I may actually be firing up my 451 soon and would like to know what primary and secondary jet sizes (and power valve) I should start with.

10.3 CR, ported iron heads, .654" Mini-Express cam (316 advertised, don't know the .050 but prob around 270), 850 DP on Edelbrock RPM, 18 initial/34 max advance, adjustable vac advance can, 1-3/4" headers, dual 3" exhaust with x-pipe. Probably idles at 1000-1100 rpm with 7" vacuum.

I have an O2 sensor bung and will be putting in a wideband sensor/gauge for dialing it in, just want to fire up something approximate for easy break-in and shakedown cruise.


Thanks for any recommendations.
-Charles

Have you looked at what Holley jets the carb out of the box? That's a good baseline as any. It'll probably need a 4.5 primary PV and a blocked secondary PV. Blocking the secondary PV means you'll need to add 6 to 8 #s to the secondary jets over what your primaries are.

And did you build this from a bunch parts you had sitting around the garage, cuz it strikes me as seriously mismatched combination?

You'll need more like 24+ initial advance with that cam, and I'd be surprised if you can get a reasonable idle any lower than 1200 RPM.

The carb isn't anywhere near new (although I treated it to a rebuild kit), but if I can look it up on Holley's site that will be a good starting point.
I figured I'd start with a more modest initial to avoid starter kick-back problems. I can always put the FBO plate in a different position if it wants that much and can still start hot.
Thanks for the tips.

About the mismatched combo: Search for "mushroom" or "Mini-Express" here on Moparts and you will find several guys who run mid-10's with iron heads in heavier cars than my A-body, for example:
https://board.moparts.org/ubbthreads/ubbthreads.php/topics/646923/2.html

Edit: your own .sig says it all: "Do what you can, with what you have, where you are." - Theodore Roosevelt

And the RPM intake seems to really perform well in every "manifold shoot-out" I've been able to find. Probably gives up some hp over a single-plane at very high RPM's but I'm not brave enough to spin it over 6500 or so, and it will be on the street more than the strip so the corresponding loss of low-end from a single-plane is a factor.

If that carb. is one of the early type 4781-? 850 CFM D.P carbs., look at the dash number and use the stock jetting from Holley to start with and go from there

You don't mention the car weight, gearing, trans type, converter stall (if applicable), or performance goal. But from what I've tested over the years, either you have way too much cam for the intake and headers, or you have the wrong intake and too-small headers for the cam.

As far as using ported OEM iron heads, sure, they'll work. I squeaked into the 10.9s with some on an RB 451 pump-gas engine in a 3700# E-body.

74's in the front, 84's in the back. Block the power valve.

Here's my old set up for my '69 Cuda just for reference>>>

Stock 440SP bottom end, BV 906's around 9.8:1, old list 4781-1 850DP, 80/80 was stock jets, I had 86/90 w/no rearPV, 50cc fr pump, 2.5pv, Team G, 324-290@.050/.650/.650sft. 2" f/wells, 14 intitial 38 all in on 98 oct leaded, HS r/rockers, tickover was a smooth count the cylinders firing 900rpm. footbraked to 4800 flash launch, ran superb back in 1990 good 60's just not very quick in the ET dept.

I assume you mean only the secondary (rear) power valve. Just curious, why would you recommend blocking that off... I could install one that opens as low as 2.5" of vac.

I don't know yet (until I get it on the road) what my cruise vac is going to be, and at what point I start getting into the secondaries. I imagine it'll take some tuning with a wideband O2 sensor...

RB446, I'm impressed that you got that "Open 24-7" flat tappet cam (290@.050) to idle at 900 rpm. Now THAT's a lot of cam for almost anybody

Edit: Came across this very detailed article on 850 DP tuning. I'm a bit intimidated... but will tackle it after the engine is broken in and the car is street-legal driveable.

http://www.corvetteactioncenter.com/tech/bbfh/big-block-from-hell-part-17.html

More like 82/88 with a 4781 850 DP, no rear PV.

Did you look at and find the list number on the choke horn? If so what is it, if not look at it and post it on here so we can look up the stock jetting for that list and dash number and give it to you
I've had five of the Holley 850 CFM D.P. carbs. with one having the original List #4781, the other four where later models with different Dash numbers like one 4781-2, another 4781-5 and even a later model with 4781-8. None of them have the exact same parts and some of them have different jetting and dual power valves also

List 4780-2 and under that, 1276 (date code?)

Turns out that's actually an 800 DP, not an 850! It's been in my parts stash a long time... wonder if I was "misinformed" by a long-ago seller I'll probably have to get a bigger carb eventually but the 800 should work fine for break-in and cruising.

Meanwhile I found this handy Holley reference table and it shows stock jetting as 70/85 with a 6.5 power valve.
Obviously the power valve needs changing to something like a 3.5 since idle vac will be 6-7 inches (from people who have run similar sized engines with this cam). There's only one PV number listed for the 4780-2 so it may not even have a secondary PV - I will check the next time I have a chance to work on it!

