Off idle hesitation/pop through carb
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Off idle hesitation/pop through carb

  1. #1
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    Default Off idle hesitation/pop through carb

    Just ran the new engine, 476 fuller roller with dual plane and 830 Holley. Anything above idle it has instant response. But if I poke the throttle right off idle it hesitates and popped through the carb 3 times over a 3 day period. I checked the accelerator pump to make sure it was pumping as soon as the throttle was engaged. Should I get a bigger cam for the pump or go bigger on the primary's? As soon as that hesitation is gone it runs great, if I go from 1500rpm to 5000rpm it screams, it only happens when coming off Idle. Idle is 1000rpm

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    Senior Member orngxtc's Avatar
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    Just a guess from me. Maybe a larger squirter size?

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    Where is your timing, do you have the advance locked out?

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    timing is 20 idle and 38 full advance. I'm thinking I need more fuel coming from the accelerator pump.

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    Chevy or Ford?

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    Chevy. If I slowly roll into the throttle until 1500rpm I can do whatever I want and it just responds. It's only quick stabs to the throttle at idle.

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    I'm thinking your advance is coming in alittle late, when we used advance (jet-boat) @ 2500 we had total timing
    it doesn't hurt to lock it out

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    Quote Originally Posted by nos1967 View Post
    timing is 20 idle and 38 full advance. I'm thinking I need more fuel coming from the accelerator pump.
    Easy enough to get the next size nozzle, the number is stamped into them.

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    Default Check "ball"

    Quote Originally Posted by nos1967 View Post
    Just ran the new engine, 476 fuller roller with dual plane and 830 Holley. Anything above idle it has instant response. But if I poke the throttle right off idle it hesitates and popped through the carb 3 times over a 3 day period. I checked the accelerator pump to make sure it was pumping as soon as the throttle was engaged. Should I get a bigger cam for the pump or go bigger on the primary's? As soon as that hesitation is gone it runs great, if I go from 1500rpm to 5000rpm it screams, it only happens when coming off Idle. Idle is 1000rpm
    Your pump circuit may be bleeding off at times... (3 times over 3 days).... Also, checking the "squirt" means as soon as the throttle shaft moves, even a few thousandths, you need to see a squirt starting. AND, if you're running a Power Valve in the primaries, check it. A "pop back" can, and does, wipe them out....
    Ray
    LOUD BOATS SAVE LIVES

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    With the engine not running or at idle, how much clearance do you have between the accelerator pump lever and the adjustable screw with the spring on it? Does the cam on the throttle shaft instantly move the pump lever when coming off idle? Running rear jet extensions?

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    Quote Originally Posted by nos1967 View Post
    Just ran the new engine, 476 fuller roller with dual plane and 830 Holley. Anything above idle it has instant response. But if I poke the throttle right off idle it hesitates and popped through the carb 3 times over a 3 day period. I checked the accelerator pump to make sure it was pumping as soon as the throttle was engaged. Should I get a bigger cam for the pump or go bigger on the primary's? As soon as that hesitation is gone it runs great, if I go from 1500rpm to 5000rpm it screams, it only happens when coming off Idle. Idle is 1000rpm
    Turns out I am having the same problem you are describing. Still working on getting it resolved. Having said that, I have found that off idle hesitation issues are sometimes pretty tough to work out.

    What you are experiencing, is a momentary lean condition. So far you have gotten some real good advice.
    A couple things I would look at trying:
    First, make sure you idle circuit is properly adjusted. If you are familiar with using a vaccumm gauge to adjust the idle mixture screws I would start there. Improper f/a mixture there can contribute to off idle problems.
    Beyond that, consider changing your 'squirter' (discharge nozzle) screws from the stock solid units to high flow hollow screws, if your carb doesn't have them already. A lot less restriction with the hollow screws, & in my opinion you get a much more accurate pump shot with those.
    If those two things don't resolve it, go to a larger primary squirter. Start with 2 sizes up from what you have & see how that works. No need to mess with the jet sizes, as they don't have any affect on the idle transition circuit.
    In addition, you could try changing your primary pump cam to a more aggressive profile. If you have a white pump cam in there now, try a pink cam. It has a bit steeper profile which gives a bigger initial pump shot. (you can buy pump cam kits with all the different profiles from Summit, Jegs, etc.)

    It's a real process of elimination tracking down problems like this. Try one solution at a time & then test.......
    Last edited by garytcosta; 06-01-2016 at 02:43 PM.

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    Quote Originally Posted by nos1967 View Post
    Just ran the new engine, 476 fuller roller with dual plane and 830 Holley. Anything above idle it has instant response. But if I poke the throttle right off idle it hesitates and popped through the carb 3 times over a 3 day period. I checked the accelerator pump to make sure it was pumping as soon as the throttle was engaged. Should I get a bigger cam for the pump or go bigger on the primary's? As soon as that hesitation is gone it runs great, if I go from 1500rpm to 5000rpm it screams, it only happens when coming off Idle. Idle is 1000rpm
    ttt
    Last edited by garytcosta; 06-01-2016 at 02:43 PM.

  15. #13
    Senior Member tigertooth's Avatar
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    Try a more aggressive punp cam. If that doesnt work you will have to change to a 50 cc pump.

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    Quote Originally Posted by Richard Till View Post
    With the engine not running or at idle, how much clearance do you have between the accelerator pump lever and the adjustable screw with the spring on it? Does the cam on the throttle shaft instantly move the pump lever when coming off idle? Running rear jet extensions?

    From what I'm understanding, Richard has a good suggestion.

    With the engine off, there should be absolutely no clearance between the accelerator pump arm and the plunger for the accelerator pump. (Don't make it to tight either) Another suggestion is slightly push the accelerator pump spring and arm down and make sure the accelerator pumps squirt fuel instantly. If the accelerator pump diaphragm or poppet valve are damaged, fuel isn't being activated like it should.

    More than once I've adjusted my idle and without realizing it, changed the accelerator pump arm creating a clearance that shouldn't be there.
    That adjustment is probably one of the last adjustments that should be made when working with a carburetor.

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