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Discussion Starter · #1 ·
I am working on a friends 1999 Sportsman 500. It ran fine a few weeks ago and he saw the temp light come on so I got it for that and of course as he brings it over it started to backfire and cut out at full throttle so I acquired that issue to work on as well.,

I know a lot about ATVs I prefer Yamaha's but work on anything. I have read other posts about this and am no stranger to forums so throw ideas out there......

ISSUE: Idles perfect, goes at very low speed then any kind of throttle it sputters and bogs.

I initially took the carb out and cleaned it completely soup to nuts....re installed....no change (Idles perfect, but bogs when throttle is applied)

Replaced fuel pump since figured not getting fuel.....Same issue

Cleaned and drained gas tank blew out all fuel lines... no change

Ran with airbox lid off........ran Worse

Ran with box back on.... got better...even better when I shoved two fingers down the intake tube...almost back to normal response with that....

I checked the ETC.....not an issue its making contact

Checked exhaust Cam....it has plenty of lift, not a worn cam.....

SO I figure its not getting gas or enough gas.....what could it be?
 

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I'm sure you checked but since it's not listed I have to ask. Any cracking in the airbox or carb boots. Even small ones have been known to cause problems.

Since you work on these frequently I'm sure you know this already, but if you spray a little carb cleaner around the boots while its idling it will surge a bit if there's a hole.

Good luck.
 

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Discussion Starter · #4 ·
No cracks, but the lid to the box has those 6 studs and someone hot glued them back in, they have broken off again. I am wondering now if that is not sealing correctly causing the problem.

And would the newer 500 airbox fit in the older machine.....This is a 97 and not a 99 as previously stated.
 
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