I have the float adjustment at dead-bottom since I rebuilt the carb...there's only enough fuel in the bowl to allow idle, let alone enough to backfire through the carb. And, remember this...it's not fire shooting out of my air horn, it's raw fuel and air. I can't imagine a situation in which an improperly adjusted float would allow unburnt fuel and air to belch back out of the intake, but I'd be happy to entertain any diagnostic ideas that you have if I'm missing something here.

 

Alrighty...the passenger-side head is off, and the gasket, while not in horrible condition, looks pretty burnt up and worn out. Also, an eyeball-check says that the no.1 intake valve is hanging a bit more than the others, so for about three hundred bucks, I can have both heads cleaned up and re-done to spec., probably with new valve guides, seals, etc. installed as well. I may just order up some new components from Summit, hand them over to the machine shop, and have the heads completed that way. I'm also tired of this POS Holley that's on the stock manifold, and I'm tired of the manifold itself. Worthless...even when this thing was running like a bat out of hell, the sucking noise from the intake told me that the carb and manifold were not meeting the easy-breathing requirements of this engine and the cam in it. As I said in another reply to a post here, I only want to do this one time in terms of the top-end of the engine. I plan on pulling the other head as well, since they should be done in pairs anyway, and perhaps upgrading the valve train as a whole...nicer rods, better lifters, and I may as well examine the camshaft if I don't see any evidence of compression leakage between cylinders when I yank the other head. It's a '78 400M...there aren't many things that could go wrong that would cause UNBURNT fuel and air to belch from the carburetor.