750 or 650 carb?

[Rdchallenger]

So as the title states. I'm curious as to which carb would be better for my 340. It's pretty much stock with
Comp cams Xtreme energy Hydraulic Flat Tappet, Advertised Duration 268/280, Lift .477/.480,
Eddy Air Gap intake
TA J/U heads
70 HP manifolds
904 Tranny
2.5 exhaust
3.91 gears

I have an eddy 750 performer carb #1411 currently on it and I'm wondering if that is too much and should go with a 650? Or possibly buy a calibration kit and adjust the jets and meriting rods.

It seems to be running a little rich at the moment at idle.

[Rdchallenger]

So as the title states. I'm curious as to which carb would be better for my 340. It's pretty much stock with
Comp cams Xtreme energy Hydraulic Flat Tappet, Advertised Duration 268/280, Lift .477/.480,
Eddy Air Gap intake
TA J/U heads
70 HP manifolds
904 Tranny
2.5 exhaust
3.91 gears

I have an eddy 750 performer carb #1411 currently on it and I'm wondering if that is too much and should go with a 650? Or possibly buy a calibration kit and adjust the jets and metering rods.

It seems to be running a little rich at the moment at idle.

[Bullitt-]

       I would think proper tuning should solve the rich issue.... 
  Seeing how the T/A/AARs ran 990CFM six packs I don't see how a 750 is to big.
         The '72 up came with 800CFM Thermoquad 

            What indication do you have that it's running rich? 

[Rdchallenger]

       I would think proper tuning should solve the rich issue.... 
  Seeing how the T/A/AARs ran 990CFM six packs I don't see how a 750 is to big.
         The '72 up came with 800CFM Thermoquad 

            What indication do you have that it's running rich?

The rich fuel smell, black smoke out of the exhaust when revving, as well as stumbling on its self.

[blown motor]

I have a 650 on mine with Lunati cam, X heads, TTI headers and 2.5" exhaust. Lots of power and good gas mileage. Was getting about 18 on the tour without getting on it much.

[Bullitt-]

       I would think proper tuning should solve the rich issue.... 
  Seeing how the T/A/AARs ran 990CFM six packs I don't see how a 750 is to big.
         The '72 up came with 800CFM Thermoquad 

            What indication do you have that it's running rich?

The rich fuel smell, black smoke out of the exhaust when revving, as well as stumbling on its self.

may be floats set to high or needs lighter springs to hold the metering rods down
         
    Your not running an electric fuel pump I trust? 

[Rdchallenger]

       I would think proper tuning should solve the rich issue.... 
  Seeing how the T/A/AARs ran 990CFM six packs I don't see how a 750 is to big.
         The '72 up came with 800CFM Thermoquad 

            What indication do you have that it's running rich?

The rich fuel smell, black smoke out of the exhaust when revving, as well as stumbling on its self.

may be floats set to high or needs lighter springs to hold the metering rods down
         
    Your not running an electric fuel pump I trust?

Correct on the mechanical fuel pump. From what I'm gathering not just from here but from other threads is that most run 750s. I think I'll stick with that and study up on the 1489 calibration kit that I had delivered today.

[Chryco Psycho]

Well I totally agree that 750 is better but I don't agree with using a 1411 carb ,
with the Comp Cam you have lower vacuum than normal & cannot adjust the air bleeds for idle & jetting to compensate so it will be a compromise at best tuning it .

[Bullitt-]

  I see lot's of info on the www about tuning with the metering rods, jets  & springs but very rarely do they address the float level which IMO is where you must begin... If the level is significantly off nothing else you do will solve this.
  Found this video.. While I don't agree with everything he says his float adjustment technique is good.  https://youtu.be/N4_I_loQ0Wo?t=170

[Rdchallenger]

  I see lot's of info on the www about tuning with the metering rods, jets  & springs but very rarely do they address the float level which IMO is where you must begin... If the level is significantly off nothing else you do will solve this.
  Found this video.. While I don't agree with everything he says his float adjustment technique is good.  https://youtu.be/N4_I_loQ0Wo?t=170

Awesome! I will start off with the floats. Thanks for the great info!

@Chryco Psycho with the lower vacuum of that cam what would your suggestion be on a carb if I can't get my current one to play nice?

[Chryco Psycho]

I have a tuning guide for these carbs on here , I also agree you have to start with the floats , after shipping they are never right or even close often .
I like Proform carbs , reasonable cost & great performance , Holley HP & Quickfuel are similar with adjustable bleeds but usually cost more , Mancini used to have the best prices on Proform but check around .

