler and Sportsman 500s absolutely do not. They do exactly as you describe yours is doing without the factory setup on it.



Sent from my SM-G930R4 using Tapatalk
 

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Champion spark plugs are only good if used for a plumb bob or a fishing weight - Bosch, AC Delco, Nippon Denso, NGK, Autolite, Motorcraft and Lodge are all good plugs.

The slide rises from vacuum which is internally ported to the diaphragm chamber - the air box is restrictive and increases vacuum so it will not respond correctly without the restriction.
 

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Discussion Starter #27
Here's the specs on the OEM carb:
View attachment 143080
Here's the breakdown on the carb
View attachment 143081
There is a thick plastic spacer under the needle e-clip and a spring seat on top of the e-clip

If it was a Mikuni BST-34 or 40 carb, then the assembly is as pictured - if it is a Chinese counterfeit, it's anybody's guess as to what is right. Only the carb manufacturer knows and they are many times defective. For my money, I would find one or two Mikuni carbs and get a quality Shindy or All Balls carb rebuild kit as a starting point.

I see you do not understand the operation of the CV carb.

As the velocity of air passing through the venturi increases, the slide rises - if you are holding the engine speed steady and the slide has risen 1/4 inch and you 'snap' the throttle open, the slide drops - this is in an attempt to keep the velocity of the air passing through the venturi constant - as the engine speed increases (and thus the velocity of air) the slide will rise - at idle the slide is usually only slightly 'bouncing' or is bottomed out and stationary - as you increase engine speed, the slide will rise and drop with the 'pulse' of air passing through the venturi as the intake valve opens and closes.

Being a mechanic is one thing, being a tuner is another thing and being able to solve a puzzle is extraordinary.

You have made excellent progress, not it's just a matter of getting it tuned - don't waste time on a POS Chinese carb, but don't get me wrong - there are quality Chinese carbs to be had - I have had success with KF Standard of Japan, Kun Fu and on small two strokes Sun, but it's easier to tune with Mikuni and Keihin because you can get parts - with the Chinese counterfeit carbs, they might look the same, but Mikuni and Keihin parts usually will not fit - the main jet for a Mikuni may not have the same threads as the Chinese carb and the main jet in the Chinese carb does not usually have a size stamped into it - this is because the Chinese size the jet to the flow characteristics of each individual carb - their quality control is not good enough for every carb to be jetted the same as the next one - thus 10 (or more) of the same brand, same size and same model of Chinese carb may all have different sized jets and since the drilled air passages are not precise, the settings of the slide needle and fuel screw are all different - there is no published factory setting - the setting is the setting that gets the carb from the assembly line to the box that it is sold in. When working with Chinese carbs, they must be calibrated for the engine they are installed on and that is the job of the mechanic installing the part and tuning the engine.
Oh btw i have the choke way to loose right now so it can not be a problem , if the carb body around it is right, i will adjust it properly though, but to loose makes it not work so it's like not having a choke at all , in my mind that should no effect the running of the carb once it's warmed up , but like i said i will adjust it properly and i meant to do so before i put the top back on since its much easier that way than guessing if i feel spring tension or not.
 

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Oh btw i have the choke way to loose right now so it can not be a problem , if the carb body around it is right, i will adjust it properly though, but to loose makes it not work so it's like not having a choke at all , in my mind that should no effect the running of the carb once it's warmed up , but like i said i will adjust it properly and i meant to do so before i put the top back on since its much easier that way than guessing if i feel spring tension or not.
That carb does not have a choke - it is an enrichener and it works the opposite of a choke - if the 'choke' is not installed properly, it opens an air passage that bypasses the butterfly throttle valve and if installed and opened, adds fuel to the air - having the 'choke' passage open reduces the vacuum pressure between the engine and the carb.
 

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Discussion Starter #29
Yes, you have to have the airbox lid AND "snorkel" on and just removing the snorkel alone messes up the vacuum needed.

I'm not sure exactly why, because the CV carb used on the Z400/KFX400/DVX400 responds just fine to no airbox lid, a pod filter & rejetting. But these on the Scrambler and Sportsman 500s absolutely do not. They do exactly as you describe yours is doing without the factory setup on it.



Sent from my SM-G930R4 using Tapatalk
do you have a picture of a snorkel so i can try to close off the hole in the air box lid to match the snorkel opening ? i guess i could just experiment with it with duct tape and when and if that works just make a plastic or sheet metal plate or make something to restrict air flow enough. I know exactly what you are saying with how it effects the vacuum and slide operation, i almost would cut off some of the diaphragm / slide spring to make it easier to raise up if i had an extra spring, in one performance upgrade kit i bought from sigma 6 they suggested increasing the little hole size in the bottom of the slide or wherever it was on that carb, to make it open the slide faster , a weaker spring would have the same effect , but i really don't want to completely ruin anything and go further backwards on this.
 

