[w72]

Wondering if a box-stock 850cfm double pumper would be close enough to fire a fresh mill. I plan to use a Sniper EFI unit with timing control, but am hesitant to crank over a new (expensive) powerplant with so many variables.

I have a suitable factory HEI sitting around, and all I would need to do on the fuel side is change the regulator. The only purpose of this would be to ensure a reliable break-in period for a HFT cam, and also as a backup should the Sniper experience a failure.

Engine specs are 400 .060" over, 4.25" stroke. Flat tops, zero-decked, ported iron heads with 95cc chambers. So 467ci, just a tick over 9.5:1, and an 041 cam with Rhoads lifters and 1.65:1 rockers.

Would any changes need to be made to the carb? I'm comfortable working on Holleys, and feel free to start a new thread to fight over the cam choice Thanks for any input.

[mysticmissle]

I think it would be close. Jets w power valves in the mid to low 80s I ran mine plugged and I think the jetting was approx 86 -88 primary and 88-92 secondary but that was a drag oriented set up

If your close and your timing is in the 33 range it should run well enough to break it in

[25stevem]

That Carb should be fine for that purpose.
It may be a little on the rich side, but after 1/2 hour of Cam run / break in time your gonna be changing the oil anyway!

[PunchT37]

All day. Put it on.

[Skip Fix]

I think on my 455 with a 4781 on the 78 after tuning I am 1 step leaner than out of the box.

[i82much]

do you have another car you can run the holley on to make sure everything is right? get the float levels and choke adjusted and such before running on the new one.

[Tom Vaught]

A few questions:

a) New or used/played with #4781 Holley carb?

b) Age of the Carb

c) How did you acquire it?

Ideally you would want an older #4781 that had #80 (P) and #80 (S) jetting if a stock (as sold) carb from Holley. If the secondary PV was removed then #80 (P) and #86 (S) jetting will work fine.

Bowls turned upside/down and the Top of the float adjust like the video below

https://www.google.com/search?client...owl+adjustment

Mixture screws (two corner idle carb = 1-1/2 turns out)
(4 corner idle carb 3/4 - 1 turn out) each screw.)

Tom V.

[w72]

Thanks for the replies. I do not have another vehicle to test the carb on, nor have I gotten a hold of the carburetor yet. Are the older units better? I don't mind rebuilding one, but would rather avoid a carb that has been subjected to previous modifications.

[i82much]

Do you have the Sniper already? I love my 850 double pumper. It's a Quick Fuel but very similar to a 4781 (850 double pumper).

[Tom Vaught]

Originally Posted by i82much

Do you have the Sniper already? I love my 850 double pumper. It's a Quick Fuel but very similar to a 4781 (850 double pumper).

Marvin and Marty did a nice job on their Quick Fuel Carbs over the years.

Marty passed away a few years ago. Marvin retired and the last I heard he was fighting older age. Sold out to Holley.

Marvin's daughter started her own carb company. Hear they build fine carbs.

Tom V.

[i82much]

Originally Posted by Tom Vaught

Marvin and Marty did a nice job on their Quick Fuel Carbs over the years.

Marty passed away a few years ago. Marvin retired and the last I heard he was fighting older age. Sold out to Holley.

Marvin's daughter started her own carb company. Hear they build fine carbs.

Tom V.

Looks like this company:

https://www.atminnovation.com/about

Probably the biggest limitation I found in selecting a new carb was that I wanted a choke. There are a lot of great carb offerings on the market these days but so many of them do not have chokes!

[w72]

Yes, I already have the Sniper along with the Hyperspark distributor and ignition box.

I would want a choke on a carb, which does limit options somewhat. In the end, I'd like the car to be fuel injected as it will see some elevation changes as well as high G-loads.

[i82much]

sounds like EFI is right for you. why not use an 1850 or 3310? should be easy to find a good used one cheap. 4781 is tougher to find and won't do any better at breaking in your cam.

[77 TRASHCAN]

Originally Posted by i82much

Looks like this company:

https://www.atminnovation.com/about

Probably the biggest limitation I found in selecting a new carb was that I wanted a choke. There are a lot of great carb offerings on the market these days but so many of them do not have chokes!

