It sounds like it is a carb model where you re-jetted it for elevation. Usually when we see a sled backfire it is because it has raw fuel in the pipe or silencer. This can happen when the sled is run hard and shut off without allowing the engine to come down to an idle. When that happens, the spark is shut off, but the engine continues to pump its air/fuel mixture through it and the air/fuel mixture will end up in the pipe and silencer. Then when the engine is fired up again, the unburnt air/fuel mixture that is in the pipe will ignite and backfire.
If the sled backfires after it is running when you take off, it is most likely a mis-adjusted throttle cable/lever. What happens is the throttle over-ride safety switches cut the spark for a second or less and the un-burnt air/fuel mixture is pushed into the pipe and silencer. Then when the spark returns, it is ignited and backfires. A properly adjusted throttle lever should only be able to move away from the throttle block approximately the width of a credit card prior to pulling on the slides of the carbs. Too much slack in the cable will cause the throttle over-ride safety switches to cut spark. I’ve also seen where a dual throttle can cause this problem when the throttle lever pivot point is tightened too tight, not allowing the lever to pivot smoothly and engage the switch properly. A faulty throttle over-ride switch could also be the cause.
If the engine is allowed to come to an idle before being shut off and it backfires when it is started, then I’d have to say the idle air/fuel mixture is too rich on fuel and the unburnt fuel is pushed into the pipe and silencer where it ignites the next time it is started. Other common things that can cause this are a choke cable that is out of adjustment, a choke plunger that is stuck or leaking, a fuel pump diaphragm that is leaking slightly, a needle and seat within the carb that is leaking or a mis-adjusted float level. I’ve also seen it happen with a back spark plug cap or wire that gives intermittent spark. Usually these items show up as more than just a backfire. They typically will result in a rich condition or misfire. If the sled has a low idle, or difficulty idling, then one or more of these items may be the culprit. – Jerry Mathews, Startingline Products