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OK gang, new to ATVs, brand new to forums so bear up with me please.

So we decided on ATV's on our family toys knowing full well that we would have to learn mechanic'n because there is no way we could afford to drag them to the dealership every time something broke. So, we bought this one knowing full well of the issue that I speak of with the assumption that it was a simple carb clean or rebuild. OOPS, what an idiot I was. I'm too embarrassed to say how many hours I have in this thing thus far but suffice it to say I'm at my wits end.

Through the help of the factory service manual and these forums previusly as a spectator only, this is where I stand:

2004 Polaris Magnum 330 4x4, apx 2000 miles.
Starts, idles, perfect (even when blipped or WOT is cut it drops back to perfect idle) but very boggy (not missing like rev limiter? IMO) under throttle from idle and or WOT.
Does not backfire or die unless you simply don't let off throttle at deep bog, but doesn't smoke. You can at times nurse it up to a fairly high rev but soon will begin to bog and not run smooth. This is not an intermittent issue, it happens consistently every time the machine is run, hot or cold.
Determined (IMO) a rich condition exists; removed air filter, condition improved, partially covered air intake and condition worsened, gas could be smelled coming through back side of carb...
Replaced spark plug (2x). It comes out black, somewhat sooty but not wet looking at all that I can tell.
Emptied tank, rinsed (with fresh gas), blew out with compressed air, filled with fresh 100% gas.
Cleaned air filter.
Replaced fuel filter.
Vacuum tested fuel pump and it bled down from 5" to 0 in 10-15 seconds so fuel pump was replaced and tested solid.
Vacuum tested needle and seat at fuel inlet fitting, solid.
Carb rebuild kit and cleaned (factory jets, settings, float height= .55 inches, level).
Tested with 120 and 117.5 main jets, no difference, maybe worse.
Then went the other way (just to say I tried everything even though it ran counter to all the signs) and tried it with 125 and 127.5 jets and the boggy condition worsened. Finally just returned to factory 122.5 main jet.
Each time I've removed carb for cleaning or adjustment I've re-clamped all lines and boots and tested both with and without air filter attached and condition absolutely worsens when air filter (K&N) is attached.
Adjusted valves.
Removed muffler drain/cleanout bolt, no exhaust restriction but did discover a couple minor holes at rear of muffler.
Cleaned all ignition/electrical push plug/connectors with contact cleaner.
Checked all battery/other electrical lugs & connections for tightness, investigated for broken /damaged wires.
Battery tests 12.80V at idle, 14.30 at 1/2 throttle, 14.40 at WOT.
Compression test jumped to 90 PSI on 2nd or 3rd stroke, then to 160 PSI steady after 5-6 strokes.
Removed valve/head cover and checked every component for wear or damage, none present and camshaft mic'd out perfect.
I have not replaced any electrical or ignition parts, but battery faithfully spins the starter strong every time the ignition is engaged.

Sorry for marathon post but I just figured I'd include my entire journal in order to save a few Q&A volleys.

Really the only thing that I know left to do is to replace the ECM but I question whether that would help me out at all on a normally aspirated engine (Carburetor, no EFI)? Hate to spend the money on a no return part only to find out my condition still exists ... Any ideas?
 

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So the carb kit included a new needle and seat? Did you inspect the slide for smooth movement and any tiny holes? Inspected in/out carb boots for tiny holes?

Sometimes 1 or more of these parts has small amounts of wear/holes that cause big problems like this. Luck. :cowboy:
 

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Discussion Starter #3
Yes to all of the above, but will tear carb back down and re-inspect with magnifyer asap.

Thanks for your response...
 

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Have you pulled the brass plug on bottom of carb and adjusted air bleed? Most say turn to lightly seat and back out 2 full turns as a starting point and go from there.

I removed both mine in our trail bosses by slooowly and lightly drilling 1/8" pilot hole just deep enough to use a drywall screw to thread in and vice grips to pull the brass plug out.
 

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No I did not, only reason being everything I read or was told said that the brass plug/screw affected idle only and believe me this thing starts and idles so beautifully I didn't want to mess with the one thing that was right... Thoughts?
 

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If its idleing clean then that should be good...have you tried raising the clip on the needle that will affect the transition from idle to wot.

I know with cars many times guys think their stumble is due to too much fuel/rich... but more often than not it ends up being a lean stumble...so try different positions on your needle and see if allowing more fuel sooner helps.

