Got a brand new carb for this thing but there are no screws for adjusting anything except for one on the very bottom on the engine side of the carb. No idea what that actually adjusts, as it seems to make no difference at any position, all in, all out or anywhere in the middle.
It has a dead spot right off idle where it'll actually die if you try to just feather the throttle up to about 20% or so. If you quickly go past that point off idle, it'll run and rev up like it should but it also seems to have issues at close to max throttle. It only likes midrange.
It starts and idles fine.
FYI the machine is new to me and I got it from a guy who said it needed a carb. It seemed to have the same issues with the original carb as well which I figured a new one would remedy.

 

I suspect it's either a Mikuni or a Keihin. Should have a PMS for fine tuning the idle mixture. I've had great results putting Dynojet stage 1 kits in my Arctic Cats with Keihins. See if you can find one on eBay for your Scrambler 500, shouldn't be expensive. Directions in the stage 1 kit are very good. I think their needle and improved spring and they give yo a bunch of main jets. Probably say screw the PMS screw in all the way clockwise, then back out 1 or 2 turns, then which E clip to use on the needle, lastly what main jet for max power at wot.

 

It's a Mikuni. What is a PMS screw? Is that the one on the bottom ? It's the only adjustment screw I see on it.
I've adjusted it all the way in, all the way out and several positions in between but nothing seems to make a difference.

 

What year model Scrambler? May not be fuel or carb related depending on model.On this I'd check that the valves are set at .006 intake and exhaust and the exhaust cam lobe is ok. Early model 500s had problems with soft cams. Check the pilot jet and main jet size in the new carb against the old ones. May be different. Normally the screw on the bottom close to the intake,fuel screw or pilot mixture screw as some call it sets around 2 1/2 turns on these.The fuel screw is mainly for idle.The jet needle clip sets normally at the third position from the top,but you can lower and raise the clip to see if it helps also. Is the new carb an actual Mikuni or a Chinese knock off or used carb? Normally Mikunis have this fuel screw blocked off with an aluminum plug.

 

It's a 2000 model. Are those susceptible to the cam issue?
Check on the valve clearance.

It ran the same with the old (Mikuni) carb as it does now with a chinese knock off. Original owner told me it needed a carb or at least a rebuild as he had it sitting for quite a while. I slightly disassembled the Mikuni but didn't go too far as I know nothing about carburetors but it didn't appear to be gummed up. I can work on fuel injection systems on $80,000 BMWs but carburetors might as well be the space shuttle to me!

 

If it's doing the same thing with both carbs, it's probably not the carb. It's just me, but I'd rather have an old Mikuni, than a new Chinese knock off. OPT hit the nail on the head for you.

 

2000 model had the HO cam and normally not a problem,but wouldn't hurt to check valve clearance. May not be carb or fuel related,possible can be electrical related. Best to have a manual that you can go through the trouble shooting guides and resistance checks.http://visit.tradebit.com/visit.php/...75/264163829/9

 

that's what I was thinking.

 

Cool thanks! I changed the plug already. Someone had put a BKR7 plug in there, which is at least 1 step colder than spec. No idea why.
I"ll look into valve clearances and hell I'll buy a plug wire for it, it can't be too expensive.

 

Colder operating plug shouldn't have that much effect on the way it's running. I've installed the BKR7ES plugs along with customers with no problems or complaints that I'm aware of.