Bachtold mower with 8hp B&S engine

Hi Folks,
I'm restoring an old Bachtold mower ("The Whipper") with 8hp Briggs and Stratton engine (cast updraft carb). I'm a novice at small engine repair and have two questions:
1) I've tinkered extensively with crab adjustment screws (watched lots of YouTube videos) and can get the mower running well enough to operate. However, I can never get a low idle. What's my next step? Tear down and clean carb? Are adjustments to throttle necessary?
2) Maybe because of the idle issue, the drive shaft is ALWAYS spinning when the engine is running. As a result, when the clutch is disengaged, the shaft is spinning against the mower belt and burned it in two. Is this a belt tension issue, or an idle speed issue?
Thanks!

If you have the same carb as two of my old generators. Those carbs
were used for Years and years and are simple enough. There is a paper
gasket that gets all wrinkled up and messes up how the float works.
Take it apart and replace the gasket. You can still get them or cut
your own out of gasket material. Here is the WARNING. That carb is a
little tricky to get apart. Look at the bottom front of the carb and
find the run jet adjuster screw. Take it out. Next you MUST remove the
long brass emulsifier tube. If you mess with the carb you WILL break
it. The rest is easy and straight forwards. That needle valve is
usually 1 1/2 turns out from seated.

I have three Bachtolds, but none are the Whipper. On mine, the shaft that attaches the blade/head to the mower frame has a bolt, I think 1/2" that you use to adjust the length that the shaft extends when you
engage the lever. Just loosen the bolt, shorten the "length" a bit and try that. May have to do that a few times. I love those machines, they are NOT dangerous if you stay behind them where you should be.

You say you can't get a low idle...

As in the engine refuses to slow down, even with the idle stop screw backed out, and hand holding the throttle shut?

If so, sounds like the throttle plate may have come loose from the throttle shaft, or it was removed and incorrectly installed. The plate must be centered on the shaft and in the bore before the screws are tightened. There is also a beveled edge that must be installed so it seals off the edges in the carb bore. It will only fit properly one direction. Also a worn throttle shaft or shaft bushings will cause poor idle control, and vacuum leak around the shaft.

If the idle can be slowed by hand, but revs back up when released, throttle cable and the governor adjustment. The governor should be able to push the throttle closed with slack in the spring and the engine running.

If you're saying the engine dies when trying to get it to idle, that could be the idle circuit clogged or sucking air between the top cover and the bowl. The idle fuel is drawn up from the main well through a small passage beside the venturi bore. If the gasket doesn't seal at that hole, it won't idle. Also low compression will cause dying and poor performance at low RPM.

I have had belts that refused to completely
release. I bolt a small rod on both sides of
the belt to squeeze the belt away from the
crankshaft pulley. It takes some fancy
bending to make it work.