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Discussion Starter #30
That carb does not have a choke - it is an enrichener and it works the opposite of a choke - if the 'choke' is not installed properly, it opens an air passage that bypasses the butterfly throttle valve and if installed and opened, adds fuel to the air - having the 'choke' passage open reduces the vacuum pressure between the engine and the carb.
i still call it a choke, i know how it works and it is closed for sure so it isn't pulling extra fuel as long as the carb body around it is good. Oh btw yesterday when i put the carb back together and got it running the way it is now it would not start at all , not even with ether until i reached down and for a split second only covered / choked the carb air inlet with my hand, then after that it was fine, started very easy and ran, just no power and won't rev much and wont open that slide as much as it is supposed to.Maybe if i'm lucky a new xpark plug will cure it, but like i was responding to theCATman i will try covering that missing snorkel hole also and spray some ether around the intake boot while its running to try to check for leaks there. i cant see any intake leaks but that doesn't mean they arent there.
 

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The Scrambler did not have a true snorkel - here's what the air box looks like - the lid is solid except for the little intake spigot on the front of it - part #1 is a 'pre-cleaner' - it's a box of screen wire just to help keep out blades of grass, June bugs, sticks and larger debris - it's presence or absence will not affect the performance - the inlet is about the same size as the venturi of the carb - the carb cannot flow any more air than can pass through the venturi, so the enlarging the hole does not affect performance - it's the turns the air has to take to get to the carb that affect it the most and the more times the air has to change direction, the higher the vacuum and the quieter the intake is.

143104
 

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Discussion Starter #32
The Scrambler did not have a true snorkel - here's what the air box looks like - the lid is solid except for the little intake spigot on the front of it - part #1 is a 'pre-cleaner' - it's a box of screen wire just to help keep out blades of grass, June bugs, sticks and larger debris - it's presence or absence will not affect the performance - the inlet is about the same size as the venturi of the carb - the carb cannot flow any more air than can pass through the venturi, so the enlarging the hole does not affect performance - it's the turns the air has to take to get to the carb that affect it the most and the more times the air has to change direction, the higher the vacuum and the quieter the intake is.

View attachment 143104
I will try to fab something to make the effect of the snorkel / intake spigot if the new plug doesn't help, other than that if i do figure out thi8s carb has been ruined and will not work i will buy a 40 mm carb instead of the 34mm carb , as i understand this would be a later year model carb , which one is it that would interchange?
 

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The BST34 is not necessarily a 34mm and the BST40 in not necessarily a 40mm carb - the difference between a standard model Sportsman with a BST34 and an HO model with a BST40 is the 40 is 2mm larger than the 34 - I believe the spigot of both where it inserts into the carb holder is the same OD, so the manifold does not have to be changed.

When you go to a larger carb, you will gain some top end and lose some low end - the throttle response is less crisp on the 40 vs the 34 and the 40 is a bit more finicky at idle, but it's a better choice than a Chinese POS counterfeit.
 

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Discussion Starter #34
The BST34 is not necessarily a 34mm and the BST40 in not necessarily a 40mm carb - the difference between a standard model Sportsman with a BST34 and an HO model with a BST40 is the 40 is 2mm larger than the 34 - I believe the spigot of both where it inserts into the carb holder is the same OD, so the manifold does not have to be changed.

When you go to a larger carb, you will gain some top end and lose some low end - the throttle response is less crisp on the 40 vs the 34 and the 40 is a bit more finicky at idle, but it's a better choice than a Chinese POS counterfeit.
well as i said it is for my girlfriend so i may just be better off sticking with the 34 then, i don't foresee her or want her running wide open down a road to the trail , WOT is seldom used in most riding conditions, would you agree?
 

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Yes - an experienced rider will stab the throttle open for a short distance, but few get out where they can hold it wide open for more than 100 feet
 

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Here’s my current project . It was a total basket case when I got it . Iv about got it ready to sale tho . So I feel your pain buying stuff someone else jacked up Lol this wound up being a huge project from engine build to re painting the plastics and new decals . And all new bearings and seals. Luckily I got it for almost nothing
View attachment 143075
Wow that looks Great!! What kind of paint is it a special plastic paint? I would love to clean mine up 👍🏻
 

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I seen them carbs on EBay pretty cheap ! And they have a ton of good reviews ..I was thinking of getting one for my scrambler since it’s a little hard to start..
Definitely would like to get the bigger carb can’t go wrong with more power haha!
 

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Wow that looks Great!! What kind of paint is it a special plastic paint? I would love to clean mine up 👍🏻
I just cleaned it very thoroughly scuffed it with scotch bright and cleared it. That’s actually what I do for a living . Well cars and bikes not quads but same principle. Iv had good results for a lot of years using ppg dcu2021 clear and bulldog adhesion promotor to make it bite. It works good. These materials are very expensive but it can be done with a cheaper Brand product with about same results . Hope you guys are having a great thanksgiving !
 

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I just cleaned it very thoroughly scuffed it with scotch bright and cleared it. That’s actually what I do for a living . Well cars and bikes not quads but same principle. Iv had good results for a lot of years using ppg dcu2021 clear and bulldog adhesion promotor to make it bite. It works good. These materials are very expensive but it can be done with a cheaper Brand product with about same results . Hope you guys are having a great thanksgiving !
Ouch! That stuff is indeed expensive. The results are great though. (y)
I'm more of a no-shine matte finish kinda guy though lol.
 

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Ouch! That stuff is indeed expensive. The results are great though. (y)
I'm more of a no-shine matte finish kinda guy though lol.
Me to but shinny attracts buyers 🤣 ppg makes a awesome product called flex n flat . It’s great for atv plastic and it’s not shinny. Actually makes it a little more durable than the gloss
 
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