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post #1 of 19 (permalink) Old 08-10-2019, 02:14 PM Thread Starter
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1999 Sportsman 335 hesitating at mid throttle

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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post #2 of 19 (permalink) Old 08-10-2019, 03:37 PM Thread Starter
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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 didnít change anything in the carb from factory, and itís the factory mikuni carb that I cleaned.
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post #3 of 19 (permalink) Old 08-11-2019, 06:44 AM
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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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post #4 of 19 (permalink) Old 08-11-2019, 06:29 PM Thread Starter
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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 donít 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). Iíll try removing the air filter and see if anything changes. Otherwise I guess Iíll disassemble the carb.... again... to see if one of the things you mentioned doesnít 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 youíre 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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post #5 of 19 (permalink) Old 08-12-2019, 11:19 AM Thread Starter
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Does anyone know what size main jet and metering rod came originally in the 1999 Sportsman 335?
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post #6 of 19 (permalink) Old 08-12-2019, 11:24 AM
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Main jet was a 150 - slide needle (metering rod) was (J8-5F14) (5F81-3)

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post #7 of 19 (permalink) Old 08-14-2019, 07:23 PM Thread Starter
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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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post #8 of 19 (permalink) Old 08-14-2019, 07:26 PM Thread Starter
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More pics

A few more pics of carb components
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post #9 of 19 (permalink) Old 08-14-2019, 07:44 PM
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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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post #10 of 19 (permalink) Old 08-15-2019, 05:56 PM Thread Starter
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Angle pics

I did a few different angles and made sure the float wasnít depressing the spring loaded pin before each one.
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