Also, dont know how a guy would check it on an atv, but sounds like all bases have about been covered, wonder if spark is getting weak...coil going bad, wire going bad, corrosion inside boot...wifes 330 had coil goi g bad and when i went to remove boot from plug and connector inside boot crumbeled and wire waz dislodged where it tied into the 90* contactor that slides over top of spark plug.....just throwing shit out to look at/check.

Guess worst case, call it a great effort and find a shop or somebody that knows these things and maybe theres something they know the boom dont?
 

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Great point, even though I failed to document it I had actually tried moving the clip both up and down one notch at a time and couldn't tell any difference.
I will say this though, as I have it torn back down again I'm going to inspect the float, diaphragm, and all other parts with a magnifier to see if possibly there is a pinhole, crack or tear that I am missing or something. To be honest The coil, wire, and boot crossed my mind as well. If my carb inspection doesn't glean any imperfections I may try replacing them as my next step.

Many thanks, I'll post follow ups, let me know if any other wiz ideas cross your minds....
 

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So a complete disassembly and inspection of the carb netted no new information or evidence of defect, everything is clear and functioning smoothly.

After reassembly I tested 6 different main jets from 115 up to 127.5 with no improvement, still as boggy as ever.

As I move towards ignition (having exhausted every possible air/fuel/exhaust solution I can think of) I'm ordering new coil/wire/cap, hoping to hit on something there.

Hey on that subject, does anyone know exactly what functions the ECM serve on a carbureted engine? Does it have anything at all to do with ignition or only charging system and electric fan functions...? Just trying to determine whether there is any way possible that could be causing the problem before ordering a $200 no return part to throw at it as a hail mary..? Thank you all...
 

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I'm with DoubleJ - its time for OPT. Before you spend more $, give him a quick read, you'll see he knows these older quads inside/out and is willing to help (just don't double post in both Expert and regular within Polaris section of ATVConnection). Let us know what ya find. Luck.
 

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Yes sir just signed up, logged in and re-posted there, anxiously waiting on response.

Thanks for the referrals to another resource, I appreciate it. Will post follow-ups...
 

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The general rule of repair is to diagnose before replacing parts. As you continue to replace parts with no satisfactory results, you ultimately will miss what the issue is. The BST CV34 is a fairly simple carb to diagnose once you understand its fuel introduction process.

My suggestion... STOP!! don't buy parts because you can not find the problem. You're just going to get yourself more worked up than you probably already are.

OK so I have read through your "journal" of attempts and see you most likely passed over the problem thinking you must of eliminated the possibility.

Lets backup and get a handle on what your machine is doing and the reasons why.

Lets start with what your carb does at different throttle positions. Closed throttle; your fuel is being introduced primarily through the fuel mixture and pilot jet. The engine is making minimal vacuum but a running engine strives for more air so it will pull air in from any source it can which is why vacuum leaks usually cause poor idling. You say you don't have this problem. Therefore your motor should have a steady rhythm to its idle. If you hear any irregular rhythm them make sure you inspect for leaks. A can of carb cleaner can assist in checking vac lines for leaks by spraying to see if the RPM increases. As you increase your throttle the carburetor will begin to draw fuel from the main jet as the jet needle rises from the main jet allowing for increased vacuum to the diaphragm. This in turn creates low pressure allowing the vacuum slide (jet block assembly) to overcome the compression spring that is constantly applying down force to the block. At about 1/4 throttle the pilot jet is at its maximum for fuel introduction and the main jet is now the main influence on the engine with the needle jet assembly having some measure of influence. ( What is the engines performance at this point?) As the throttle is increased through to 1/3 and beyond the main jet is now working at full capacity but being regulated by the position of the jet needle. From this throttle position and greater the venturi effect and the fuel introduction work is unison to increase RPM, increasing vacuum, which opens the slide until maximum RPM is attained.

Because these engines can run off of the charging system with minimal battery reserve and you have the engine holding an idle, I would continue to search for your issue within the fuel system and not be concerned with spark. By locating the throttle position relative to the "bogging" you can further look at what part of the carburetor is responsible. Bogging is only one of two possibilities, too much fuel or too much air. If you're not fouling the plug then you have a lean mix problem. Lean mix is either a blocked fuel flow or a secondary air introduction. The same goes for a fouled plug. there is one caveat to this issue and that is restricted air flow before the carb. You said the air filter was cleaned. However if the K&N filter was never cleaned before you bought the ATV then a replacement may be necessary.

