Snowmobile Repair: snowmobile carb setup, fuel mixture, fuel mix

Question
I have a 1971 yamaha gp 396 that is giving me some problems.The sled starts o.k. but runs for a while then stalls(typically 3-5 min.)when running the sled will bog down and syall if you increase throttle , unless you increase the throttle very slowly . The sled has two carbs , each having a front and back screw. Which screw is fuel and which is air and how many turns out from closed should be your starting point to adjust? Also what else can you suggest to try to eliminate my problems.           Your help is greatly appreciated, Mike Carlisle

Answer
Hello Mike,

  The GP is a real nice sled. The carbs you have, Keihin's 406, remind me of my 72 Yamaha SL338c. Those carbs are real touchy, but if you get them just right they'll go all day. I actually had the same problem when I first got my Yami. As for the screws, the little one, that is closest to the engine is the screw that controls the fuel mixture when the sled is idling. The other screw, farther out from the engine, and larger, is the high-speed mixture screw. Both of the screws are fuel mix screws, which as you know means that they lean the mixture when turned in, and add more fuel to the mixture when un-screwed. The recommended starting positions for the screws are as follows: 1 and 1/4 of a turn out for the little idle mixture screw, and 1 turn out for the larger high-speed screw. I always did both out 1 turn. Whatever works well for you. These carbs have a real big problem with sticking inlet needles. One way to help would be a carb kit. As a word from experience, the inlet needles in the kits made today are not too good. They tend to leak and make things worse. If do decide to rebuild the carb, you may want to clean and reuse the inlet needle, unless it has deteriorated. If the sled continues to have this problem, a set of NGK B6HZ spark pugs will cure it for good. Hope this helps. Let me know if you have any other questions. Good luck with the sled! --Arcticatman