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Hot Rods Sizing a carburetor for a 350 chevy

Discussion in 'The Hokey Ass Message Board' started by ramrod2624, Apr 30, 2018.

  1. ramrod2624
    Joined: Dec 19, 2006
    Posts: 648


    i have a 350 chevy in a 59 Willis panel. The engine has a ca in it not sure what it is exactly , it has a larger race carb on it right now.
    I had the engine Dyno tuned but it still seems to run rich ( the tuner said it was leak) but you can smell a lot of gas when idling. Currently getting about 8 miles to the gallon on this.
    The tuner re jetted the carb and said it was running lean...... my question is do I need to put a smaller carb on this and if so what's best , size. Will u see better mileage?
  2. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,600


    Exactly what "race carb" is on it now?
  3. indyjps
    Joined: Feb 21, 2007
    Posts: 4,076


    Any idea on the cam, or what was the result of the dyno tune. Is this a driver/ cruiser or a racer. It could use a 600 vacuum to 750 double pumper or any variation in between, depending on what it is.
    If you have additional details it will narrow it down.
  4. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,239

    Mike VV
    from SoCal

    The FIRST question...what's the makeup of the engine, trans. and rear end (gear ratio)..?
    Is a 195hp wonder, or is it more like 550hp ?

    Sorta sounds like you've taken the car to people that aren't sure what they are doing.

    Frankie47 likes this.

  5. Q-jet. One size fits all.
    Note : Notice the o/p would like to improve his gas mileage..Hence, no DP's :eek:
    upspirate, j-jock and squirrel like this.
  6. Torana68
    Joined: Jan 28, 2008
    Posts: 1,239

    from Australia

    Find a new tuner , A “dyno tune” should have left you with a clean running car but I don’t know what you asked of them ....
    Truck64 and oj like this.
  7. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,071


    8mpg at 1/4 mile at a time sounds like a fun and inexpensive day of fun to me ;)

  8. oldolds
    Joined: Oct 18, 2010
    Posts: 3,116


    Sounds like you may be chasing 2 different problems. Running lean and smelling gas do not go together. I think you may be running lean because of a vacuum leak. The tuner may have over jetted to compensate for that. Just a guess from too far away to really know.
  9. Yes.
    Smaller, with vacuum secondaries.
    Buy a new one.
  10. oj
    Joined: Jul 27, 2008
    Posts: 6,257


    For economy, ease of installation and reliability a 600cfm Edelbrock would be the best for you, thye just take up the space between the manifold and air cleaner. Be sure to add a phonelic spacer under the carb. Don't get the fancy one with the tunable whatzits and thngamajigs, get the most basic one with electric choke and bolt it on.
    Just Gary and hrm2k like this.
  11. 97
    Joined: May 18, 2005
    Posts: 1,689


    The only place you NEED a bigger carb than a 600 vacuum secondary on a 350 Chevy is the drag strip , and nobody measures miles to the gallon on a drag strip . Go to Holley's website and see the carb sizing graph... Hell even NASCAR ONLY used a 390 cfm carb.
  12. Clay Belt
    Joined: Jun 9, 2017
    Posts: 381

    Clay Belt

    500 CFM Carter AFB. Used to have one on a 390 big block
    samurai mike likes this.
  13. Misconception number one. Carb size is based on airflow. If your engine needs lots of air you got to a bigger carb and then jet appropriately.

    Misconception number two. Its an eyeburner at idle thus the jets are too big. Tow things usually are the culprit on that one and jets is not one of them. Most of us do not use a carb with idle jets.

    First improper float adjustment is a problem. Second is that the secondaries are coming open or as in the case of a Holley are propped open. That is adjustable some are easier than others.

    Misconception number three. Bad mileage means you need a smaller carb. No bad mileage means that your engine is not operating efficiently. If your engine needs a lot of air for instance and you go to a smaller carb you are only going to starve it not increase your mileage. Get your air/fuel ratio correct, then your basic tune and you will make your engine efficient. Once you are running efficiently you are getting the best performance and mileage that you can ever expect from the old beasty.

