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Carbs: 750 -- 650 -- 600?


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My engine currently has a Holley 750cfm carb on it, old and tired and thirsty.   My H code  351c has an M code 4v intake on it, installed at least 22 years ago by a prior owner.   I want to install a new carb and was wondering if the 750 cfm is the way to stay or drop it a bit to a 650 or so.  Also are the carbs like the Edelbrock AVS2 650, Holley 4160 600 or Summit Racing M2008 Series Carburetors SUM-M08600VS  a plug and play, especially with throttle and kick down?  Any other suggestions?

 

Thanks,

Jim

 

Edited by jscott
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Jim, just to start the ball rolling, it had an M code intake with huge ports added, but were the heads changed as well to M code do you know?  The reason is there would be a huge mismatch at the ports. I'm thinking that could be an issue for your performance.

On mine, M code, I have a Holley 4150 SA 670. that needed a bit of tweaking to get it to run right, but if I were to do it again, I would look more towards the Quick Fuel HR series. They used to have a 735cfm, but that has been dropped which is unfortunate because I feel it would have been damn near perfect. I don't like the 4160's personally and I really don't like Edelbrocks for no other reason than they "look like they're on sideways", plus I know people who have had them and did not get the performance they wanted and got rid of them. But that's a personal choice, some really like em, some don't.

The summit M2008 seems to be a favored carb at a good price. It comes down to what will work on the build you have.

 

Edited by Stanglover
corrected typo.

Geoff.

 I learn something new every day!

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2 hours ago, jscott said:

yes,  my heads have a 4* on the corner which denotes a 4V head with the larger ports and valves.

That's great, but I would still check the date codes between the rockers, just to be sure of what you have. I believe (and I'm not 100% sure) 4* heads could be open or closed chamber depending on when they were made. It would be an asset to know that without having to take the heads off.

Going back to the carb, I think most feel the 750 cfm would be fine on a Cleveland, but if it were mine, I'd definitely be looking at jet sizes and timing along with plug and wire choice. For secondary's, I'm lazy, so vacuum with a correctly chosen spring, would be mu choice.   Basically, it all works together so finding the right combination is where the experts come in.

Have fun and build another beautiful car.

 

Geoff.

 I learn something new every day!

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Good questions.

First it looks like you have the correct heads and someone did good to upgrade to roller cam and rocker setup.

My car is finally on the road and I am trying to debug a few things.  In the carb world I am unsure if I am undersized or maybe have another problem.  I have the Holley 670 ultra on mine and I notice a slight stumble during more aggressive acceleration.  Whether at lower speed or on the highway, it tends to hesitate or stumble noticeably during “more than normal” acceleration.  I am not carb savvy enough to know if that is jetting, carb sizing, or ignition.

It seems very challenging to size a carb for our 351 4v cars.  According to Summit and Holley, a 600 to 650 is supposed to be fine.  But on this forum and other Cleveland sites, I hear 750 all over the place.

Kinda makes me wonder what the OEM carb would run like.

 

kcmash

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That's good to know your heads are a match to the intake.

One of the heads on my known original early 71 motor, has a date code of 0M1, 70-Dec. 01 and is a D0AE-GA. For whatever reason I only took a pic of the one side. At least that gives you information on what you have.

Geoff.

 I learn something new every day!

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18 hours ago, kcmash said:

Good questions.

First it looks like you have the correct heads and someone did good to upgrade to roller cam and rocker setup.

My car is finally on the road and I am trying to debug a few things.  In the carb world I am unsure if I am undersized or maybe have another problem.  I have the Holley 670 ultra on mine and I notice a slight stumble during more aggressive acceleration.  Whether at lower speed or on the highway, it tends to hesitate or stumble noticeably during “more than normal” acceleration.  I am not carb savvy enough to know if that is jetting, carb sizing, or ignition.

It seems very challenging to size a carb for our 351 4v cars.  According to Summit and Holley, a 600 to 650 is supposed to be fine.  But on this forum and other Cleveland sites, I hear 750 all over the place.

Kinda makes me wonder what the OEM carb would run like.

 

kcmash

Just chiming in here to help out, but I thought I'd offered my thoughts on this in a post you started. I have the same carb on mine and yes it took some sorting out. Drilling a 3/32" hole in each primary throttle plate, front of shaft in the middle. That helped big time with AFR at idle. Other than that, jets are stock, accelerator pump cam and cam are stock, but I changed the "squirter" from the stock #31 to (if I remember) a #35. That pretty much stopped the bogging you are experiencing once the motor is up to temp. You may also need to play with the secondary vacuum spring. It has a quick change cap. so very easy to do. Go to Holley.com and download the manual for the SA 670 if you don't have one.

NOTE: if you do decide to drill the plates, you MUST do this off the motor and plates removed. Make sure to secure the screws with red locktite or punch.

Edited by Stanglover
Added note.

Geoff.

 I learn something new every day!

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was looking into the new carbs and saw mechanical secondary vs. vacuum so I went to look at my current set up.  There is no vacuum line running to the carb other then the brake booster but there are 2 blocked vacuum ports.   I am assuming the bottom, angled one in this picture would have been for the distributor (now a Mallory so no vacuum needed) and the other????   Would that have been for my secondary and if so does that mean I am running without the benefit of them?

carb vacuum ports 021.JPG

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35 minutes ago, jscott said:

was looking into the new carbs and saw mechanical secondary vs. vacuum so I went to look at my current set up.  There is no vacuum line running to the carb other then the brake booster but there are 2 blocked vacuum ports.   I am assuming the bottom, angled one in this picture would have been for the distributor (now a Mallory so no vacuum needed) and the other????   Would that have been for my secondary and if so does that mean I am running without the benefit of them?

