I sold my 1988 FLHTC to my brother in law and he lives in New Mexico at about 5000 feet above sea level and the bike ran fine here in Kansas City MO. It now back fires like crazy until it gets to operating temp. and will back fire some then--need suggestions on jetting, it has a constant velocity carb, true duels with v & h mufflers, and a kuryakin Twin Velocity™ Plus air cleaner.
Help please

 

Tell him to chaeck the exhaust, make sure eveything is tight.

If so its likely the tune, Air is thin up there.............

 

I would check for the obvious first, air leak at carb/manifold, loose carb etc. If no problem is found remove the carb bowl and check for water trask etc. if clean I would replace the pilot jet with a #46 to start and remove the airscrew cover and adjust the mixture with the engine hot. CV (constant velocity) carbs compensate for altitude. There are many articles on this and other sites that go into great detail. Normally, the mixture richens as you gain altitude.
I hope this helps.

 

+1 with Mikey 1450 on this one.

 

Conditions such as an altitude change can cause a bike to backfire. The air is much leaner. I suggest a re-mapping or a dyno run to get the fuel mixture correct prior to spending big loot on other parts or troubleshooting.