When I use my Weber grill with the cover on, my food will have a greenish ashy or sooty surface. This does not happen uncovered. It didn't always happen. I noticed it a couple years ago. I always use Kingsford charcoal, a chimney starter and newspaper. Every time before I grill I scrape the sides of the kettle and the cover and wipe then down with wet cloths. I clean, scrape and oil grill rack. I clean the bottom ash catcher before I grill. If I use the cover, I've tried the vents all variations of open/closed. All the same result. Before I buy a gas grill does anyone have suggestions .