friend just bought a '03 Rev. 800 that was fresh rebuild last year, Runs and rides great but averaging 6 miles per gallon. I'm thinking could be pourly installed PTO seal but DOO's Arn't my specialty.
Any ideas or help would be apreciated. Thanks in advance.

OK. after thinking further about problem looked over reed valves, they were shot so put in V-Force 3's and lowered jets, runs at about 8 mpg. now. Still not impressed, what other issue could it be besides hard riding?and yes, now it's much faster and still runs good but a little fat at idle and gives a sputter when continued rpm of 6500 for more than 1 mile.

You should have #20 pilots in, about 1 to 2 turns out on the air screws. This will help with your high idle problem. The pilot is to lean from the factory. The mid is a little lean and I think people were installing an additional .010-.020 shim under needle. Main jets should be 360's. Stock was 380's.

I dont think a PTO seal will cause your low mileage. Usually if your seal is bad you will suck in air and cause a lean condition. This will cause you to rebuild your motor.

You really need to know more about the rebuild and what was done. Was the 2nd pto crank bearing replaced. The bearing seal goes bad then causes preassure to start pushing out isoflex grease out the pto seal. If the bearing was not replaced, keep an eye on any grease behind the primary clutch. The 03 were bad for this. If you see grease you need to replace the 2nd pto crank bearing and outer seal before you blow it up.

I dont think you will get any better than 10 miles per gallon if you ride hard. I have an 830 kit and can only get 10 miles per gallon.