Engine stalls when you pull number 5 spark plug wire.

Hi,

Maybe someone has an idea and will point me in the right direction. I noticed a lean mixture on my afr. Somewhere in the 14's when i step on it. There is also a lean pop at constant speed. Checked spark plugs and number 5 seems to be the culprit. When I pull off the spark plug wire the engine almost stalls. Huge vacuum leak??

What I have already checked;
*Vave train checked and everything is fine
*Compression test done and everything 190 dry and about 194 wet
*replaced intake gasket
*replaced spark plugs
*ignition parts checked.

Engine;

416 stroker
10.5:1
Hughes solid flat tappet cam
Worked over speedmaster cnc heads with harland sharps
Weiand xcelerator intake
MSD 6al ignition with blaster 2 coil with summit/firecore distributor.