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RTV - a carbs worst enemy - I've pulled or blown RTV out of air passages, fuel passages and fuel screens of certain carbs. RTV cannot be found in my shop - it is not gas or oil resistant and there are so many more better sealants on the market. RTV is OK for sealing water out of something in an area where it is not exposed to sunlight and may last for a few years, but it's uses are very narrowly limited.

If it is running rich at idle - the fuel screw needs to be turned in or the pilot jet needs to be decreased in size - depending on the brand of carb, the pilot jet may not be marked with a size, may not be replaceable or may be removable, but a different size jet may not be available as many Chinese carbs have a different thread than the Mikuni carb they counterfeited.
 

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Discussion Starter #62
4s carbs are fuel flow screws to fine tune idle-1/4 throttle mix, 2s are air flow screws.

My thoughts are a vacuum leak if it's racing at idle and no real adjusting seems to help. Or the pilot jet in this aftermarket carb is way too lean.

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its not racing at idle, it almost will not idle, its to slow and sputtering and if you adjust the idle screw up to where it will idle continuously then it is obviously opening the butterfly valve as if you are giving it throttle to go , I'm thinking it is way to rich most likely , with the lid on the airbox it really just wont take gas and sputters at any RPM , in fact it belches black smoke , and misses and sputters with the lid on, take the lid off and it runs great Id say its way to rich if anything, they advertised it as fitting all of the polaris 500 line up from them years and we have talked about the Sportsman being jetted richer from the factory, i guess thats why usually a scrambler will not run right without the air box closed because of lean jetting, I'm guessing just like a jet kit i bought for my arctic cat 650 from sigma 6 instructed to drill the vacuum hole in the slide bigger for faster throttle response this carb has that done because this thing responds fast, almost to fast for my girlfriend to react to in tight spots, sorta like a really hot 2 stroke , lol. Back to the idle problem and air screw. i am thinking the air screw is to rich and i know it only effects idle and just a really small effect across the board , if i can get it to idle i will fab my own intake box somehow to protect the filter and open that exhaust up to take the richer carb if i have to , i like the power its impressive
 

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Discussion Starter #63
RTV - a carbs worst enemy - I've pulled or blown RTV out of air passages, fuel passages and fuel screens of certain carbs. RTV cannot be found in my shop - it is not gas or oil resistant and there are so many more better sealants on the market. RTV is OK for sealing water out of something in an area where it is not exposed to sunlight and may last for a few years, but it's uses are very narrowly limited.

If it is running rich at idle - the fuel screw needs to be turned in or the pilot jet needs to be decreased in size - depending on the brand of carb, the pilot jet may not be marked with a size, may not be replaceable or may be removable, but a different size jet may not be available as many Chinese carbs have a different thread than the Mikuni carb they counterfeited.
i don't like using RTV either but it was my only way to know if the diaphragm was sealing properly, it seemed a little loose around the plastic washer part in the middle., it could spin freely around it , i was really careful and meticulous to not get the RTV anywhere it could get sucked into the carb. and it already wasn't working right so there wasn't much to lose in that situation. As far as that new carb from maxspeedingrods , i am trying to get customer support to tell me how it is jetted and adjusted from them, it does seem to me to be to rich since it runs great without the air box lid on , but will not sustain an idle , if i adjust the idle screw until it actually opens the throttle as if i were pushing on the throttle just a little it wants to idle to high, When adjusted where it should be it idles way to slow and rough. Thats why i think it's just to rich, either the pilot is to heavy or the air mix screw is to rich , so in is more rich and out is leaner correct?
 

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so in is more rich and out is leaner correct?
Fuel screw - in is leaner out is richer - on a quality carb adjustments are noticeable in 1/8 turn increments - I'm guessing you are 1/2 to 1 full turn out too rich and about 4 sizes too big on the main jet - if you go all the way in on the fuel screw with no change, either the pilot jet is 1 or two sizes too big or the air passage for the idle circuit is restricted or obstructed.

