Is the new carb a Polaris? If not, you're probably better off rebuilding the old one if it's not physically broken.

 

 

What year model? Possible if a pre 2000 model Scrambler 500, a ground down exhaust cam lobe as this was the symptom. Idle good,but backfire,won't take throttle. Check the cam and valve clearance anyway. .006 intake and exhaust. IF the cam isn't the problem then I'd rebuild the better oem Mikuni carb with a good Shindy kit.http://www.shindypro.com/carb_repair_kits.html

 

Try your air filter .You have a dry one and pre filters . Take prefilters clean let dry. Then lightly put air filter oil on and keep adding till it stops backfiring .owned a 98 425 magnum that was touchy like that. Would ride for awhile then slowly it would start backfing more and more especially if you were working it .used to pull a drag and do yards for horse arena's and it would start drying out and this would happen. Pull carb clean same thing rebuilt it same thing adjust clip on needle same paid shop many times to work on it some times get back last long time sometimes u load and load right back up and bring to shop .spent lots of money figuring that one out. And alot of that might have been due to the filter up by gas tank would dry out or rip so didn't always have them two on. But now running K&N washable with a dry or lil oily prefilter that covers main filter and love that set up vs the dry one. Hope it helps

 

If I'm not mistaken it came with a paper filter with a stocking-type pre-filter. I think all 4-stroke Scramblers and Sportmans ever made use the same filter including the new ones. Why would you oil a paper filter? If mine gets oily I throw it away and buy a new one.

P.S. Do Scramblers really have another filter up by the gas tank? I don't know much about them, except that they're fun to ride.

 

Some models of the Scrambler 400 had foam precleaners over the paper filters that you could oil which were really needed for these engines. 500s just had the fine mesh prefilter.

 

I know the 2-strokes had their own thing going on, but other than those, all the Scramblers and Sportsmans I've ever seen had a paper filter with a "sock" on them, like this pre-filter from Polaris' site. "This Pre-Filter fits securely over the air filter to enhance its performance and extend the life. Water repellent and can be cleaned multiple times." I've seen a black one at least once, but don't know if it was OEM or aftermarket. It looked like a generic Outerwears with no name on it.

 

What's strange is that first 96 Sportsman 500 and the first 99 Ranger 500 used gray foam prefilters.After that they went to the mesh.https://www.ebay.com/itm/Polaris-96-...Jaz3aO&vxp=mtr
https://www.partzilla.com/catalog/po...rf50aa/air-box

 

I've seen foam pre-filters like that on other brands of ATVs too. My first ATV was a '97 Sportsman 500 and the filter looked just like the new ones, except the plastic on the back end changed color sometime over the years. Without looking at one, I think the new ones are blue and the old ones were black. The mesh must be better than foam somehow, or just cheaper. When I think of foam filters, lawn mowers are the first thing that comes to mind.