my 302 idled with 11 inches of vaccuum with the 30-30 set at .030
when i put it down to .026 i lost 4 or 5 inches HG, exacltly in the ballpark that Glenn just said......subsequently, the Powervalve had to be changed, and I'm sure other things would need to be adjusted for optimum performance... i.e. jetting, timing
this effectively makes that big arse cam even bigger... especially in a no torque 302.
now... on top of that... a 64 or 65 FI vette (which also used the same cam) will not idle properly on that low of vaccuum... I tried it on that car as well...
seems like .004 couldn't possibly make a difference, in the lash setting... but if it makes enough of a difference to kill 5 inches of Vaccuum???
.004" tighter lash added 12 degrees of set-to-seat duration with your 1.52 rockers. It's not the lift that gets you, it's the duration.
With the stock stamped rockers (lower ratio), you should not see such a huge drop in vacuum when lashing at .026".