I've seen Chinese carbs that were undrilled for the idle circuit air passage - the operation during manufacture was simply missed, skipped or a drill broke during a previous operation and the passage simply was not drilled full depth.

Quality control would have caught it, but the Chinese have lax if any QC.
 

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Discussion Starter #65
RTV - a carbs worst enemy - I've pulled or blown RTV out of air passages, fuel passages and fuel screens of certain carbs. RTV cannot be found in my shop - it is not gas or oil resistant and there are so many more better sealants on the market. RTV is OK for sealing water out of something in an area where it is not exposed to sunlight and may last for a few years, but it's uses are very narrowly limited.

If it is running rich at idle - the fuel screw needs to be turned in or the pilot jet needs to be decreased in size - depending on the brand of carb, the pilot jet may not be marked with a size, may not be replaceable or may be removable, but a different size jet may not be available as many Chinese carbs have a different thread than the Mikuni carb they counterfeited.
Fuel screw - in is leaner out is richer - on a quality carb adjustments are noticeable in 1/8 turn increments - I'm guessing you are 1/2 to 1 full turn out too rich and about 4 sizes too big on the main jet - if you go all the way in on the fuel screw with no change, either the pilot jet is 1 or two sizes too big or the air passage for the idle circuit is restricted or obstructed.

I've seen Chinese carbs that were undrilled for the idle circuit air passage - the operation during manufacture was simply missed, skipped or a drill broke during a previous operation and the passage simply was not drilled full depth.

Quality control would have caught it, but the Chinese have lax if any QC.
thanks for the reply, since you know about carbs pretty good i would like to ask you your opinion on a mikuni TM 42 carb, a guy on marketplace is selling one for $50.00 . I will need some carb work done or a new carb for a 2007 predator 500 that i am waiting for the jug and big bore kit to come back to me. it will have 12.5 compression and be about a 560 , i didn't want the kit but the cylinder was junk without it, it had a cracked water jacket, at least that's what Kustom Craft told me. I'm osing faith in them, ive caught them already misleading me about them doing nikasle recoating and they have had my jug and a good amount of money for about 3 months now. But anyway how can someone even find out how much a carb can flow or how to even figure out what our engines needs are? I tried looking up specs but can't find anything at all on the subject .
 

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Discussion Starter #67
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.



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i think its because the scrambler is jetted at 125 on the main and the sportsman is jetted at 145, im trying to put together a Chinese carb that wouldn't idle and when it ran with me revving it it ran like a scalded dog but way to rich, so i took it apart and it had a 1.56 main jet in it, so i am putting a 145 i had from the rebuild kit i had bought , the pilot seems the same from the kit to the carb , the air fuel screw was adjusted to about 1.25 or 1.5 out, but as i search through the years i see that was what a 99 plus should have, i really dont know what size the pilot is , im just using a cleaning kit to compare sizes but if it is a smaller jet like the 99 plus is supposed to have i am wondering if it will ever idle, or if the air screw adjustment wil help, its a crap shoot and im sick of taking this thing apart, lol.
 

