Has anyone experimented with drilling the holes in the secondary plate out a tad larger on a 600 cfm Holley 1850 4 bbl? Somewhere, I read about someone trying this, albeit with poor results.

I know about the metering block conversion kit, which I will do, but this questions is more of a curiosity.

if you have the small "pin" drill bits, it is an easy task. It is a one way path though. Once you get too big,,,,,
i would try one bit size at a time.

I've drilled out butterflies before on an engine that it really had no effect on. When I put that carb on a milder engine and it was too much, I used JB Weld on the holes to cover them completely.

While it might not flow the exact same as if they were all smooth, the carb is working fine.

What is the reasoning for you to drill the secondary butterflies?

I think he is referring to the sec jet plate. There are several company's out there ( Quickfuel for one ) that sell jet plates that accept screw in jets. They are pretty cheap and solve the problem of drilling too big and not being able to back up to a smaller size. They take standard thread holley jets.

I get it now.

Yep, that is the exact reason for my question.

For those who know, aren't the plate holes as they are from the factory equivalent to 72 jets?

no there are different jet sizes in the plates.you have to look at your plate and the number that is on it,and then go to the tech archives and find the translation list there to find out the actual jet size in the plate.