351C Carburetor Questions

[kcmash]

I know this has been discussed by a lot of people before, but I am still unsure which way to go.

Issue: Carburetor selection for car.

Engine: 351C 4-v/ 4-Speed/ 3.25:1 locking rear. Engine has quenched chamber 4-V heads/ stock 4-v standard bore intake, and a Motorsport M6250-A341 Cam. The engine has not been started yet.

Carb Choices:

1) On the engine right now is a Holley 600CFM Vac-Sec that was on the car from my college days before I tore it down. I recently rebuilt it for the sole purpose of starting and breaking the engine in. I understand this carb will likely be a choke point for the engine, so I am looking for the right carb to run the car.

2) I have 2 original Autolite 4300 carbs that need rebuilt. I have a straight bore intake, and these are straight bore carbs. I would need to find the correct kit and rebuild one of these, but i am unsure how it will do for overall performance and driveability.

3) I can buy a re-manufactured Autolite 4300 from Rock Auto --http://www.rockauto.com/en/catalog/ford,1971,mustang,5.8l+351cid+v8,1132692,fuel/air,carburetor,5904?a=Referer+www.google.com+URL+%2Fen%2Fcatalog%2Fford

4) I can buy a new carburetor - Holley 750 DP/Carter/ Edelbrock/ Quickfuel per forum recommendations.

My biggest question is this. If I send one of my 4300 cores to Rock Auto to get a re-manufactured carb, I am only in for $192. Will I be unhappy with an OEM carb?

What do the super mechanics recommend out there?

Thanks! kcmash

 

[goodnigh]

Not a super mechanic myself although my current mechanic is pretty good.

Have a setup similar to yours. Used a Holley 670 Avenger on a rebuilt engine

and the car seemed to want more air. Switched to a Holley 770 with better

results, "but" still seems to want more air.

mike

 

[rocket366]

Holley 750 DF DP is what I run and Recommend.

Paul

 

[will e]

Can you tell us a bit about how you plan on driving it? How many RPM's will you spin it?

Double pumpers are cool but usually not a great choice for street driving. Vacuum secondary carbs are easier to set up.

750 cfm is big for a 351 CI engine spinning 5000 RPM. Do some google searches based on CFM, CARB, Cubic inches, RPM.

 

[Stanglover]

I'm no mechanic either, but from what I've learned recently with my own 71 351C 4V with a mildly upgraded cam, my choice for a carb would be a Quick Fuel of around 650 cfm. I currently have a Holley Street Avenger 670 with 65 primary jets, 67 secondary's. I had trouble with it at idle, it was running too rich and the idle mixture screws were ineffective and I could not get a smooth idle. My tuner guy drilled a small 3/32" hole in each primary throttle plate and regained control. Probably could have gone a few thou bigger for a better result. It needed more air!!

From what I understand, people immediately think they need a bigger cfm carb. Not so. There is a good article on Mustang 360 website that was recently posted here, all about carb sizing. Even my 670 is likely just a bit too big, but as tuned, I get good mileage and that is more important to me with our much higher gas prices.

The Quick Fuel carbs are way better for tuning as they are fully adjustable. My local speed shop has dropped the Holley line for Quick Fuel if that says anything.

Geoff.

 

[will e]

Stanglover, check your power valve as well. Take a vacuum reading at idle and cut the value in half. This is what you should use for your power valve.

Holley has a lot of good video's on how to tune their carbs:

 

[Stanglover]

Stanglover, check your power valve as well. Take a vacuum reading at idle and cut the value in half. This is what you should use for your power valve.

Holley has a lot of good video's on how to tune their carbs:

That's going off topic, but that has all been done, more than once. Even tried other rated power valves, but ended up at 6.5 which is the factory recommended PV for the 670 as my vacuum is above 13" hg.

What it comes down to is the fact that these 4 V manifolds with huge intake ports do not run well at idle speeds. The trick is to get the best balance between fuel and air, but it's never going to be perfect.

You have however, raised a good point and something that all too often gets missed. I appreciate your come-back non the less.

Geoff.

 

[72HCODE]

with power valves the best is 1/2 idle HG +1

so 17HG [17/2 +1 = 9.5 (instead of 8.5)] the "+1" is a good all around fudge factor for temp variation and octane variation in gas.

the higher the number the earlier the valve opens when you stomp on it. there are also duel and triple stage valves you can also compensate with the accelerator pump. I changed over to the 50cc accelerator pump from the stock 30cc to prevent lean miss fire, in addition to pump shot cam changes and increased shooter size.

yup "4 V manifolds with huge intake ports do not run well at idle speeds" you have to go off the reservation in setting up clevelands they all tend to run better with larger then normal cfm and you have to pour fuel into them with a funnel.

you also have the option of reducing timing advance to compensate for not being able to introduce more fuel into a system.

many people install the mr gasket 3000rpm timing set on the mechanical advance then run into major issues where they feel they need to run a 850cfm or larger to compensate or they start drilling out carb trying to get more air or fuel into the system. when the solution was better to go back to factory springs and then adjust them either making them tighter or looser to hold back the timing longer before the mechanical fully came on.

the V4 open chamber heads are a bastard under 1200rpms.

