Engine hesitation upon acceleration

Discussion in 'Engine Topic' started by ratpatrol71z28, Jul 18, 2016.

  1. ratpatrol71z28

    ratpatrol71z28 Veteran Member

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    So when accelerating as of putting the foot down to the ground something is causing a hesitation such as a cough, bog, or sputter then the power comes right after. I have a Holley double pumper and tried adjusting the floats as I know how critical that can be. That hasn't seemed to change anything...... I have a feeling I may need fresh spark plugs.... even though they looked good when I checked them today and they have been recently done about a few hundred miles ago.

    I have a 350 sbc with MSD billet dist, msd street fire plug wires, and msd digital 6al box...... 650 DP carb...... around 360 hp range.....

    Any suggestions to what I could check or tune up?

    Thanks
     
  2. Z28zz383

    Z28zz383 Veteran Member

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    I would start by checking timing and more specifically where the timing advances. Un plug any vac adv and check initial.set at 10-12 Rev motor to 2500-3000 while checking timing. Should go up to 32-34. If you need more than that then a vac adv system will have to be used. Next check the accelerator pumps on the carb. They should start to shoot fuel as soon as you even slightly move the throttle. Google holleys website for a detailed tuning video to adj the carb correctly.
     
  3. ratpatrol71z28

    ratpatrol71z28 Veteran Member

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    Thanks I will take a look more into the carb adjustment.... I figured the timing could have changed a bit too.
     
  4. harbone66

    harbone66 Veteran Member Lifetime Gold Member

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    Check the front bowl accelerator pump or the lever to make sure it moves as soon as the gas pedal moves
     
  5. MotionClone

    MotionClone Veteran Member

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    Is this a new problem like it was running fine then all of a sudden started to hesitate ? Or did you just get this carb new or used and trying to tune it to the engine?
     
  6. ratpatrol71z28

    ratpatrol71z28 Veteran Member

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    New problem....... I played with the accelerator pump today and checked them with the feeler gauge.
     
  7. Damon

    Damon Veteran Member

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    Forget the feeler gauge- that's old wive's tale bullshit that has somehow propagated into modern day folklore. The key is that the accelerator pump must start injecting fuel in INSTANT the throttle arm is moved.

    That means, basically, you want to adjust the nut/screw on the accelerator pump arm spring adjustment so that it's got no slack in it at idle. Tighten it up (compressing the overload spring) until it's got slack in it, then loosen it up until the slack goes away, plus a smidge more just for good measure.

    Even if there is NO measurable clearance in the WOT position it doesn't matter. The overload spring on the accelerator pump arm will absorb it even if the pump itself is completely bottomed out.

    INSTANT hit of fuel when you open the throttle arm is what you're shooting for here. INSTANT! Capiche?
     
  8. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

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    You have a power-valve issue. The Power valve opens as vacuum drops when you put your foot down. It covers up the hole before the main jets flow at 100%. The reason you are bogging is a lack of fuel which causes misfire. Then the mains start up about 2500 RPM and the bog goes away, but will come back again ( but less ) as RPM gets into the 5,000 range because that missing power-valve fuel is still missing, so the engine goes lean again.

    Check for a blockage in the power-valve channel restrictions and/or in the high-speed air bleeds in the main body ( the brass restrictions with the small hole). Run some stiff wire down in there to clear the passages. Holleys are VERY sensitive to dirt and clogs. I've seen gasket material flake off and clog the internal restrictions far too many times.

    Check to see if the power-valve is not blown. They are sensitive to backfire, and they just get stiff with age and malfunction. You can suck against the spring to see if it holds vacuum. I generally just replace them every few years anyhow..they get stiff.
     
  9. ratpatrol71z28

    ratpatrol71z28 Veteran Member

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    Oh this is all new to me but i am learning even more! I will definitely take a look at those areas guys! Thank you! These carbs can get very tricky at times.
     
  10. rocket dawg

    rocket dawg Veteran Member

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    Ethanol leans out a carb. Seems like I started having problems is when they put 15% in the gas. I went with larger squirters and it solved my hesitation.
    In a 2 stoke engine, they suggest to go up on jet sizes when running ethanol gas
    (snowmobile)
     

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