    Misconception number four. My engine has a cam in it. All 4 stroke engines have a cam. All your are saying is that it is a 4 stroke engine and not a 2 stroke. Has no bearing on carb size.

    Most mildly built small block Chevies will run just fine with 600 CFM if the intake is sized for that size carb. Even with an intake sized for say a 700 CFM carb they will still run fine. To run around town and the occasional trip down the track a 600 CFM Vac secondary carb is all most of us will ever need.

    To actually size a carb for an engine to optimize it you need a whole lot more info than we have been given and what information I have given on carb size is just from my own personal experience with small block chebbys. It is not optimal for a given engine just a general rule of thumb.
  14. Torana68
    Joined: Jan 28, 2008
    Posts: 1,239

    from Australia

    And GM fitted a 750 cfm , standard, why would they do that? NASCAR used the smaller carb to restrict HP ..... so fit a smaller carb and reduce your HP.... or tune what you have to run right (only works if the carb was sized for the combination you have)
    72yenkonova likes this.
  15. ramrod2624
    Joined: Dec 19, 2006
    Posts: 648


    Let me try and answer a few questions asked above
    The combo is a 350 motor , 350 trans combo, at this time I'm not sure of the rear end ratio.
    The carb is a carterAFB competition series 9635SA 1565. Which if I'm right is a 625 cfm, the jets have been changed
    Manifold is a performer RPM
    Initial timing 15 38 A total 49 I have a pertronix ignition on this
    Currently has 205 to the rear @ 5000rpm
    again it smells like its running rich to me, and as I said before burning through a lot of fuel. Roughly 8 miles per gallon
    This is more a daily driver local streets / weekend cruiser
    Trying to get better milage and the running rich issue addresses
  16. Marken
    Joined: Jun 21, 2010
    Posts: 38


    Have you pulled and checked the color of your spark plugs?

  17. Everyone seems afraid or cant tune.......learn how a carb works, and what the engine needs.
    The factory stuck big Holley's on stuff, then stuck 2bbl's on stuff.... The old= use a 600 cmf is a stand by- to get you by....... Tune a lil, maximize things...... That's unless your watching the gas pump per dollar.......
    I can't wrap my head around the ol'e 600 cfm is the savior to all carb issues.....ugghh
  18. Tickety Boo
    Joined: Feb 2, 2015
    Posts: 1,310

    Tickety Boo
    from Wisconsin

    Do you have an internal or external fuel pressure regulator and can you check amount of fuel pump pressure to the Carburetor. Might also clean/check the Carburetor needle/seat for dirt.

    A lot of fuel problems show up in spring after a winter nap, the ethanol stored contaminates the Carburetor and or regulator .
  19. Torana68
    Joined: Jan 28, 2008
    Posts: 1,239

    from Australia

    Find a dyno tuner that can tune not just do a HP print out, you need to put your hand in your pocket and pay someone that has a good rep . Look at it as an investment in your years fuel bill.
  20. I run a 700 Holley on my SBC, recommended by my cam manufacturer. With the modifications and steep rear, I have gotten 11-12 mpg out of it. No fuel smell at all, the plugs are clean. Look at your plugs, see if they are full of gas. You may have something else going on, like a bad float level.
  21. carbking
    Joined: Dec 20, 2008
    Posts: 2,961


    The "Carter 9635SA" is NOT a race carburetor. Yes, it says Competition Series, but that was Federal Mogul's marketing, attempting to cash in on the real Carter Competition Series carbs.

    Yes, it is a 625 CFM. The real Carter Competition Series 625 CFM was 4759S.

    The carb you have is one that was re-engineered by the F/M accountants (does that make it re-accountanted ;) ) to sell for less than the 1850 Holley.

    When Carter was still making carburetors (the 9635S is the carb that was modified by F/M to make the 9635SA) Carter recommended the 9635S for use on a medium performance Chevrolet 350 CID street engine.

    There are many reasons why it could be running rich.