 

[Hemikiller]

I had that exact cam in my old 71 Mach. Exact same setup, other than I used a variety of gears from 3.00 thru 3.91 over the years and had a Ford D1ZX dual plane aluminum intake.

My recommendations would be:

1) forget the M-craft 4300 option. Too much of a PITA to tune and they have a bad rep anyway. Sell them on eBay.

2) Get a Holley double pumper in the 700~750 cfm range. I had an 800 on mine and it only became an issue when I put the engine in a 70 Torino GT with a C-6 for a summer cruiser. Needed a higher stall converter.

3) you'll need a carb spacer for the Holley in order for the throttle plates to clear a stock iron intake, 1/2" will do. Better option would be an aftermarket dual plane intake, the Edelbrock RPM Air Gap is supposedly good if you have the hood clearance and do a port match. Blue Thunder dual plane is the cats meow for a 351C4V.

 

[c9zx]

The Autolite 4300s that fit you OEM "square bore" intake are all likely 600 cfm so, as Hemikiller said, sell them. The -L (last letter in casting number) intake you have should be opened up to a larger diameter. A 1 inch open spacer will likely improve power above 4500 RPM. As others have said, the Quick Fuel offers far more adjustability than the Holleys and for less money. A 750 CFM should work well.

 

[Don C]

Cleveland's, especially 4Vs, seem to like more carburetor than the "C" brand engines that most online carburetor sizing programs are geared towards. You can easily go 50 cfm more than what they tell you. My 4V with a Shelby (Blue Thunder) intake is quite happy with its 760 Holley with vacuum secondaries.

 

[kcmash]

I had that exact cam in my old 71 Mach. Exact same setup, other than I used a variety of gears from 3.00 thru 3.91 over the years and had a Ford D1ZX dual plane aluminum intake.

My recommendations would be:

1) forget the M-craft 4300 option. Too much of a PITA to tune and they have a bad rep anyway. Sell them on eBay.

2) Get a Holley double pumper in the 700~750 cfm range. I had an 800 on mine and it only became an issue when I put the engine in a 70 Torino GT with a C-6 for a summer cruiser. Needed a higher stall converter.

3) you'll need a carb spacer for the Holley in order for the throttle plates to clear a stock iron intake, 1/2" will do. Better option would be an aftermarket dual plane intake, the Edelbrock RPM Air Gap is supposedly good if you have the hood clearance and do a port match. Blue Thunder dual plane is the cats meow for a 351C4V.

Thank you for the great information.

I am running the standard engine and a factory Ram Air system. I think there is room for the 1/2 inch spacer in there.

I keep hearing the Quick fuel option but am totally unfamiliar with those carbs. It sounds like users on this forum are very satisfied with that option though.

How did you like the cam when you ran it? Too much for the street, or just fun and powerful?

Thanks

Kcmash

 

[kcmash]

So are you guys recommending the Quick fuel HR750 for me? The Web site has that at $519. Is that cheaper than the holley 750 dp?

 

[rocket366]

I got a nice used Holley 750 DF DP off ebay for 220 delivered and spent 50

on a quickfuel rebuild kit.

No complaints

Paul

 

[kcmash]

I got a nice used Holley 750 DF DP off ebay for 220 delivered and spent 50

on a quickfuel rebuild kit.

No complaints

Paul

That was the route I have been planning to take too. I just rebuilt the 650 that is sitting on the car, so I have the tools and the talent. Just need t know how to buy a decent used Holley off the internet.

Quickfuel rebuild kit? why not a Holley kit? What does the DF stand for?

Thanks!

kcmash

 

[rocket366]

Duel Feed

I like the quickfuel gaskets they are better than Holley IMO

On the ebay carb check the list number stamped on the horn to this page;

http://documents.holley.com/techlibrary_carb_numerical_listing.pdf

When you get a carb see if the jets match that page as well

Paul

 

[c9zx]

The QF is an improved version of the Holley for less money. The same is true of their rebuild kits and replacement parts. QF was founded by former Holley employees.

 

[Hemikiller]

How did you like the cam when you ran it? Too much for the street, or just fun and powerful?

Thanks

Kcmash

It's a great street cam. Well mannered when you want it to be and comes alive over 3K. I had that engine in three cars, two 71's and the as mentioned 70 Torino. Fuel mileage ranged from 18 w/3.00 to 12 w/3.91. Quarter mile was in the 13.6 ~ 13.8 range. I did have a problem with lifter pump up at around 5800, but that was because I made the mistake of using stock replacement lifters and not a performance lifter.

 

[basstrix]

Here's a thread on a great site for cleveland info that talks about your cam and may have some useful info regarding carb selection.

http://pantera.infopop.cc/eve/forums/a/tpc/f/5650045562/m/4181021846