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Discussion Starter #1
My 335 was gone through pretty thoroughly over the winter, and was running great with the exception of a hesitation and miss at 1/4 to 3/4 throttle. At first, it was just a slight issue, but it seems to be getting worse. It idles fine and for as along as I want it to. Starts fine also. Once I get past the miss going up a hill at 3/4 to full throttle, it runs strong and pulls hard with little to no miss or hesitation during the pull up the hill. Once I let off at the top, and get back into it below full throttle, it starts to hesitate and miss again. No backfiring or popping, just hesitation and miss. I put a new plug in over the winter. Using new gas. Carb was THOROUGHLY cleaned over the winter. There were no clogged orifices at that time. Any thoughts on what this could be or what I should do to troubleshoot?
 

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Discussion Starter #2
Picture of spark plug

I took out the plug and pic is attached. Obviously fouling out. I put a new plug in, and it runs like brand new (for now). No hesitation or miss. So it seems to be obviously a carb issue. The problem is, I didn’t change anything in the carb from factory, and it’s the factory mikuni carb that I cleaned.
 

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Obviously over fueling - possible causes; (outside the carb) dirty air filter or obstructed air intake duct (inside the carb) incorrect float level, damaged slide diaphragm not lifting slide properly (pin hole in the diaphragm can do this), clogged internal vacuum passage to slide vacuum chamber, jet loose or fell out inside float bowl, leaky o-ring sealing float needle seat to carb body, damaged main or pilot jet, slide needle not assembled correctly, damaged, incorrectly installed or missing vacuum slide spring.

There are other possibilities within the carb, but it depends on the definition of 'thoroughly cleaned' - for some, thoroughly cleaning is taking the carb apart and spraying it with aerosol cleaner, for other is taking the carb apart and soaking in Simple Green, others wash it in gasoline and call it done. To me thoroughly cleaning is total disassembly (removing every non-pressed in part) and cleaning all the parts in a heated sonic cleaner with a specialized cleaning solution. The cleaning solution I use cost $100 a gallon. Heated to 180 degrees F, it will remove paint from the exterior of a carb, but it will not harm rubber. It removes varnish, oxidation and algae (algae grows in side the float bowl due to alcohol carrying water into the chamber).

Alcohol can be derived from many sources. Methanol alcohol was distilled from wood, Ethanol is produced from grains or other renewable agricultural and forestry feedstocks. Isopropyl alcohol is primarily produced by combining water and propene in a hydration reaction. And there are many other types of alcohol, but the three aforementioned are the most common, produced in large quantities and commonly mixed with gasoline. To learn more about alcohol, go to https://www.sciencedirect.com/.

Before alcohol can be blended with gasoline, it has to be dried - distilled alcohol contains a small percentage of water - ethanol must have at least 99.2% purity [119], which is obtained by dehydration or drying of the ethanol after the distillation. As little as .4% water in a gallon of gas will cause ethanol to separate from gasoline.

I digress - check the intake and re-rebuild the carb if no restrictions are found to the air induction.
 

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Discussion Starter #4
My version of thoroughly cleaned was to disassemble every non-pressed in part, soaking in one of those gallons of carb cleaning solution, meticulously brushing every piece and orifice, blowing every one out with compressed air until what seemed good flow was detected, making sure each removed part was completely free of dirt and varnish. I don’t have the sonic cleaner, but it ran fantastic for a couple of months of backyard oval track fun compliments of my 13 year old (yes I had to re-seed the yard). I’ll try removing the air filter and see if anything changes. Otherwise I guess I’ll disassemble the carb.... again... to see if one of the things you mentioned doesn’t look right. I re-adjusted the fuel-air idle mixture (which is a royal pain on this model due to lack of room between the engine and the bottom of the carb) to achieve best idle. It ended up exactly where it was before I re-adjusted, and will still idle all day without loading up. I think you’re right. Something must have changed inside the carb. This problem only got worse when we started riding at Mines & Meadows, which is serious hilly trail riding.
 

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Discussion Starter #7
Carb off

Well, I was hoping to find a loose main jet or something else wrong after taking off the carb, but it looks perfect to me. The slide moves up smoothly when I blow air into the slot, no holes in the rubber membrane, correct jet and metering rod, nothing loose, nothing dirty. I attached some pics, and will attach a few more in a separate reply. Do you see anything wrong?
 

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All looks good, but I would like a pic of the float about a foot back from the carb so I can see more of the body and the carb should be tilted about 45 degrees so the weight of the float is not resting completely on the spring loaded pin in the float needle.
 

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Discussion Starter #11
There is nothing coming out the bowl overflow at any time, and I made sure the tube for it was clear during my initial cleaning. Not sure why those photos are flipped 90 degrees, but it’s sitting on a counter at the requested angle. It’s got a K&N permanent air filter on it that looks pretty clean. I didn’t run without it before disassembly, but unless you see something wrong in the carb that I don’t, I guess my next step is to re-install and run without that filter to see if there is any change.
 

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Upon viewing the pics I believe the float level is too high which will cause a rich condition at all speeds. Check the manual and try to be as precise when setting the float as you possibly can. I use a vernier caliper when setting the float.
 

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Sorry, my error - the pic of the carb setting on the inlet bell as depicted in the manual looks like the float level is correct - there is an allowance of +- 1mm & most floats are slightly heavier than when new, so I tend to set to the + side (14mm).

So I no longer believe the fuel level might be a source of your problem - it's to the point now that the only way I could be of any more help would be to have the carb in my hands.
 

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Discussion Starter #16
I think it might have been at 12MM before I made the adjustment. Now, it’s definitely at 13.03. The ATV is at my RV site at Mines & Meadows. I’m going to take it out there, put it on, and fire it up to see what happens. I also still haven’t fired it without the K&N air filter off in case that is somehow clogged up that I can’t visibly see, so I’ll try that as well. If none of that works, (unless you’re up for a trip to Ohio/Western PA ;-) I’m thinking I’ll get a $40 carb off the Internet and give it a shot. I know they’re hit/miss, hopefully I’ll get a “hit” one.
 

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Discussion Starter #17
Resolved

Turns out your first call was right. Put the carb back on, and it runs flawlessly. Drove it on the trails for about an hour with no issues. That 2mm float adjustment was the difference between loading up and missing, and running like brand new. Go figure.
 
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