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carb gurus please come inside

Discussion in 'The Hokey Ass Message Board' started by shifflett, Jul 30, 2013.

  1. shifflett
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    west columbia tx

    shifflett Member

    Hey guys, I have a problem I hope you all can help me with? im running dual carter yfs on my 235 and they have a stumble of idle, they have been rebuilt with auto parts store generic kits (I didnt know any better at the time). I have synced them with a snail style vacuum gauge, im running 5/16th line which is just a hair smaller than stock. it idles pretty good and runs really well at higher speeds. if anyone could maybe point me in the right direction it would be greatly appreciated!
  2. OldColt
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    OldColt Member

    If you have a stumble coming right off idle, there is probably a lean condition right at that point. This can happen if the butterflies are open slightly too much to reach the desired idle speed exposing the little holes in the main circuit. If you advance the initial timing up say 5-10 degrees and close the butterflies some to get your best idle speed and the stumble goes away, that is the problem. If not, you may need a larger shot of fuel from the accelerator pumps. This is assuming you don't have a vacuum leak anywhere in the intake system. Hope that helps.

    --- Steve ---
  3. carbking
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    carbking Member

    I wee bit more information would be helpful.

    What YF's are you using?

    If you have a basically stock 235 and trying to use two YF's originally designed for a 235, then the stumble may be very difficult to eliminate.

    However, often a stumble right off idle is a direct result of having the idle set too rich, resulting in puddling in the intake. When the throttles are opened, the puddles are swept into the cylinders creating an instantaneous OVERRICH condition, hence the stumble. This condition often is caused by using too large a carburetor(s).

    Jon.
  4. Ricks Garage
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    Ricks Garage Member

    Bog ?....or.....hesitation?
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  5. shifflett
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    shifflett Member

    My motors a 54 235 bored 60 over with dual exhaust and dual carbs the head is a 53 235 with hardened valve seats the rest is stock. The carbs are carter yf 964, if I slowly accelerate its ok but if I try and give it any gas quickly it almost dies then comes to life.
  6. carbking
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    carbking Member

    The 964s should work well.

    Before doing ANYTHING to the carburetors, make certain your distributor is advancing correctly, and the dwell and timing are as they should be.

    Then, acquire a Uni-Syn (brand name I am familiar with, there are probably others) and synchronize the carbs:

    http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

    If these two procedures do not eliminate the problem (or at least minimize it to an acceptable level), it may be necessary to redo the idle circuits on the carbs. If so, give me a call at the number and hours posted below. Will be glad to explain. Or this link may help:

    http://www.thecarburetorshop.com/Idlemixturescrews.htm

    Jon.
    Last edited: Jul 31, 2013
  7. shifflett
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    shifflett Member

    The vacuum advance moves just not sure how much it should move also I have synced the carbs with a gauge not to sure how well the motors timed
  8. carbking
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    carbking Member

    Just a suggestion:

    Set the dwell first, then the timing, then think about the carbs.

    Jon.
  9. shifflett
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    shifflett Member

    Alright ill give that a try could those really be that serious of a problem?
  10. hoop98
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    hoop98 Member

    That funky 235 Combustion Chamber need lots of advance, try it with manifold to the vacuum advance. Here is a 59 235 Timing Spec, 3-5 initial. Dont mess with the carbs until you have the advance right, thats opinion.

    [​IMG]
  11. shifflett
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    shifflett Member

    How much should the vacuum advance move when accelerating
  12. saltflats
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    saltflats Member

    Listen to carbking for the carbs to work wright the engine needs to be in proper tune and the engine needs to be sound.
  13. saltflats
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    saltflats Member

    The dist sould move around 8 to 14 deg
  14. 31Vicky with a hemi
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    31Vicky with a hemi
    ALLIANCE MEMBER

    There's no way to answer this.

    Under light throttle ( incidentally you said yours runs good) the vacuum can should advance the timing proportionately to the amount of vacuum being generated by the engine. There is plenty of vacuum at light throttle. Look at hoops chart.

    Under heavy throttle ( incidentally where you are having trouble) there is very little vacuum ( high load) so the can does little to zero on the advance. You are relying solely on the mechanical curve built into the distributor. This curve is based off of rpm and is very adjustable by changing weights and springs. Again look at hoops chart.

    Picture this-
    You are sitting at a light, the ignition is advanced via vacuum.
    Light turns green and the pedal goes down, throttle plates open, vacuum goes to zero, all vacuum advance drops out and your relying on mechanical advance only. But since it falls on its face the Rpms go down instead of up and you loose your mechanical advance too. This really compounds your problem.

    Ever hear the saying " 90% of carb problems are of an electrical nature"

    Your distributor may very well be perfect and you may very well have a carb problem. But you need to make sure that your distributor is working as it should or tuned to your engine first. It's the hierarchy that reduces the tail chasing. You can ignore the tried and true hierarchy but you will get dizzy chasing your tail.
    Last edited: Jul 31, 2013
  15. shifflett
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    shifflett Member

    Alright ill get all the timing stuff situated and go from there
  16. carbking
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    carbking Member

    And always address the dwell first, as changing the dwell will change the timing. If you set the timing first, and then change the dwell, you will modify the timing.

    Like 31Vicki stated, doing things in order can eliminate a lot of chasing your tail.

    Jon.
  17. hoop98
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    hoop98 Member

    You need a "MytiVac" pump or similar. You use the chart above and apply vacuum to the cannister and make sure it is inside the spec. So at 12" of vacuum you should advance 16-21 degrees.
  18. shifflett
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    shifflett Member

    Ok timing has been checked and seems right on, when the pointer is on the bb in the flywheel then rotor points to cylinder number 1, points are gapped to .018 is this correct? If so the car is still acting the same.
  19. mustang6147
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    mustang6147 Member

    If you can adjust the accelerator pump. I think it needs to come in earlier...

    You got good vacuum right?

    There is that moment when the throttle is opened. That split second the manifold vacuum is gone, and your engine needs fuel. The accel pump makes up that fuel deficit.... or perhaps your pump is bad all together.
  20. shifflett
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    shifflett Member

    Vacuum at my carbs? If that is what you mean when I synced them it was 20 inches of vacuum. Im no mechanic by any means so some of this doesn't make alot of since to me so mind my ignorance
  21. 52HardTop
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    North Haven Ct.

    52HardTop Member

    I know your in Texas and maybe don't need my question, but, your running two carbs? Do you have heat to the intake? I know, I know it's warm in Texas! I figured it's worth a shot...
    Dom
  22. shifflett
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    shifflett Member

    Im in an extremely hot and humid area in texas been 100s last few days so no intake heat as I feel when the motor warms up there's plenty of heat how would I isolate one carb
  23. Ricks Garage
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    Ricks Garage Member

    Block off the fuel. But it may be a combination of both carbs either giving it too much fuel or too much air. Your carbs have mechanical "power" valves that pull a metering rod out of the main jet. You need to determine if you are bogging (too much fuel) or hesitating (not enough fuel). Too much fuel the engine will labor until you back out of the throttle slightly. Hesitation, it will fall off until you hit the throttle again.
  24. shifflett
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    shifflett Member

    Im guess im bogging it wants to almost die if I push the gas fast until I back out of the throttle

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