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Discussion Starter #68
thanks for the reply, since you know about carbs pretty good i would like to ask you your opinion on a mikuni TM 42 carb, a guy on marketplace is selling one for $50.00 . I will need some carb work done or a new carb for a 2007 predator 500 that i am waiting for the jug and big bore kit to come back to me. it will have 12.5 compression and be about a 560 , i didn't want the kit but the cylinder was junk without it, it had a cracked water jacket, at least that's what Kustom Craft told me. I'm osing faith in them, ive caught them already misleading me about them doing nikasle recoating and they have had my jug and a good amount of money for about 3 months now. But anyway how can someone even find out how much a carb can flow or how to even figure out what our engines needs are? I tried looking up specs but can't find anything at all on the subject .
Well i gave a bad review and bad feedback on the ebay carb, they said return it and no one else has had any problems , but anyway they issued a refund and don't want it returned so i am going to try to use it, i can't find another carb on ebay or amazon that i would trust, they say they fit all the way through some 700cc machine, lol. But anyway the jet in the ebay carb has 1.56 stamped on it, i assume thats a 156 or 155 , the ebay rebuild kit has 145 jets, i know the sportsman 500 with the same engine has 145 jets, but the pilot i have no clue on, the rebuild kits and the ebay have the same pilot, and the same jet needle and the same part that the jet needle goes into and the main jet screws into, it looks like the BST 34 carb, so i know that carb ran before but wouldn't idle unless i turned it way to far up like partial throttle, it ran great with tons of power but was to rich, so im going to put in the 145 since my snorkel like piece is missing and i dont know what to do about the pilot jet , but i adjusted the air screw to two turns out like a 1997 is supposed to be.
So now i am asking what would you do if you were trying to do this? I know the main is primarily for the top end 3/4 to WOT but if it feeds the jet needle it has to make a difference right? do you think this will work? i don't mind slightly rich but it was exhaust clogging rich before . Or should i try the stock jets from my mikuni in the chinese carb? i mean a 145 runs in the sportsman so why not a scrambler? Like i said im really low altitude and i did see achart somewhere on recommended jetting for altitude and temperature and even that chart said the scrambler needed a 145 at my altitude. I can not find this chart now, and thought it was in my clymbers manual buti can't find it. there is a chart with the sportsman and scrambler listed with all this somewhere i just can't find it right now. Anyway if i get this going i will take you up on sending you that carb, i just want this thing running and i can't find a used carb anywhere so i have to do something
 

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Discussion Starter #69
@latebird transitioned this discussion to private conversation
Well i gave a bad review and bad feedback on the ebay carb, they said return it and no one else has had any problems , but anyway they issued a refund and don't want it returned so i am going to try to use it, i can't find another carb on ebay or amazon that i would trust, they say they fit all the way through some 700cc machine, lol. But anyway the jet in the ebay carb has 1.56 stamped on it, i assume thats a 156 or 155 , the ebay rebuild kit has 145 jets, i know the sportsman 500 with the same engine has 145 jets, but the pilot i have no clue on, the rebuild kits and the ebay have the same pilot, and the same jet needle and the same part that the jet needle goes into and the main jet screws into, it looks like the BST 34 carb, so i know that carb ran before but wouldn't idle unless i turned it way to far up like partial throttle, it ran great with tons of power but was to rich, so im going to put in the 145 since my snorkel like piece is missing and i dont know what to do about the pilot jet , but i adjusted the air screw to two turns out like a 1997 is supposed to be.
So now i am asking what would you do if you were trying to do this? I know the main is primarily for the top end 3/4 to WOT but if it feeds the jet needle it has to make a difference right? do you think this will work? i don't mind slightly rich but it was exhaust clogging rich before . Or should i try the stock jets from my mikuni in the chinese carb? i mean a 145 runs in the sportsman so why not a scrambler? Like i said im really low altitude and i did see achart somewhere on recommended jetting for altitude and temperature and even that chart said the scrambler needed a 145 at my altitude. I can not find this chart now, and thought it was in my clymbers manual buti can't find it. there is a chart with the sportsman and scrambler listed with all this somewhere i just can't find it right now. Anyway if i get this going i will take you up on sending you that carb, i just want this thing running and i can't find a used carb anywhere so i have to do something
 

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The jetting is as individual as the carb and you have to work with them making changes to each of the 3 circuits to get each one right and overlapping correctly - this can be done on the vehicle (engine) or electronically on a device called a flow bench. With a flow bench you can electronically detect the amount of fuel mixed with the air and make changes to get a linear mixture based on the flow characteristics of the engine being tuned.

The setting on the fuel screw is the initial setting - the initial setting is just to get the engine started and warmed up to operating in preparation for the final setting. The final setting is obtained by connecting an accurate tachometer to the engine and turning the fuel screw in by 1/8 turn increments pausing 10 to 15 seconds between changes - when the engine idle drops by 50 rpm, the fuel screw it turned out one full turn from that position and that is the accepted final setting, but sometimes the throttle response can be improved slightly by adjusting in or out about 1/2 turn - this is to compensate for fuel quality and air density, but the average rider will not be sensitive to this change.

