Excess carbon build up is a sure sign of a rich mixture.
Operating the set at altitude requires re-calibration of the carburetor - which must by done by an authorized repair facility. Special tools are required to change the jets.
At 4000 feet, you could get away without the carb modification IF you do not use the -eco-throttle feature AND you always run it to at least 75% of the rated capacity.
If the engine has that much carbon, it would be a very good idea to have it de-carboned.
Excess carbon creates a lot of problems with the set, including: excess heat build up, pre-detonation, and premature wear on the cylinders and valves.
It sure is!
But you can not be afraid of getting dirty. You WILL get dirty.
There are 2 ways to de-carbon an engine. The first way is the absolute best, XXXXX XXXXX it is not too bad, the second will work - especially if you do it 2 or 3 times.
1. Remove the valve cover. Loosen the rocker arms and remove the push rods, keeping the rocker arms and push rods marked as to which is intake and which is exhaust. Remove the head. Rotate the engine until the piston is all the way up. Using a steel brush (a small one that looks like a toothbrush works best), remove all carbon from the head and piston. If carbon buildup is extremely thick, a small scraper (a small screwdriver) will work - just be very careful not to scratch the head or piston.
Reinstall the head with a new head gasket (do not reuse the old one). Insert the push rods into the proper position. Reinstall the rocker arms and set the valve lash. The intake should be set to .006" (.15mm), and the exhaust to .008" (.20mm).
2. Obtain a can of combustion chamber carbon remover (I recommend Onan 4C). Remove the air cleaner. Start the set and put a load on it. Let it run for 15-20 minutes to make sure it is good and hot. Spray carbon cleaner according to the directions. Usually, you will spray it until the engine quits. Let it sit for 15-20 minutes. Start the set and put a full load on it. Let it run until the smoke clears (there will be a LOT of smoke - and it stinks, so don't do it next to your grill or your neighbors - unless you don't like them!).
If you are going to do a second or third application, let the engine completely cool first.