Carb. size for 350

Discussion in 'Small Block Tech' started by MNBelv57, Nov 10, 2018.

  1. MNBelv57

    MNBelv57 Well-Known Member

    Hello all. My 75' Regal has a 350 a KB c118 cam, headers, TH 350, and 3.73 gears. The engine was a replacement short block bored .040 and the pistons are a stock type replacement without the relief holes at the top. They are probably 8.5 - 9.0 compression. The car is only street/highway driven. The literature for
    the cam said not to use anything smaller than a 750cfm carb. and I presently have a 750cfm Holley 3310 with vacuum secondaries. The problem I'm having is a fuel in the oil and a strong rich smell from the exhaust.
    The smoke from the tailpipes at startup seems normal and not black. My friends seem to think that the carb. might be too big. The car otherwise seems to run well with good power especially on the highway. I have a stock HEI with an Accel coil and timing is set at around 10-12 BTDC.

    Any advice is appreciated. Thanks, Mario
  2. hugger

    hugger Well-Known Member

    The carb isn't "too big" that's an old term that needs to die. You have a stuck or sticking float, in correct float level or a blocked air bleed . The carb needs to be serviced/ adjusted is all.
  3. MNBelv57

    MNBelv57 Well-Known Member

    Thanks Hugger. That's what I keep telling my friends but they are Chevy/Ford guys and don't always understand Buicks.
  4. alec296

    alec296 i need another buick

    750 is fine. Get an air fuel meter. It’s a universal carb, so you need to rejet , set float levels and tune, possibly a Power valve change , accelorater size and squirter changes. The 350 would like a mechanical Holley secondary over vacuum.
    You would be better off with the right Quadrajet built by Quadrajet Power or Everyday Performance. Fuel curve much more accurate for a Buick 350.
    The 118 is a decent cam but needs high compression and 3000 convertor.
    Most 350 replacement pistons are very low compression, figure 7.8 to 8.0 compression.
    Idle timing is ok , but you need to check total mechanical advance timing and what rpm it comes in. I have that cam in an engine with the pistons with valve reliefs (hyperutectic). Barely 240 hp. Needs a 3 series rear gear.
  5. Stevem

    Stevem Well-Known Member

    It's not a matter of understanding Ford Chevys Buicks or whatever , it's a matter of understanding Carbs!

    Confirm this for use to help you better.
    Look down into the primary side of the the Carb with the motor up at normal temp and at idle in gear if it's a auto car.
    While doing this do you see any fuel dribbling out of either of the booster hanging in each throttle Bore?
  6. Fox's Den

    Fox's Den 22 years of racing the same 355 Buick motor

    If you have the stock intake with that 3310 Holley you have to use a adapter plate under the carb for the square bore Holley. This gives the engine trouble in the fuel department. You need to have the TA intake on there so the 3310 Holley will work properly, and 750 is not too big.

    If you have the stock intake on there your best bet is to have a Q-Jet on there as those mate up to the intake correctly.

    Or a spread bore Holley or a Thermoquad.

    That is the rule of thumb with the two intakes.

    TA intakes, any carb you want to use.

    Stock intake, Q-jet, Spread bore Holley dbl pumper, Thermoquad.

    You probably need about a 70-72 jet for the front and take that screw off on the side of the fuel bowl and see how high the fuel level is. It should be just to the bottom of the hole with the screw off.

    The shooter on the top is usually about a 28. That is stamped on the front of it which size is in there. If you have those two in that area you will be in the ballpark for tuning the Holley. If it had backfired at any time the power valve has blown out and that will cause flooding in the engine. So make sure that is replaced no matter what, it only takes one backfire to blow this thing out.

    A 1/2 hr rebuild of this carb and you will be as good as new. Easy to work on. The hard part is the cleaning the old gaskets off.
    alec296 likes this.

    DEADMANSCURVE my first word : truck

    Correct on the rebuild . Easy . They are a modular carb so just think of it as a front , a rear and the body . Keep the sections separate , keep track of jetting etc as you go . Check Holley web for stock spec info for the carb . Or let me know what series 3310 that is and they should be in my book .
    And as mentioned be sure the air bleeds are clear . Also check info concerning throttle blade setting and the transition slots .
  8. cruzn57

    cruzn57 cruzn57

    here is a very informative site to calculate almost everything

    based upon the info you provide
    90% efficiency ( doubtful that high) ( 80 to 85% is normal)
    6000 rpm
    550 to 710 cfm is what it says,

    bigger carb does not atomize as well as a smaller carb,
    the air does not flow as fast thru bigger carb, and does not
    sent a strong ( correct) signal to idle bleeds, tip in ,etc.

    not there to argue, just think, how often does your engine reach 6000 rpm?
    where does MOST of your driving take place? ( street or strip)

    the reason a Q jet worked so well, was it has very small primaries,
    which had excellent air speed (velocity) to send good signals to all the
    necessary atomizers, bleeds, tip in slots, etc.
    vonhef likes this.
  9. Dano

    Dano Platinum Level Contributor

    I had same cam/gear but with 10.5 SCR & a M-21 & big valve ported heads and it ran incredibly well on a QJ and even that carb was never optimized so that'd be my recommendation. Not to say it won't run great or even better on an optimized /rebuilt Holley. Others on this thread have more expertise than I do and there's some great advice here.

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