If you make it a mechanical secondary I would think it would have made the problem worse....because you now are dumping the throttle blades open without the pump shot provided by the rear pump on a 4150 :?:

Make sure your idle mixture screws are adjusted properly.A turn and a half will be close but it's best to set them with a vacume gauge..adjust it to the point that gives the most vacume at idle.

idk
but it fixed the origional problem

and i did that
i went threw the hole range on the mix screws with no change

i set the floats properly and it fixed the problem for about 5 minutes and then right back to how it was

so i thought is might be an air leak
and i sprayed carb cleaner all around the carb and there was no change in idle
im stumped

Bowhay:

Just an idea... but had similar symptoms once & it was the needle & seat which had a small particle stuck in it not allowing it to operate. Had the high idle, small hesitation, sounded very lean. Good Luck!