Here is the real deal with 4 corner idle. It will improve idle on any engine by making the un-adjustable secondaries adjustable.
It is up to you wether you want a secondary block with a powervalve. The important part is to have the mixture screws obviously. If using a powervalve, you will possibly need to resize the PVCRs, (power valve channel restrictions) but this can be done later and will not effect idle.
This is what you need to do:
-Get a primary metering block. Any carb will do, but the best thing is one from the same carb you have now. I have bought broken or core carbs off Ebay to get them. Thank god for the torque monsters breaking their baseplates. A core carb will help too because you can practice everything that I mentioned in this post before drilling the carb that you will run.
-Drill the baseplate on the secondary side to match the primary. You will see how the primary idle circuit is routed. It is kind of tough, but you have to drill the baseplate from the underside so the the passages meet up through the baseplate. Use your baseplate gasket as a template. You need two holes to intersect. One from the top and one at an angle from the bottom. There is usually a bump just under the throttle plate where they need to meet. Try and match the primary side hole sizes. You will notice on the baseplate that the primary only has one hole and the secondary has two. That is where you will be adding the passage from the top. Don't drill either hole too far. Go a little on each until they meet. Use a drill bit to check the angle that came stock on the primary side.
-Block the old secondary crossover channel in the baseplate using JB weld. It is a channel between the primary and secondary on the sides looking from the top. I plug it about 1/4" and use a file to carefully machine the JB weld flat.
-Look at the carb main body. There are holes where the idle circuit passes through on the primary side that aren't on the secondary side. They will intersect the top of the baseplate on the secondary side where you just added your new holes. Looking at the bottom of the main body, again use you gasket as a template and mark where the new hole needs to be. Now this part is critical just like the last intersection... On the side of the carb at the secondary side of the main body, there is a hole that needs to be drilled that will intersect the hole you drill from the bottom. Use the primary side again as a reference. The depth of these hole must meet up the same. I go a little at a time until they meet. Makes sure you are drill straight and level when drilling both holes. Don't go too far. Sometimes the hole from the side has been started for you as on some 1850s I have done. Most carbs you will need to start from scratch though.
-A good drill index will help for this part. You need to check the sizes of your IFRs (idle feed restrictors) and IABs (idle air bleeds). Use the butt end of the drill bit for this. When you feel a snug pull and the next size up won't fit you know you found it. The IFRs are on the metering blocks. They are brass restrictions about 1 1/4" up when looking at the main body from the side. They are on either side of the powervalve at about the same height to center. On top of the carb, the IABs are the outer bleeds at the entrance to the venturi next to the fuel bowl vents. They are brass restrictions. The IABs are the larger diameter holes on the outside. Don't touch the HSABs (high speed air bleeds) which are located on the insides closest to the fuel vents. They have nothing to do with the idle circuit for the most part.
I would shoot for .033" on the IFRs both primary and secondary. For the IABs go with .070". This is just a baseline. The main thing here is the IABs on your carb will be the wrong size for 4 corner idle. You can get brass hex head setscrews to make you IABs and IFRs adjustable. You will need to drill and tap the IFRs and IABs. I would use 10-32" for the IABs and 6-32" for the IFRs. Get the 1/4" length or 3/16" if you can find them.
This step is what 99% of people miss in doing the secondary conversion. The IAB/IFR relation makes or breaks the benefit of 4 corner idle. A wideband 02 sensor is your best bet at fine tuning everything when you are done. That will help find the timing and tuning of your bleed ratios.
Once you get all the conversion finished and the restrictions done, set your transition slot to .020" on primary and secondary if possible. If you can't get the idle you want you can close down the secondary some. It is more important to get the primary set right. I always adjust my idle speed at the secondary and leave the primary at .020"
Your advance curve must be set right. I like 18-20o initial on my engine. I wouldn't go any lower than 14o. Make sure your mech. advance is set up for proper total. This will be somewhere in the 32-36o range approx. Vacuum advance can help a street car too. Just don't overdo it. Make sure the starter doesn't kick back and that you don't detonate at cruise or under load.
This website will help more than anything else out there. Do some searches for drilling and tapping bleeds. This is the best info on carbs I have found anywhere. http://www.innovatemotorsports.com/f...isplay.php?f=7
There are plenty of books out there showing how to add 4 corner. Most of them miss one or two steps though. You can look though to see where the passages are that I am talking about.