Yes, tuning a carb is time intensive as can be adjusting fuel injection - the difference is a carb requires physical and material changes - EFI can be adjusted electronically requiring a tuning device (computer and software) and a trained technician to make the adjustments based on dyno output recordings or rider input. I can rebuild a carb and get satisfactory results, but I refuse to waste my time performance tuning a utility vehicle. Getting an engine running right is sometimes as simple as an adjustment and sometimes requires replacing worn parts, but when replacing standard parts with non-standard parts just increases the number potential variables.
 

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Discussion Starter #71
The jetting is as individual as the carb and you have to work with them making changes to each of the 3 circuits to get each one right and overlapping correctly - this can be done on the vehicle (engine) or electronically on a device called a flow bench. With a flow bench you can electronically detect the amount of fuel mixed with the air and make changes to get a linear mixture based on the flow characteristics of the engine being tuned.

The setting on the fuel screw is the initial setting - the initial setting is just to get the engine started and warmed up to operating in preparation for the final setting. The final setting is obtained by connecting an accurate tachometer to the engine and turning the fuel screw in by 1/8 turn increments pausing 10 to 15 seconds between changes - when the engine idle drops by 50 rpm, the fuel screw it turned out one full turn from that position and that is the accepted final setting, but sometimes the throttle response can be improved slightly by adjusting in or out about 1/2 turn - this is to compensate for fuel quality and air density, but the average rider will not be sensitive to this change.

Yes, tuning a carb is time intensive as can be adjusting fuel injection - the difference is a carb requires physical and material changes - EFI can be adjusted electronically requiring a tuning device (computer and software) and a trained technician to make the adjustments based on dyno output recordings or rider input. I can rebuild a carb and get satisfactory results, but I refuse to waste my time performance tuning a utility vehicle. Getting an engine running right is sometimes as simple as an adjustment and sometimes requires replacing worn parts, but when replacing standard parts with non-standard parts just increases the number potential variables.
yes i know, im not expecting any miracles , but judging from what was happening i would have to think since it would not idle and the air screw was only like 1 1/8 turns out it was to lean at idle, so i would assume 2 turn out would be in the right direction, as for the mid range all the way to WOT since when i could keep it running by revving it it was way to rich and carboned everything up i think the smaller main jet would affect how much fuel is let through as the jet needle raises and on the WOT , so i would assume i am on the right track. That could be a possiblity right? To lean on idle circuit , it took gas easily so i think leaving the pilot alone is ok, it seems like they used a BST 34 carb body style that is ever so slightly bigger diameter since it was extremely tight in the intake boot but the jetting from the BST 40 carb for newer and bigger machines, at least thats what i am thinking. How much difference would it really make to go from a 156 to a 145 main jet? is it enough ? If you were doing this , and i know you wouldn't be in real life since you don't play games with junk, what would you do about the pilot jet ? I've got my plan on the main and air screw . it might even work, and like i said i can't find a good used stock carb anywhere.
 

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yes i know, im not expecting any miracles
Back the fuel screw out to get the idle right - if you exceed more than 4 full turns to get it to idle, then go one size larger on the pilot jet.

The main jet only comes into play between 3/4 to fully raised vacuum slide - between 1/8 and 3/4 slide opening it's the needle and needle jet - if it's too rich in the mid range, lower the slide needle. If you get it running right in the mid range and it falls on it's face near top end, then go smaller on the main jet.

Only make one change at a time so if it has to be changed back, you know what to change.
 

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Discussion Starter #73
Back the fuel screw out to get the idle right - if you exceed more than 4 full turns to get it to idle, then go one size larger on the pilot jet.

The main jet only comes into play between 3/4 to fully raised vacuum slide - between 1/8 and 3/4 slide opening it's the needle and needle jet - if it's too rich in the mid range, lower the slide needle. If you get it running right in the mid range and it falls on it's face near top end, then go smaller on the main jet.