    Montana1 likes this.
  22. Yes and no! Not to confuse you but, I run a 500 Edelbrock on my 433 cu. in. BBC in the motorhome, but it never sees over 4000 RPM, requiring only 501 CFM.

    On the other hand, I run a 750 Edelbrock on the hot rod with a 385 cu. in. blown SBC, which only turns 5500 RPM, requiring only 612 CFM (unblown). Let me explain.

    The CFM of any motor is easily computed based on it's displacement and the estimated maximum RPM it is capable of. In your case, a near stock 350 cu. in. should be capable of approximately 5000-5500 RPM with a mild cam.

    However, a 4-cycle motor only moves 1/2 it's cu. in. in air through the motor per revolution. A 350 will pump a maximum of 175 cu. in. of air per revolution at 100% volumetric efficency.

    Let's assume your 350 could turn an over estimated 6000. That means you would pump 1,050,000 cu. in. of air through the motor per minute. (175 x 6000 = 1,050,000)

    One cubic foot is equal to 1728 cubic inches (12" x 12" x 12"). This means that 1,050,000 cu. in. equals 607 cu. ft. That's the most air you can ever pull at 6000 RPM with a 350. 607 CFM.

    Here's the formula... (1/2 displacement in cu. in. X estimated RPM) divided by 1728 = total CFM needed.

    A 350 with a 700 CFM carb could have enough air flow for 6912 RPM, and a 350 with a 750 CFM carb could go all the way up to 7405 RPM, with the right cam, heads, exhaust, etc., of course.

    That is way more than you will ever need for a mild 350. An over-sized carb will usually cause a loss of throttle response and performance, and be very difficult to find the correct tune.

    Although some people like to trash the Edelbrocks and Quadrajets, they do have two cool features. Their primaries are small for good throttle response under normal driving, and secondaries have a variable venturi.

    That means the secondaries will only open far enough for the motor to reach maximum CFM (what ever the configuration of cam, heads, exhaust, etc will allow), up to their maximum air flow in CFM.

    Nevertheless, I prefer the Edelbrock because it is super easy to tune and parts are everywhere. Holley's are not quite as easy to understand, but some guys are good at them, and Quadrajets are way too complicated for the average guy.

    I say all that to say, that a 500 Edelbrock would be perfect for your application on the street, but your 625 CFM Carter will be just fine once you get it tuned and adjusted right.

    There, you just had a crash course in automotive engineering on how to find the right size carb! And you thought I was just a painter... :rolleyes:;):cool:
    David Gersic, pat59 and bobss396 like this.
  23. My recommendation for the 600 CFM carb is that I know it will work in nearly all situations. I could have just as easily recommended a Q Jet (725 CFM?) and he would be fine too. We are working with an unknown entity here or at least we were at the time.

    We still don't know much about the engine other that it is currently running an AFB (not a big carb @ 625 CFM) on a Performer RPM. Edelbrock recommends 650-800 CFM for that intake.

    We are also working with a guy who is not a tuner nor is his dyno man obviously. And we are working with an engine that no one knows anything about. The owner and certainly none of us.

    His current carb should be fine if he or someone that he is associated with learns how to fix the problem.

    @ramrod2624 not to bust your chops and just for future reference 625 CFM is not a big carb. 800-1000 CFM is a big carb on an SBC.
  24. missysdad1
    Joined: Dec 9, 2008
    Posts: 2,986


    An Edelbrock 1406 600 cfm vacuum secondary electric choke is a bolt-on-and-go carburetor for SBCs...and Edelbrock has spent a lot of time and energy making it that way. Why try to reinvent the wheel? Go to your local parts store, buy one and bolt it on. Save yourself all the aggravation and hot air...unless, of course, you've got nothing better to do than try to beat Edelbrock at the game they play best.
    tfeverfred and Gasoline Junkie like this.
  25. Deuces
    Joined: Nov 3, 2009
    Posts: 19,621

    from Michigan

    I'm sorry, I'll try not to laugh... :rolleyes:
    Montana1 likes this.
  26. Yeah, something basic is wrong with the carb that he has. High float, sticky or hung up float, bad parts changes, bad adjustment.

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