Backfire through carb, runs very rough

[Higglypiff42]

My brother just bought an 06 GS500F that sat for 5 years and wasn't running. We cleaned the carbs and she fired right up! But we ran into a new problem, it runs very rough. At idle it runs rough almost like its misfiring. It also backfires through the carb when idling and refuses to rev up. If you try to rev it it just sputters. It will also rev up on its own to 3k or 4k rpms for a second or two then go back down to 1.5k rpms. I'm not sure what it could be but I'm thinking its electrical. We checked the voltage going to the coils and they were both about 4 volts with the engine running and it would go up when the rpms went up. I would think it should be around 12 volts at all times? Checking the spark outside the engine you can see a very nice and consistent blue arc so I think the spark is good. We're both so flustered and can't figure out what the culprit is.

[Higglypiff42]

I also forgot to mention that the bike has the stock exhaust, new air filter (stock replacement, not a high flow one), and stock jetting.

[Higglypiff42]

So we just got done giving the carbs a really good clean. Turns out we just didn't clean it good enough the first time. She runs like a champ now! We also cleaned the tank really good because it had a litte bit of old gas and some junk in it.