A vehicle with mud grips, would really need a choke, no doubt!!!

[Tom Vaught]

IF YOU GET THE RIGHT AIR CLEANER, the choke does not hurt you one bit.

Plus on a street car it does a great job in cooler locations.

Tom V.

[i82much]

i have a manual choke holley on an old truck, it makes such a huge difference on cold days. took a little messing around to get the fast idle and choke flap relationship right but after that it has been great. that sort of adjustment would ideally be done on another engine to get the carb right before using it to fire a fresh engine, imho.

[Cliff R]

I had a 4781-2 for quite a while and used it on my first 455. Tom helped me get it dialed in and it was flawless in all areas.......except.......no choke.

Not having a choke just doesn't cut it for me. As many threads as we read where folks recommend blocking heat crossovers and not using a choke I will tell anyone reading this that deal sucks!

ALL start ups involved adding consideraby MORE fuel to the engine than you'd ever need with a working choke. As long as it was warm or hot outside the "ordeal" I went thru wasn't too hateful, but you still had to pump the accl quite a times and sit in the drivers seat "feathering" the throttle until enough heat got into the intake so the engine would idle on its own. Tolerable but certainly NOT the ideal scenario. I think this is why many folks recommend no choke and a cold intake as they use these vehicles most of if not all the time in warm or hot summer months and if they do live in a colder climate don't start them up very often and for sure aren't driving them a lot when it's really cold outside.

When the cold weather moved in the amount of fuel to get the engine to fire and stay running became rediculous. If for any reason I had to move the car off the lift in the Fall or Winter it was a serious PITA. Below freezing it would require 10-15 shots of fuel from the carb just to burble for 5-8 seconds (too rich), smooth out when it cleaned up the excess fuel I just dumped into the intake, then die out when it went lean again. I'd have to repeat this process over and over till it got some heat in the intake and for sure you were STUCK in the drivers seat "feathering" the throttle to keep the engine running because it was NOT going to idle on it's own for quite a while.

Jump ahead to about 2005. I had shelved the big Holley by that time and moved to the 1977 Q-jet. It ran equally as well everyplace and a much better fit for the Shaker and associated parts. It didn't have a choke either and still a serious PITA to fire up the engine in cold weather. So I decided to put an electric choke on it. From that moment cold starts were instant and I had fast idle to keep the engine running. One of maybe two shots from the carbs accl pump would set the fast idle cam and it would roar to life and run flawlessly until warmed up. I'd just "blip" the throttle once along the way to get the carbs fast idle cam to drop to a lower RPM, then one more time when I was ready to drive off and the engine would drop to curb idle speed and sit there idling on it's own.

So there's my take on the subject and right up until the day I sold the Ventura the only difference between starting it and a modern FI vehicle was having to blip the throttle once or twice to set the choke and put a shot of fuel into the intake......FWIW........

[Tom Vaught]

I drove my 64 GTO in college in the Michigan winters.

It had a heat stove pipe (factory) going the the center carb, open intake/exhaust cross-over, and functioning "flapper valve" in the passenger side exhaust manifold.

It always started but had a good battery and I always had a heavy blanket over the engine that I removed when I fired the engine the first time in the morning (5:00 am) prior to going to work. I still sat there and let the engine warm up each morning.

When you have to get to work and it is sub zero weather and you have ONE vehicle, you do what you have to do.

Never tried a Q-Jet on the car, engine was basically a stock Tri-Power in those days.
I did install a AFB on the car for a short period of time (for a college class related to fuel economy) and it also performed well in the winter time.

A older #4781 Holley carb with a factory air horn and correctly working choke is a very nice driving carb.

Tom V.

[i82much]

I have a 770 street avenger on a 396 in an old truck. I plow my driveway with it. I have the functional heat riser valve in the passenger side exhaust and the carb has a manual choke. It's not quite like fuel injection but it's not far off. I can pretty much start the truck up and get going without letting it warm up very long.

[ponchonlefty]

Originally Posted by 25stevem

That Carb should be fine for that purpose.
It may be a little on the rich side, but after 1/2 hour of Cam run / break in time your gonna be changing the oil anyway!

the 850dp i have has annular boosters is there an advantage to that?or does it matter?