Another possibility is that the DC fan motor that is suppossed to blow the smoke out of the stack is wired backward. This will cause the smoke to be sucked back into the locomotive, the momentary puffs you get may be caused by the fan momentarly stopping allowing the vapor to escape on its own. A gentleman named Jim Barrett addresses this exact problem on a DVD called Backshop Volume 11 for sale by OGR publishing. Simply Google OGR publishing to find their Web site. Hope this helps....
Scott
Take the top off the F7 in question and look at the smoke unit to see if your fan might be slipping or you need to push the miniture connector into place better. If it is a PS-2 unit you may need a new battery for the memory to work properly.
Lee F.
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