Hey guys, need some help. I have two srvs that ran great but now have the same big problem. After they warm up for about 20 minutes they backfire about mid range, when they are doing this you can pull the starter and they will smooth out and take off to high range but will either backfire again or just plain die. Carb electrical is unhooked, been thru the carbs several times, have good plugs and spark. Both ran great all last season and this season until this. Oh yeah and they have fresh gas also. Please help me thanks in Advance, Eric