I have an older Holley 4781 that does not have the adjustable secondary idle screws. Is it worth it to purchase a new secondary metering block to gain this?

This will be on top of a little 468 Big Block. Cleaned up heads with 2.19in and 1.88ex, 9.8 to 1comp,cam of which I am going to bug UDHarold for lol. 255/263 at .050, .626"/.626" solid flat tappit. Behind that a 3000 stall converter with a turbo 400 trans with a 3.73 gear.

As for intake I am on the fence between the Edelbrock RPM Air-Gap , Weiand Stealth, Edelbrock Victor Jr, Any opinions on these would be great. :)

Thank you everyone in advance.

I'm no expert, but with that cam I don't think secondary idle screws would do much. My secondary idle screws are always kept completely shut, (it's a similar #4779 on a 408ci bbc). This seems to allow the front screws to have a little more effect. That's with a cam that's slightly less radical than yours, and an otherwise very similar combo.

As for intakes, I'd use the RPM Air-Gap. I like the idea of keeping that incoming air as cool as possible.

Go for the RPM Air Gap

IMO, your ahead w/o the secondary screws. Their a PIA to setup correctly, and don't (for me) seem to have enough of affect to warrant having it.

Use what you have and go with the Air Gap. Depending on how early the carb is it might have really too small idle restrictions in the metering block.

Hello,

I would not worry about having secondary screws. For years my 468 had an old (very very old) 850 4781 atop. No secondary air fuel screws. Had solid lifter flat tappet cams in .600 range, and also hydraulic flat tappet cams a bit smaller. Was able to get the carb to work with every combo. Street car....so I always had power valves front and rear. With the big cam I had 3.5 power valves as the motor did not make much vacumn. With smaller cams 6.5 power valves worked. My car is also a stick. Never had holes drilled in butterflies.

Best tip I got over the years was to tweak the rear butterflies open a bit. It is done with the set screw that is screwed into the base plate from the bottom. You can take it out and screw it in from the top to get better access to it for when carb is on the engine. Using that screw, I had secondaries opened just a bit with allows some fuel and air into the engine from the secondary side. If I remember right, you only want to open them until .035 of the "transfer slot" is exposed. Any more than .035 and the carb may act funny.

If you open the secondaries a bit, you might need to readjust pump shot levers using the spring loaded bolts that drive the accelerator pumps. Not a big deal.

With some trial and error you can get your 850 carb cookin on your engine.

Good luck, Joe

Thank you so much for the info people. :)

Any one else with more information?

What those other guys said. the sec idle usually end up screwed all the way in in any type reasonable street deal anyway. If you live in the northern half of the country you'll want a working choke if you run an air-gap. If the carb doesn't have a choke you might want to consider a non-air-gap if you want to drive it in cold weather. No choke + air-gap manifold + cold weather = PITA. Or, you could stick a heating pad under the manifold for a couple hours before taking off.

Tom a heating pad lol. Well I am in San Antonio TEXAS so there is no real need for a choke.

But for now I think my problems are taken care of for awhile lol. A friend had a Edelbrock Victor Jr That he no longer needed since he went with a square port and Dominator Carb. And he was working on a car that needed to be old school and I had an old Edelbrock Torker 2-O that he feels he can use. So for either good or bad I will have the Victor Jr and the 4781 Holley 850 DP. Crossing fingers :)

And just a metering block won't make a four corner idle carb. Mods are required to the throttle body to make it functional.

I've had both and like everyone else has said, if it's an anywhere near streetable combination the four corner idle is overkill.

Gary Anderson