Loss of HP

What is the most likely cause of loss of HP. I have a 1996 BMW F650 motorcycle. It is a carbureted single cylinder engine. The lower HP is noticeable at lower altitudes, but is extremely noticeable at high altitude (5000 ft). The loss is the same with the stock exhaust or a performance exhaust. The engine starts fine and runs fine.

-carb tuning?

-valve adjustment?

-engine ware?


Note: a possible related or separate issue is it backfires on deceleration.

Being altitude related I would focus on timing and timing controls.

Found the answer if anyone else has the same problem. It was the carbs. I rebuilt the carbs and found the diaphrams were broken. Replaced the diaphams and cleaned the slides. The bike runs like new!!

Typical. The only engine problems I’ve ever had in motorcyles has been with carbs. Probably the nature of carbs, plus the fact that motorcylces sit dormant for periods of time.