I'm new to snowmobiling, and recently purchased an 87 Polaris Indy Trail with 4k miles. The sled runs pretty well, starts on a pull or two, and rides nicely (for the $400 I paid). I have two questions, though. At startup, it smokes pretty well, I'm guessing that's a normal condition. A friend says "smoke is good, it means your engine is getting oil." But how much smoke is normal, and when does it indicate a problem? Once started and warmed up, it will smoke while riding, then not smoke at all, then sporadically it will start to smoke, then stop, etc. For a sled of that age, when should I be concerned about the amount of smoke? Secondly, when running at low rpm, there seems to be a sporadic metallic clanking (seeming to come from the side of the engine with the belt). At high rpm, it seems to not be noticeable, but with no throttle, it reappears, makes he engine sound like it's not firing evenly. Could there be a problem with one of the cylinders? Or, again, is this fairly normal? Thanks for whatever insight you can offer.

The smoke is the oil your burning....you can try another type of oil. The synthetic lubes are all ashelss oil and they burn much cleaner than the natural based oils.
The other thing you can do is check your plugs to see if they are looking like your using too much oil....they will be black,sooty maybe slightly wet,and may have build up on them.
Too much oil won't hurt your engine , it will use a few more plugs and you'll be adding oil more often.

You might want to check your cylinders compression also.

The noise you hear under the hood could be 100 things.....I'd say it might be your clutches.

I'll try the synthetic, and will probably need to get a technician to check cylinder compression, unless that's something I can do myself. I just bought the sled, and the previous owner said it had been sitting unused for two or three years. I doubt he has started it over that time. I don't want to sink too much money into reparis, and would like to attempt carb adjustments, maybe a minor tune-up. Engine looks pretty simple. Is this wise, or should I just have a tech tune it up?