My edge 600 pops and backfires at idle. Sled seems to have no power. I drained the gas and put new gas in. Cleaned the carbs .everything looked good. Pilots were clean. Put in new plugs and ran it . Pulled plugs and the pto side plug is black and mag side is fine. Checked and cleaned exhaust valves. Stock exhaust and slp can. The top end was rebuilt not long ago and compressionthe is reading 115lbs on each side. There was an old post on here with the same problem but the post never said what the fix was. Please help!