I have a 79 moby which starts and idles it used to go 30mph, but will only get up to 15 mph presently. I have changed the petcock, put a brand new fuel line with a new fuel filter, cleaned the carb., and put brand new spark plugs. This did not fix the problem. It sputters and backfires in the muffler. Do not know what to do next. Any suggestions would be greatly appreciated.