Your fouled plug(s) being carboned up would have me going straight to the choke. The choke plug in this carb will stick over time as fuel weeps past the chamber towards the cable side of the plug and becoming varnished. If you didn't or couldn't get it out when doing your carb cleaning then look no further. The plug will fail to return to its fully off position and cause everything you have been experiencing. Removing the choke plug may take lots of soaking in or with a strong cleaner and time. My last one took 3 days to pull.

You mentioned cleaning the carb, however sometimes what you think is clean has only loosened contamination. There is a guy on eBay that sells this carburetor jetted for the 2004 330 for $150. If he still has stock then it's a quick way of solving what I believe is a bad carb issue. When doing a carb rebuild I do a three process three day flush. Yamaha offers a good cleaning solution for soaking carb bodies that we can't get up here that you may want to try. You're going to need an air compressor if you're to blow the chambers clean as well.

Hope this all helps. Don't forget, Edison knew over 700 ways how not to build the light bulb.
 

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OK, very well, all good stuff.

For the record every single thing that you have read that I've done has brought me back to exactly the same condition as when I first started, which is:

Will idle through an entire tank of gas if you let it (with or without air filter installed), sounds like a sewing machine.

With air filter installed, it really doesn't matter whether you attempt to roll it gently or go straight to WOT it bogs. It bogs consistently straight off of idle but you can sometimes feather it gently and attain some RPMs but at some point it's going to bog at which time if you let off completely it will drop back to this beautiful idle. Never dies unless you just stay on throttle when it begins to bog.
During all my hours of testing it has rarely backfired unless I just stay on throttle till it dies, even then very rarely.

So I take off the air filter and it will run measurably better, if you feather the throttle you can eventually get it to run at near full throttle with some response if you blip it a few times or roll off then back on throttle like once it's kind of wound up it has decent throttle response, just don't try to get it up there to quickly or it will fall on it's face.

So there you have it, the couple of things that keep me awake at night are with regards to the air filter, if it were in need of a new air filter would it not run near perfectly by simply removing it temporarily for test purposes as I have done?
On top of that, as I mentioned I hooked my shop back up to the filter and the vacuum never changed pitch, air just seemed to be whooshing through the filter at a really nice rate so I jumped past that as a possibility. Wrong...?

Also, if you truly feel that soaking my completely disassembled carb body and Jets for several days would make a difference I would begin disassembly tonight, I've just been in and out of that carburetor so many times I'm experiencing some serious burnout absent almost guaranteed results, you know what I mean? This choke deal you spoke of I'm pretty certain I've never had out, so I'm open to trying to free it up and removing/cleaning it if you thought that held some hope given my expanded description of the bike's behaviour.

Sorry to whine here but please just appreciate that with forums, service manual reading, and other research added to the actual wrenching time I'm sitting right now between 40 and 50 hours invested, what was once a quest has become quite the nightmare so I do really appreciate your thoughtful responses in an attempt to help me out....
 

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A carb rebuild isn't a rebuild if you didn't service the choke system. These chokes will gum up over time. Everything you have described regarding lack of performance can & maybe contributed to your choke being stuck partially in the on position. Your plug(s) when pulled are black. Not oil but heavy carbon, symptomatic of a rich burn. Increasing the airflow by removing the filter results in a modest positive response because the carb has more air to mix with the fuel. Even if you find the choke functioning, your problem is in the fuel system. Did you put the jet needle back to the middle setting? Best if you do till you solve this issue.

As for soaking the stripped body; if the carb sat for some time with fuel in it then three days to soak is the minimum IMO, unless you use an aggressive cleaner and repeat several times. If you pull everything from the carb body then you could use muriatic acid as a cleaner. Should bring the shine back as well. Just don't leave it soaking in this stuff. We've had to soak two strokes off of outboards for a week or more and still had them need pressured fluid to move the varnish out. I used to double boil my Holly spread bore back in the day too.

Polaris ATV Carburator Fits The 4 Stroke ATVs Mikuni Carb Magnum Sportsman | eBay

I have bought from this seller for my Magnum 425 with 100% satisfaction. The carb listed is jetted and set for your 330. He hasn't changed is price since I bought mine either. $125.00 and you're back to riding.

With 50 hours in just consider how well you now know your machine, and how much quicker you will find an issue in the future as a result. :happy:
 

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Ok, you seem to be a voice of experience, so I'm going with your wisdom.

