I recently bought a replacement base plate for my 4778-4 holley. I have a two corner idle and a square bore. I purchased a base plate off eBay from allstate carburetor and the guy on the phone told me that the baseplates are the same for a two/four corner idle circuit. Now I got the one in the mail and it has two pegs blocking off the passage where the circle is in the picture. I'm thinking it's for a four corner idle and I already installed it. It was the same part number on both but my old one was different. I also have to say holley must just make garbage, because my new base plate does the same thing as my old. Their is just too much slop in the shafts which causes idle problems unless I use certain throttle return springs. I can explain more but I need to get going. Here's the pic, the circled areas are where the pegs are in the new one. I was also at a local speedshop today and they had a plate without the pegs.

I think the base plates are the same, though I don't have one infront of me to look at. I say they are, becasue I bought a ProForm base awhile back, and they say they are for 650-800 cfm carbs (DP specifically)... and my 650dp is not a four-corner idle carb like my 750dp is.

I wouldn't be surprised but it wouldn't make sense.
If they are different should I only use the primary throttle side for idle and leave the secondaries closed? I'm not buying another base plate. I just want the damn thing to idle as it should. Right now i'm having cold running problems and I didn't have them before. When I push the clutch in to come to a stop the car stalls, it wasn't this bad before. When it's warm it doesn't happen.

AFAIK, the only difference between 2-corner and 4-corner carbs, is that the rear metering block's idle circuit is adjustable on the 4-corner, and is set and non-adjustable on the 2-corner. They both still flow fuel at idle. Just bolt it on as it is and see.

Have you turned the allen screw under the linkage for the secondary plates?
Try closing the secondary plates completely so that you are only idling off the front plates. See if this helps.
-Jesse

Idling only off the front plates causes other problems you don't want.

Adjust it the way it's supposed to be.

I have been running the new base plate since I put the car back on the road. Mine was leaking from the throttle shafts.
Since then I messed with the idle/jetting/and squirters. Now i'm worse off then I was before and i've just about had it. The thing runs strong when you stomp it but other then that it's annoying and it really isn't a joy to drive unless you're floored.
I think before I was doing the wrong thing by jetting up to mask a hesitation when you step on the gas while cruising. If you step smoothly and not fast it's fine, other then that it just stumbles once then catches up unless you stomp it fast then it stumbles many times. I'm close to just jetting back up, I wasn't fouling plugs.
I had 74 primaries and 81 secondaries with a pink cam in the first hole up front and a 31 squirter. Now I have 70 and 79 withe the pink cam #1 hole and 31. I can't get rid of the stumble off acceleration and I can't get the damn thing to idle right. I change from a 28 to 31 nozzle with little noticible difference. I even went out and bought 35 nozzles when I had it jetted down to 68 and nothing, it was barely noticable. I didn't mention the secondaries because when I open them there's no hesitation. I've spent hours reading about tuning holleys and I can't get it right. This is pathetic, i'll admit it. I don't know how I got it to run good last year, i'm lost right now.

I just went outside and changed the pv back to an 8.5 inch valve. I forgot that I switched back down to a 6.5 inch as per the holley tuning instructions. I will see tomorrow when I drive it, but i'm pretty sure that was what got rid of my stumble the last time in the end.

Today I went and talked to someone that knows their stuff and he said right away before I got to it. You have to watch the passages in concern because of differences. So i've been chasing my tail. Now i'm going to put the stock one back on. It barely leaked from the throttle shafts, there was a little buildup of hydrocarbons but when I sprayed it with starting fluid the idle was steady. I have to say allstate carburetor on eBay is a bunch of idiots, he told me they're the same, and gave me an attitude. I'd like to drive there and throw it through their front window, but i'm not motivated enough. Now when I call we'll see what they have to say.

Just in case anyone is interested the blue pump cam in the number two hole works wonders. I'm shocked at how much of a difference it can make. I was able to tune my carb and it's good now. I have an 8.5 pv, 31 shooter,72 jets, and a blue cam. All is good now, better then it's ever been.
I was told when I called allstate carburetor, that i'm fine. Supposably you can't put a 2 corner baseplate on a 4 corner idle carb, but it will work the other way around. And he just told me to barely crack the secondaries.