The Yamaha VK Pro can be an excellent grooming snowmobile and is an excellent choice for a low budget operation or for someone just getting started. It is heavy, has a 156 x 20 track, 3-cylinder water cooled 900cc 4-stroke engine, Hi/Lo transmission with reverse, and has 3 carbs with carb heat. Its most glaring problem is that it dumps the exhaust on the track. At low grooming speeds under load, the heat from the exhaust melts the snow which will re-freeze in the tunnel. You'll eventually have a tunnel full of ice and will be stranded on the trail! If you didn't bring a hammer and an ice pick, you'll be walking back to the shop.
The fix for this problem is simple - just re-route the exhaust to the side as shown in the photos below. Disassemble the top of the tunnel and haul your machine down to the local muffler shop. Our shop put new pipes (stainless steel) on the existing stock muffler. It took them about 20 minutes and was $100 in October 2017. The passenger seat is above the muffler. Cut holes in the side of the seat support as needed.
The 2nd problem with the VK pro is the carbs can either vapor lock or ice up depending on the conditions of the day. Watch that carb heat valve position for the conditions and this machine will be an excellent reliable grooming sled.
The 3rd problem may be the one that gives the VK Pro a bad reputation - hard starting! My machine would hardly start, much less run after it was warmed up. If I could get it started and get it off idle without stalling, it would run fantastic with lots of instant smooth power. When I let it idle it would often die. If it had run more than a couple of minutes I just gave up on trying to start it until it cooled down. Per a hint from a very experienced snowmobile mechanic, I removed the fuel cutoff valve from within each of the two vacuum-operated fuel pumps. If it didn't require removing the pumps, this would be a 5 minute job. The first time I hit the starter after removing the fuel cutoff valves I could tell it was immediately improved. After it was warm, it started instantly as I expected. I think and hope this is the final solution for the VK Pro's hard starting reputation. I'll let you know after we have snow and I can get it out a few times. The fuel cutoff valve is a curved dome white plastic disk on top of a 3/8" diameter light coil spring. I believe the fuel is shutoff if there's no vacuum. That means if you have a vacuum problem you'll never get fuel and will never get it started. Disclaimer: This is still a theory until proven over this winter - 2020/21.
TBD: VK Professional History and model year changes