Could the tappet adjuster have fallen into the cylinder head and is jamming the intake valve, hence, air/fuel going to opposite direction out of the carb?
I see you are an optimist.
One of the "popular" nuances of the 77-78 CB750F model is it's propensity to wear out valve guides much earlier than the rest of the 750 line. Some with as little as 25K miles.
These heads have bigger valves and to do that the valve geometry had to change along with domed pistons.
The valve angle placed a lot of side force on the guides and they wore out oblong. The valve would then wobble at the stem end and the seat. The seat would wear and ruin the valve seat and valve face, leaking compression. The stem end would wear on the valve adjuster increasing clearance, and the hammering and wobble would mushroom the valve stem and the adjuster. Wouldn't surprise me if the hammering and rotation didn't knock the tappet adjuster loose.
The valve spring compresses when the valve is opened by the camshaft. Jamming an adjuster post in the spring might keep the valve closed. I don't see how it would keep the valve from closing, but I haven't seen every failure there is to see.
If your engine has the dreaded valve guide issue. It will almost certainly be cheaper to find another engine. If you want to avoid the guide wear entirely, then get a K model engine or an F model prior to 77 and paint it black for a disguise.
Shop for all new valve guides, and valve seat grind, Either new valves or refaced at both end existing valves and lash cap to restore the proper valve height for proper seat pressure, all new adjusters, and a gasket kit.
For an estimate of cost for JUST the head work, contact Mreick, a forum member here.
One of my Cb750F models already had a 78 K model transplanted into it. FYI. I don't have much riding time on any of my 750s.
Cheers,
