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You have a large air leak.
What is happening is there is an air leak allowing too much air into the crankcase. This is causing a very lean condition which causes the high idle and backfiring.
Get some WD40 or carb cleaner. With the engine running at idle spray around first the carburetors, then the block itself.
Make sure the carburetor shutters are fully closed and they aren't hung open. If they are they too would cause the problem. By using the spray you are locating the leak. Just listen for a change in the engine sound and rpm.
Once you locate the leak then post back where you found it and we can then fix the problem.
Don't try to operate the engine until we get it fixed, damage could result.