Sorry, I meant what thread size are the PVCRs?
I think (think) they are 10/32 but it is possible they are 8/32.
You should be able to drill the holes pretty close to the center. Especially for air bleeds and emulsion, the holes don't need to be perfect. But you can get them very close using a pin vise and a drill press. Hell I know a guy who chucks his pin vice in a 3/8 drill motor, gently clamps his drill motor in a vise and holds the blank and drills it that way. Any way you get it done without breaking tools, hurting yourself and making an acceptable hole is perfect.
You would do well to spend some time testing and tuning the power valve channel restictors and the T slot restricters. Because the IFR, air screw setting, IAB, main jet and HSAB pretty much control cruise fuel curve, if you can get the cruise A/F ratio lean as you can on the main jet and let the power valve add fuel you will have a better fuel curve. Same with the T slot restricters. With the power valve you can change how soon it opens. I no longer set power valve opening by what my idle vacuum is. When the system is right, the power valve can be open at idle and not make a tinkers damn of difference. Now, if you rupture the diaphragm in the power valve, it will drain fuel right out of the bowl and into the manifold. Two different issues. So, I pick my PV opening based on minimum cruise RPM manifold vacuum. Let's say I run a big honking cam and I have 8 inches of idle vacuum. But cruising down the road I have say, 12.5 inches. Most guys look at the 8 inches and drop a 4.5 PV in there. I would have used an 8.5 so instead of vacuum falling 8 inches to open (12.5 cruise vacuum minus 4.5 PV opening) I would only have 4 inches of vacuum to drop to activate the PV. Lots of stumbles can be fixed just by setting PV opening based on cruise RPM vacuum and not on idle vacuum.
The other thing you can do is lean out the main jet as far as possible with no power valve on cruise until it starts to get lean. At this point, you can add a little fuel through the T slot restricters. You have more control by adding fuel to the T slot restricters or, if you have an oddball like mine you can take fuel away at a the T slot to help clean up the cruise fuel curve. If it's rich at cruise, put smaller T slot restricters and clean it up.
Also, don't forget to use the power valve channel restricters. You can run a leaner main jet and if it's surging or acting lean, open up the T slot resticter. Once you get the cruise worked out now we need to work on power. You've done all the work, got all your cruise tune up stuff dead nuts on. Bitchin' Good. BUT! When you mash on it there is a stumble. I hate stumble. Most guys want to add more pump shooter. That's my last resort. Remember, if you pull the PV out you see two channels. Some blocks have the power valve channel restricters already drilled. If not, just drill and tap to 8/32. Please check and make sure you have 8/32 sized holes. I've seen some blocks that can take a 10/32. Really, it makes no real difference. Just drill and tap the hole if it's not to 8/32. If it has to be 10/32 so be it. Just make it tuneable. Now, when you are cruising down the road and you mash on the loud pedal and you get a stumble, the fix is add .010 fuel to the power valve channel restricters. If it fixes it, damn good. If it makes it better but not correct, open them up another .010 until you eliminate the stumble. If you now thing you have it by the short strokes, add a bit more. If it likes it you know you are there. If at this point it acted like it made it worse, go back to the last setting and test. If it's better, then you need to drill out new restricters but make the .005 smaller than what you had. You should have it nailed.
It takes time to sort out each system individually. But the best results happen when you tune in blocks. Focus on the smaller parts of the system, get them in shape. Then move on the the next step. When you are ready to tune emulsion, let me kmow. I'll help where I can.
Carbs are simple devices if you understand them. It's like eating an elephant. One small bite at a time.
