My friend just bought a used 86 XR600 and is trying to get it running. He had a poor spark and replaced the coil. Improved it. We took apart both carbs yesterday and cleaned them and checked float levels. Now the bike starts and runs but will not rev any higher than mid range. Motor stutters and backfires. Seems to run ok idling or running with the throttle open 1/4 to 1/2. We're trying to figure out if we have a timing problem or a carburator problem. Any suggestions?