 

The vacuum for secondary's is from internal vacuum through ports in the carb, NOT a separate line.

Geoff.

 I learn something new every day!

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The bottom is full manifold vacuum and provides advance timing at idle where as the top is what they call "port" vacuum and it only kicks in at higher rpm's and when the throttle plates start to open.   You want to be connected to the bottom.   As for sizing your carb you may want to watch the following video.  Very good info.

 https://www.badasscars.com/index.cfm/page/ptype=product/product_id=450/category_id=13/mode=prod/prd450.htm

Edited by Kilgon

Kilgon

 

 

"The only dumb question is the one not asked"

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Kilgon is right, however there is much discussion on whether to run distributor vacuum on "ported" or full manifold. My choice is ported (timed in other words) and that seems to work best on my engine. But, as you have a mechanical distributor, not a concern unless you change dizzy's to vacuum advance.

Geoff.

 I learn something new every day!

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1 hour ago, Kilgon said:

The bottom is full manifold vacuum and provides advance timing at idle where as the top is what they call "port" vacuum and it only kicks in at higher rpm's and when the throttle plates start to open.   You want to be connected to the bottom.   As for sizing your carb you may want to watch the following video.  Very good info.

 https://www.badasscars.com/index.cfm/page/ptype=product/product_id=450/category_id=13/mode=prod/prd450.htm

thanks, informative video and confirms, at least to me, I need to downsize from my 750 to a 600  

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Looks like you might have the typical 3310 vacuum secondary 750cfm Holley. What's the LIST number on the airhorn? 

For a 4V Cleveland with an aftermarket cam, 750 is a good size. Clevelands are completely different than any other small block engines, so "carburetor calculators" don't work here, nor should you take advice on carb sizing that is not specifically for a 351C.

Your "thirsty" carb is more likely a result of improper timing and the lack of vacuum advance. IMO, you could try a rebuilt kit on the existing Holley, get rid of the non vac advance distributor and pick up performance and fuel efficiency. 

 

 

 

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Please keep in mind:
No matter what carburetor size you finally decide on, none of the carburetors is plug & play. Yes, your engine will more or less work, but all new carburetors are set too rich out of the box. In any case you have to tune the carburetor to your engine.

Frank

1972 Mach 1, 351C-2V, wimbledon white, blue all vinyl luxury

born in Dearborn, grown in the district of San José, spent a lifetime in California, moved to Germany in 2010

Mustang_Mach1_wallpaper_300_150.jpg proud member of clublogozusammen.JPG.59fb4a10d15cfff9ec756235059135b8.JPG

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10 minutes ago, Vicus said:

Please keep in mind:
No matter what carburetor size you finally decide on, none of the carburetors is plug & play. Yes, your engine will more or less work, but all new carburetors are set too rich out of the box. In any case you have to tune the carburetor to your engine.

Frank

I know, only replace one and that was on my AMX, pretty easy on that one, just bolt on and fine tune.

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What is your goal? What do you use the car for? There are way too many "undriveable" cars out there due to a poor selection of parts. 

A 750 will offer more power and performance, a 650 will offer better throttle response and drivability. A smaller carb, within reason, gets better "signal" from the airflow through the engine and will be more responsive to throttle opening. The larger carb, again within reason, allows more airflow and potentially better performance. 

A carb recommendation is not a generic "this will work best" proposition. With a mild cam and 4v heads I'd suggest refreshing the 750 you have unless the throttle shafts are worn out. On a 2v with a mild cam I'd go with a 650 ish carb. It all depends on the combination. 

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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That is likely a combination of no vacuum to the distributor and maybe not enough accelerator pump. I've never really been a fan of the Mallory distributors, I put in a Duraspark with an old Ford Motorsports (MSD) box and love it. I paid a friend with a distributor machine to reset the mechanical advance to provide 26* at about 3000 RPM.  That said it's hard to diagnose running issues over the keyboard. 

Is the existing carb in good shape mechanically? If so, working with the carb you have will be the most reasonable route. 

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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I can't speak from the part of having a 351c but I would think based on your info for what you have and are wanting in performance a 650 to 700 tops would work well.  Way to many people oversize their carbs and then fight issues with running rich and foul plugs.  I bet half the guys running what they think is the right size carb have carbon on their plugs whenever they pull them and are constantly fighting a rich mixture.    The video I posted provides good info and should be used as a guide but it is not the gospel.  I have a 460 that can easily handle a 850 dp but for my driving habits and use I am running a 750 with vacuum secondary and it runs great.  It's little easier on the gas also.  

Kilgon

 

 

"The only dumb question is the one not asked"

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Just put a new Holley 600 on and she started right up.  After a few adjustments idles a lot better.  Need to get a return spring and c clip to finish kick down part.  At the same time I pulled the steering wheel to put a new one one.  I had ordered one with a billet aluminum hub, they sent a black bub so I am waiting for the correct one before I take a drive.........Unless i really get the bug to see how it runs then in will pop the old wheel back on.   When i took off the old 750 it seemed to have lot of gas in it.  How much gas sits in the carb normally.?

 

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