If I may, so you are suggesting that I do the strip, soak, and clean the carb including the choke mechanism, then once satisfied, re-assemble everything, and if it still persists, go with the new unit, correct?

As I proceed, just a couple of details that scare me a bit;
As I dis-assemble the carb and begin soak, will it hurt the white plastic looking block that the vacuum slide moves up and down in? Is that removeable, and if so is there a trick? I took the screws out of the top of it once and had intentions of removing it but couldn't get it to budge so I re-installed screws and left it be for fear of breaking it or something else.
Secondly, I'm not sure I've seen anything in exploded view or parts listing that is specifically labeled "choke this" or "choke that". As I have torn it apart in previous attempts I would swear I've taken out every removeable piece possible (other than the jet block). Can you give me direction there? Could it be called something else? The fact that it starts so easily (WITHOUT the choke interestingly enough?) has me intrigued to see what I find when taking a look at the choke assembly as I get it apart, just want to be certain that I get it covered this time.
Sorry to wear you out here, thanks for your help again.....
 

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OK; the block assembly that the diaphragm is screwed to slides out with the jet needle sitting under the compression spring. Check the needle for wear. If you feel a ridge towards the bottom from where it sits in the main jet you can try some fine steel wool to remove it. Be gentle or you'll end up replacing it. If after you clean it there is still a contact dent or ridge then replacing it will be required. After you take out the 2 screws from the block, turn it (the carb) over, remove the main jet and push the plastic frame out from the carb body. you can try using a small socket and very light tapping in the hole to get it moving. Sometimes it helps to put the carb top back on with a couple of screws so you can stabilize the carb while you tap the jet block out. Once you have it out unscrew the needle jet for cleaning. The jet block should only need some carb cleaner sprayed over it to complete the cleaning. Note that there is a bump in the edge of the diaphragm that sits in the slot of the carb body to make sure you haven't put the assembly in reversed.

Now for the choke. If you have a manual go to the exploded parts view and locate item #46 referred to as "the plunger". It will have your choke cable or handle directly connected to it. You should have a cable running from the handle bars but sometimes people will discard the cable in favor of the pull handle. Find the nut that holds in the plunger and undo it. A working choke will slide right out. If the assembly wont pull out do not force it! Getting the plunger out will need a strong but safe penetrating oil and patience. My last one took 2 days to remove.

The reason I included the link is to give you an option. When you start adding up internal parts of the carb it is easy to spend more than just buying a new one for that price. Don't know where that guy gets his supply but when you consider the time, effort, and costs, $125.00 for a drop in and go speaks for itself. Just replacing the diaphragm and needle jets will cost the same. Who knows how long his supply will last?

If your carb is fairly clean, pick up a couple of cans of throttle body or carb cleaner and do a series of spray and soaks, followed by blowing clean with lots of air. I like to use both cleaners and repeat till I run out of cleaner from the two cans I bought. I always let the carb sit over night after the first initial spray application.

One thing you should do is make a list of all the jet numbers for comparison with what is stock. I believe your needle jet should only have 6 holes and your jet needle has very little taper. We can compare numbers later if you run into problems finding the right numbers, keep in mind the Polaris manual has all the numbers listed.

I'll try to check my email during the day in case you have questions.
 

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No worries, enjoy your day at work, it will be this evening or possibly tomorrow evening before I will have time to tear back into it.

Will be in touch, thanks again, great stuff...
 

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Ok- removed and disassembled carb.

Choke plunger fell out freely once external nut was backed off (out), no signs of buildup or other issues.

But here's what I did find: as I inspected the needle, it likewise moves freely in the jet, with no roughness or ridges as I would describe it.

I did find however a very noticeable hump right at the point on the needle where it would be the top of the area that slides up and down in the jet. So much so that you can easliy feel the transition (albeit smooth) from the fatter upper part to the skinnier lower 2/3 of the needle.
I mic'd it at both the larger diameter (.0990) and the smaller dia (.0865) for an obvious .0125 disparity between the upper and lower portion of the needle.

My question is (and please don't laugh); when new is this needle the same diameter from top to bottom, and would this level of wear be sufficient to be the cause of my problem..?
 

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Ive got 2 new carb rebuilt kits in the shed for wife and daughters trailbosses. If nobody responds by tomorrow evening when i get back home ill grab one and mic a new one to see what its like ao you can rule it out or needs replacement.

Sorry i cant do it sooner but need to get some side jobs going, planned to take friday off to do them but now 60% rain so gunna try to get a good start after work tomorrow.
 
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