Holley 4150 Secondary Throttle Stop Screw

[bsprowl]

I got a Holley carb problem.

I want to remove the Holley 4150 Secondary Throttle Stop Screw.  This screw is reached from under the carb when it is off the engine.  It is a slotted head screw.  I have a couple of carbs I want to have replated and I thought I would take these screws out as I suspect the plating process will lock them in place.

I treated the screws, one with WD-40 and the other with PB Blaster over night.  Today I cannot move them.  I've twisted the end of the blade of both a Craftsman and then a Snap-On screw driver but the screws don't move.  Any suggestions?   

[SSdynosaur]

 I would caution against "backing out" the screws (toward the bottom of the plate); they are usually very tight and backing them out tends to expand the  slot on the screw and, ultimately, lock them in place via interference. I have enjoyed much more success by simply applying penetrating oil to the top and bottom of the screw and advance the screw upward, toward the carb body (as opposed to "backing out"). In a few instances I've found it necessary to apply a small amount of heat--be very careful. Good luck.

[Falcon67]

I replace those 100% with either the replacement adjuster the Holley sells or at least with a hex set screw.  Those slotted set screws are junk. Yes, run out through the top, backing out can be an issue.  If it's too screwed up (lol) you'll have to drill it.  It's way, way easier to adjust on the car with a hex wrench, even easier if you replace with the Holley parts.

https://www.holley.com/products/fuel_systems/carburetor_components/brackets_and_linkage/parts/26-137

[bsprowl]

I tried heating it today.  Semi-long story.
Holley carburetors usually have nylon bushings on the throttle shafts and since nylons melts at 210 degrees (I looked it up), I needed to completely disassemble the base plate before I heated it.  And I had to create a tight fitting small but strong screwdriver.
Disassembly wasn't hard but I did discover that the bushings were missing.  I guess someone took it apart and, like me, couldn't get the bushings back into place.
Using small allen wrenches, I measured the width of the screw slot at 3/64 or 0.046875 inches and the hole size at right at an eighth of an inch.  I took a small, four inch, oldie but goodie Craftsman screwdriver and carefully cut it down to size on a small grinder.
After clamping the base in a vise using wood spacers so the jaws wouldn’t mar it, I got out my oxy-acetylene torch and located my infrared thermometer gun (my shop is new, and some things don’t have a “home”) so I could monitor the heat.  I slowly took the area near the screw up to 500 degrees and then let it cool off.  I then put it in the freezer compartment of my shop refrigerator for a half hour until it cooled to 20 degrees.
I clamped the base back in the vise, put some WD-40 in the screw hole, with an 8” crescent wrench on the shaft of the screwdriver and while pressing firmly down on the screw driver I tried turning the screw, slowly increasing the turning pressure.  It would not move, at all, in either direction.  When the screwdriver jumped out of the hole I gave up. 
I’ll try the other carb tomorrow. 
Monday I’ll take it to a shop in Montgomery and have it drilled out.  My drill press has a bit of run out and I don’t think I can get a small bit to do what needs to be done.