Originally Posted by 8gv
That's a very interesting idea about the air door. If one were to rejet, there would be less fuel to go with the thinner air and as a result, less power. Your solution brings MORE air (albeit thinner air) to the same fuel. It seems like that way most, if not all, of the HP is retained.
NO ... HP is not retained ... but the DR650 motor will run cleaner and use less gas when more air is provided ... or a smaller main jet is installed.
At altitude ... ALL motors
... lose power, including any F.I. motor. But the F.I. has the ability to "lean out" the mixture, allowing the motor to start, run and idle perfectly even at 20,000 ft. But Power will always be lost the higher one goes.
If a DR650 is jetted properly (very lean) at sea level it will run fine up to about 12,000 ft. Higher than that and it's best to either go with a smaller Main jet (leaner) or do the simple, quick solution and pull the air box side cover. More Air or Less Fuel ... both accomplish the same goal, but rejetting is the real answer for long term use.