If it does this when cold, you ought to be able to (on a cold motor for safetys sake) spray at the intake/head joint with a can of starting fluid(either) and if there is a leak get a rise out of the idle or some such. Be safe, etc, etc.
If all is tight you can drown the exterior of the motor and air hoses and get no change in idle. Again, cold motor, for safetys sake. I have known these intake gaskets to leak, and the intake bolts to be loose over time. It isn't too hard a job to change them out, be aware of other stuff that has to be moved that might break in the process.
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