Dito with Jeffs thoughts on altitude and Vacuum. I'm at 5400 feet and pull about 15" with a healthy motor. Also your motor would probably benefit from a leaner running carb cause of the altitude. Approximately two jet sizes smaller (with rods ~.030 smaller) would be a good place to start if your carb has the original jets. However, if indeed you're experiencing lean misfire, you will need to solve that problem first. But black plugs would tend to tell otherwise. 15" of vacuum is enough of a signal for the carb to function properly once it's jetted correctly.