I would put a p/v in there a couple of "`s below CRUISE rpm`s not idle as it has no influence. That`s a wide jet spread but try it as is I guess and what size ifr`s, idle and high bleeds does it have cos that will influence the curve and overall characteristics of the car..........Then there`s the t-slot holes............

Thanks. There seems to be some disagreement as to the effect of power valves at idle... Holley says it can enrich the mixture at idle (although I don't think there is a large effect). In your experience it doesn't make any difference?

I have no idea what cruise vacuum might be with such a large cam. Guess I will have to try it and find out!

If you set that carb up to idle on the transition circuit like many people on here recommend, the power valve can and will be opened at idle if the manifold vacume is lower than what the power valve is rated at Tuning all of the circuits on any carb. on a race type motor with a "big" cam is the key to making things work properly, in other words make the motor idle on the idle circuit only, not on the transition or primary circuit I'm sure that carb is not a four corner idle carb. originally so you can open the secondary throttle blades to allow more air in without fuel in it to help speed up the idle RPM and clean up the idle mixture AFRs at the same time

Drive it w/a vac gauge you can read on a steady cruise state, then light load etc and get a feel for your richening/lean needs.......

Thanks Cab. I agree, idle circuits are for idling (and if the transition slot is too exposed I wouldn't be able to adjust the idle mixture with the screws either).

Yes, it has a setscrew underneath to open the secondaries (that the carb has to come off to reach). As a ballpark guess I have just barely cracked the secondaries a few thousandths (a previous owner has already drilled tiny holes in the primary throttle blades). The primaries are set with the idle speed screw so the transition slot is "square" appearing (around .020").

For initial break-in I want 2000+ rpm anyway, so if the idle speed is in that range with these settings, it won't be a problem.

Guess I will put in the stock jets for now (70/85, 6.5" PV). I'm certainly seeing a range of recommendations here

i would set it up to baseline, the holes drilled in the primaries will probably help.
the carb i got, which is probably not perfect for my app but seems to work fine so far, has a screw on the secondary throttle shaft so i can adjust it from the top, pretty nice.

my motor is a mild 360 with high compression and small holes in all four throttle blades from the previous owner, it still seems to work fine though i probably have very very little of my t-slot showing.

anyway, for the breakin you will be in the transition so hopefully that fuel mix will be close.

I could convert mine (with the Holley 26-137 kit). But that little bracket and screw costs $43 Probably worth it if I have to take the carb off more than once, though!

To the best of my knowledge, 4780's all came with only two corner idle.
If yours hasn't been converted to 4 corner idle, I think you're going to find it a chore to get it to idle well when in gear, and/or you'll have to have it idling at a fairly high rpm, with a cam with as much duration as what you're running.

Part of the reason for so much spread in the jetting is the 800dp's have larger Venturi in the secondaries, so the signal is a little weaker and more of the total air flow at wot is going through that half of the carb.
The "if it were me" answer is, I'd start out at 73/85 jetting, 5.0pv...... And convert it to 4 corner idle.
Quick fuel makes an adjustable secondary idle speed bracket, Jegs has them for about $18.

Thanks for the info, and the QF bracket at a reasonable price!

Car is a 4-speed, so no worries there, even if it idles best at 1200. I have a Holley jet kit, so I'll try your recommended setup first.

Can you please tell me what parts I'll need to do the 4-corner-idle conversion? Presumably a new secondary metering block with idle screws, but will I need anything else?

when i'm researching things like this, i usually do a google search then look under the images, found this pretty quickly. it's easy to find what might be good articles or forum posts based on the good pics...

https://board.moparts.org/ubbthreads/ubbt...orner-idle.html

edit: that is talking 3310 there ^ so maybe not everything applies,

here is what is probably another good post.
http://www.hotrodders.com/forum/4-corner-idle-conversion-holley-4150-a-344154.html

also this forum...
Mark Whitener http://racingfuelsystems.myfunforum.org

Just what I need, thanks.
I gave up on this board's search engine many years ago, it's practically worthless.

All that's needed are the mixture screws and gaskets, the correct size tap and a few small drill bits.
It's just a matter of "connecting the dots".

On my old 440 that was actually a 446 as it was .030 over. I ran 906 heads and it had 10.0 comp as it ran good on 92 pump since I used quench pad pistons. I used the MP .557 cam and ran an 850 DP with 1-3/4 headers. It was a street car so I ran the primary power valve. I was running 76's and 82's I believe with a 4.5 power valve. It worked good as I could drive the car on the street all the time and the plugs looked good and even after 2 years on the same plugs they did not foul. Course I only drive maybe a few thousand miles a year. I had it jetted for mph as it did run an 11.49 @ 116 in my 3700 lb 63. I still use the 850 on my 440/493 but I had to jet way up with this stroker as I run 86's and 92's right now to feed this combo and I still run a power valve as its a 3.5 in it now. Ron

The closer the carb is to the motors cfm requirements the closer the stock jetting will be to what it needs. If the carb is to small for the motor it will be rich, to big and it will be lean out of the box.Dave