Carb selection.

[klinton994]

I need some assistance in choosing a carb for my new rebuild.

The past year my motor has been in the process of being rebuilt. A local machinist is doing the work, and assembly.

The motor is an original 351c 2v. The car a 71 Mach 1

The machine work is complete. I am installing trick flow 72cc chamber heads, comp camshaft 578 lift 230@ .050"

The automatic transmission is being replaced with a toploader.

The car will only be used for street.

Based on the cam specs, and heads, Trick Flow recommended using and Edelcrock RPM manifold, and a 750 carb. They also recommended a carb they produce along with Quick Fuel Technology, or using a Holley HP series 750

What I am seeing is that none of these carbs have a choke on them. Do most people choose not to run a choke on a modified motor? Pros, cons, options for carb with a choke?

What is the best way to control the secondaries, vacuum or mechanical?

 

[Jeff73Mach1]

With a 4 speed I prefer mechanical secondaries. With a larger than stock cam, vacuum is often compromised.

This is one I would consider https://www.holley.com/products/fuel_systems/carburetors/double_pumper/classic_double_pumper/parts/0-4779CE?gclid=EAIaIQobChMI0tzV1s2Z2QIVFhuBCh32nQn5EAQYBiABEgJgi_D_BwE but it is a bit high priced

Here is another option along the same lines https://www.holley.com/products/fuel_systems/carburetors/quick_fuel/hot_rod_series/parts/HR-750

No choke is challenging if you drive in cooler weather it is manageable to a point, but slow starting is hard on the engine and the electrical system

 

[73pony]

+1 on the 750 DP (mechanical secondaries) I am running a Proform (holley knock off from Summit) it has electric choke. There are plenty of 750 DP's with elec choke out there.

 

[mustang7173]

klinton994,

Check the Carburetor CFM Calculator. You could go to a 650 D.P with mechanical secondaries.

http://www.4secondsflat.com/Carb_CFM_Calculator.html

 

[Hemikiller]

Carb calculators are pretty useless without all the rest of the info. A stock 351 is going to need a lot less carb than a 351 with .600 lift cam, AFD race ported heads and 2" headers.

That said, it's a safe bet on a 351C with 4V or aftermarket heads to use a 750. Clevelands like to be a bit "overcarbed" compared to other small blocks.

To answer klinton994's question about choke, given the size of the cam you're using, most people would op for a no-choke carb for the smoother flow into the primaries. A couple pumps of the throttle and she should light right off. You may have to hold at at a slightly higher idle (1000 ~ 1200 -ish) while things warm up for a minute, but it should then be able to sit at idle without issue. FWIW, chokes are more of a convenience item and were to help the uninitiated get things going without flooding or fouling out the plugs when these cars were daily driven.

 

[boilermaster]

I need some assistance in choosing a carb for my new rebuild.

The past year my motor has been in the process of being rebuilt. A local machinist is doing the work, and assembly.

The motor is an original 351c 2v. The car a 71 Mach 1

The machine work is complete. I am installing trick flow 72cc chamber heads, comp camshaft 578 lift 230@ .050"

The automatic transmission is being replaced with a toploader.

The car will only be used for street.

Based on the cam specs, and heads, Trick Flow recommended using and Edelcrock RPM manifold, and a 750 carb. They also recommended a carb they produce along with Quick Fuel Technology, or using a Holley HP series 750

What I am seeing is that none of these carbs have a choke on them. Do most people choose not to run a choke on a modified motor? Pros, cons, options for carb with a choke?

What is the best way to control the secondaries, vacuum or mechanical?

Klinton994,

I will kind of side with hemikiller here, as to not enough information here with a few exceptions.

what is your compression ratio going to be and what gearing ?

a part number on that camshaft would also be quite helpful.

While a 750 cfm carb will be (in the ballpark ) for one of our 351 c engines, that only describes the

AIR side of the carb and NOT the fuel side.

some of the 750 cfm carbs out there have an entirely different fuel curve than others.

Especially with some of the double pumpers you could wind up terribly RICH throughout your powerband !

Without knowing the rest of the story (specifications) why risk so much on a newly rebuilt engine ?

STREET says to me that you need the engine to idle, and do you have power brakes ?

Street also says vacuum secondary's (what is your rear axle ratio and tire size ?

I would , at the very least get an O2 sensor bund in one of your header collectors and install a wideband monitor and be prepared to swap out some carb parts.

accelerator pump cam (cam's) and nozzles, power valve and main jets and if vacuum secondary's

an adjustable vacuum diaphragm.

There are some guidelines out there as to what the A/F ratio's should be with respect to what circuit you are tuning, however it all boils down to engine configuration and what YOUR engine likes.

I have done a lot of tuning on peoples hot rods who have bought and bolted on two or three carbs and stated that it just wont run right, I actually have made a stand alone O2 wideband monitor as a tuning tool and use it quite often.

For an engine size the 351 C is quite unique in that it does like a lot of air, but most of the carbs out there that are in our AIR range are NOT in our fuel range.

do yourself a favor ! if you cannot do it yourself your engine guy should be able to.

if not , find someone who can.

            Boilermaster

 

[Stanglover]

It has taken a while to get this information, so I hope it's not too out of date.

I am not sure which thread that was discussing QF mechanical versus vacuum secondary's, so I'll post the reply I got from Summit Racing here as well as another similar post as it's relevant.

I have redacted the names and as emails tend to be in reverse order, I have cut and pasted the reply so it reads in sequence.

Hope everybody finds this useful.

Geoff.