Only make one change at a time so if it has to be changed back, you know what to change.
thanks man, i just hate all the assembly and reassembly but i guess it has to be done. I sorta think the mid is to rich since when i tried to put the airbox lid on it instantly stops taking gas well, so the slide is probably not raised up much at all, it makes carbon about like when you light an acetylene torch without oxygen , lol With the lid off it runs great but also has the carbon and black smoke i dont like free revving engines to much but when it was revved pretty high it made the black smoke also, and the slide spring seems weaker than the stock one. so many variables to think about , lol
 

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Yep - two completely different carbs - the only thing the same is the appearance
 

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Discussion Starter #75
Back the fuel screw out to get the idle right - if you exceed more than 4 full turns to get it to idle, then go one size larger on the pilot jet.

The main jet only comes into play between 3/4 to fully raised vacuum slide - between 1/8 and 3/4 slide opening it's the needle and needle jet - if it's too rich in the mid range, lower the slide needle. If you get it running right in the mid range and it falls on it's face near top end, then go smaller on the main jet.

Only make one change at a time so if it has to be changed back, you know what to change.
I finally got a chance to look at it again, i have bought a new home and have tone of loose ends to get done, and weather had been bad and i just couldn't stand to screw with the scrambler for a while , but i had the carb already on and hooked up with my adjustments , if i remember right i had the air screw at 2 turns out, i lowered the jet needle one slot , i changed the main from a 155 to a 145. I think i have the choke set right and do have the throttle cable right. So it did not want to start at first. Then the plug was dry but filmy feeling , of course it had sat in the engine not running through cold weather. I finally got it to start after tapping on the carb bowl , probably a stuck float , then draining the bowl . through running it it seems like a hanging up float might be an ongoing problem , not allowing fuel delivery from time to time . When running it seems a little sluggish low end but then seems to pick up ok through mid to top. but i don't have anything to compare to so i don't know how its supposed to run. So i might try to adjust the air screw out just a little to see if i get a little better throttle response. Also i can't seem to get the air intake boot to stay on the carb side from the air box, i can't find a hose clamp thin enough and my zip tie might need cut off and a new one installed but the hoses also come disconnected from the air box, it is stuck in 4x4 at the moment , the switch does not stay pushed in so im sure it needs a cleaning or replaced and who knows what is going on in the hubs . also the clutch seems to take to long to engage, could just be a worn belt but i have heard these are just like that somewhat. other than that i think i just needs a seat cover before my brittle seat cracks and i lose the foam and the tires are original i think and are to hard for some reason. i would like slightly more ground clearance but the gearing is tall enough so i wont change tore sizes. I know we talked about that original carb but i have decided to keep it and get a good rebuild kit for it with the correct parts and all the parts it needs, maybe one jet size bigger or something but nothing to much.
 

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Discussion Starter #76
Yep - two completely different carbs - the only thing the same is the appearance
Remember i was talking about that 2007 predator that i sent the jug off to Kustom Craft? i still haven't gotten that back and the last time i talked to them was over a month ago, they told me there bore machine broke a tool or something but they had a new one and was getting it done right then. but a month later nothing, its been over 7 months now since i sent it in, i think i will never get it back or my close to $500 that i paid them. What can i do about it? What would you do? I could call them again but i already have several times and its always some crazy story i get and another false promise. I know things happen but they have not called me one time to let to let me know anything
 

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Remember i was talking about that 2007 predator that i sent the jug off to Kustom Craft? i still haven't gotten that back and the last time i talked to them was over a month ago, they told me there bore machine broke a tool or something but they had a new one and was getting it done right then. but a month later nothing, its been over 7 months now since i sent it in, i think i will never get it back or my close to $500 that i paid them. What can i do about it? What would you do? I could call them again but i already have several times and its always some crazy story i get and another false promise. I know things happen but they have not called me one time to let to let me know anything
What do you mean you paid them? You don't pay till the job is done and they are ready to ship it out.

Sending you a PM for some personal information
 
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