[chargerdon]

Help me to understand.   I have a 360 with stroker kit which with .030 over pistons becomes a 408 cubic inches.   
Which dividing by 1728 (number of cubic inches to a cubic foot) gives .236 cubic feet.   

Now at 6000 rpm (i believe to be a practical redline for a flat tappet engine) that engine can "suck in" 1416 cubic feet IF it had an intake stroke for every revolution.   

But of course 4 cycle engines only have 1 intake stroke for each two revolutions, so dividing by 2 gives 708 CFM.   However, each cylinder does not get a "full" charge on each stroke.   Most street engines have about 85% volumetric efficiency so multiplying 708 by .85 gives 602.    So based on that alone 600 CFM carb would be almost big enough and 650 would be plenty. 

I went on Summit and they have a CFM calculator and it shows 602 for a street engine, so my calculations are correct.   

So, why would i want to put a 750 CFM carb on my 408 ???

PS a 340 by their calculator shows only 501 for street and 650 for a racing engine, which they say the efficiency can go as high as 110 %. 

[70vert]

I agree it is baffling, like all the manufacturers and sales sites got their hand slapped for recommending higher cfm carbs?? I think what we see is mathematical formulas verses real-life experience. And here's the thing, you can tune an oversize carb to work well but you can't force an undersized carb to flow more (other than major surgery) so most are better off going bigger. That is my thought process developed over ~50 years of garage machanic-ing. 

Help me to understand.   I have a 360 with stroker kit which with .030 over pistons becomes a 408 cubic inches.   
Which dividing by 1728 (number of cubic inches to a cubic foot) gives .236 cubic feet.   

Now at 6000 rpm (i believe to be a practical redline for a flat tappet engine) that engine can "suck in" 1416 cubic feet IF it had an intake stroke for every revolution.   

But of course 4 cycle engines only have 1 intake stroke for each two revolutions, so dividing by 2 gives 708 CFM.   However, each cylinder does not get a "full" charge on each stroke.   Most street engines have about 85% volumetric efficiency so multiplying 708 by .85 gives 602.    So based on that alone 600 CFM carb would be almost big enough and 650 would be plenty. 

I went on Summit and they have a CFM calculator and it shows 602 for a street engine, so my calculations are correct.   

So, why would i want to put a 750 CFM carb on my 408 ???

PS a 340 by their calculator shows only 501 for street and 650 for a racing engine, which they say the efficiency can go as high as 110 %.

[Bullitt-]

So, why would i want to put a 750 CFM carb on my 408 ???

From what I understand, at least with vacuum secondary carbs like the factory used, they self adjust.
   maybe this article could shed some light
                    https://www.motortrend.com/how-to/selecting-right-carb/

I'm not saying a street car won't do just fine with a smaller carb.... Some time ago I was given an AFB off a 383 that the owner said ran good but had replaced with an original Carter during restoration ... Looked up the # & it was a 600CFM 

[Scooter]

Help me to understand.   I have a 360 with stroker kit which with .030 over pistons becomes a 408 cubic inches.   
Which dividing by 1728 (number of cubic inches to a cubic foot) gives .236 cubic feet.   

Now at 6000 rpm (i believe to be a practical redline for a flat tappet engine) that engine can "suck in" 1416 cubic feet IF it had an intake stroke for every revolution.   

But of course 4 cycle engines only have 1 intake stroke for each two revolutions, so dividing by 2 gives 708 CFM.   However, each cylinder does not get a "full" charge on each stroke.   Most street engines have about 85% volumetric efficiency so multiplying 708 by .85 gives 602.    So based on that alone 600 CFM carb would be almost big enough and 650 would be plenty. 

I went on Summit and they have a CFM calculator and it shows 602 for a street engine, so my calculations are correct.   

So, why would i want to put a 750 CFM carb on my 408 ???

PS a 340 by their calculator shows only 501 for street and 650 for a racing engine, which they say the efficiency can go as high as 110 %.

Likely one of the biggest mistakes most street rodders make... too much carburation. And I mean streeters too... if you are headed to the track your car spends a lot more time at WOT.

I've done it to myself. I had a 750 CFM Holley on my mild 390 FE Ford and wrestled with it for years... cams.. pumps... jets... oh my. The biggest favor I did for myself was tossing on a 600 CFM vac sec Holley. The car became manageable again. Did I surrender a bit off the top? Perhaps...

Put a 625 CFM Holley on my .030 over 360LA. Had to tune it down just a hair from bone stock to get it near perfect.. the AFR gauge I installed took all the guesswork out.

My experience.. unless you are racing a slightly undersize carb is going to be far easier to deal with.


My2c.