Runs a bit lean...

The Transfer slots are your Low-speed fuel delivery system.
The Idle mixture screws are just there for Idle enrichment.
If the ignition timing is NOT factory, ie is now advanced, then to keep the idle-speed down, this will require the throttle-blade to be closed, which then closes the Transfers, which then makes idle-issues and usually an accompanying tip-in sag/aka a hesitation on take-off.
To compensate for the loss of transfer fuel, one/I will usually find the idle-mixture screws set wide-open.
This works at idle, but makes the entire low-speed system go rich.
The only cure is to put the idle timing back to stock, and with it, the mixture screws adjustment.

Having said that, slanty intakes are notorious for sucking air at the head interface.
Someone mentioned the PCV, but I have never found a PCV make trouble, unless it was an old dried out and cracked hose, or a hose was installed that was not designed for the high-vacuum purpose, which got sucked to near flat at idle....... but this makes the idle speed go slow, and so the throttle has to be opened to get more air, which opens the transfer ports too far, and the whole program goes rich.
In your case, covering/restricting the air intake is proof of one of three things;
1) the engine is getting too much air, or
2) the carb is not delivering the correct amount of fuel, or
3) the fuel droplets are not being properly mixed into the airstream, which occasionally points to the under-carb heater.

If the problem is ONLY at idle, I would go straight to the transfer-port exposure.
.