Holley accelerator pump cam
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Holley accelerator pump cam

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    Member 78hallett's Avatar
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    Default Holley accelerator pump cam

    Have a pair of holley 600's on a 460 with TR. Iam having some hesitation at full throttle and have been playing with pump cams. I changed the orange cam that came with the carbs to the pink cam on number 1 which got rid of the hesitaton but the engine is hard to start. I adjusted the cam arm tension so there is no slop but still has a hard time starting. If i put the orange cam back in, it starts right up but then my hesitation is back. Any ideas why the pump cam change would effect starting? Iam not much for carb tuning so i hope this isnt a dumb question. Thanks

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    Quote Originally Posted by 78hallett View Post
    Have a pair of holley 600's on a 460 with TR. Iam having some hesitation at full throttle and have been playing with pump cams. I changed the orange cam that came with the carbs to the pink cam on number 1 which got rid of the hesitaton but the engine is hard to start. I adjusted the cam arm tension so there is no slop but still has a hard time starting. If i put the orange cam back in, it starts right up but then my hesitation is back. Any ideas why the pump cam change would effect starting? Iam not much for carb tuning so i hope this isnt a dumb question. Thanks
    It sounds like you are dripping fuel with engine off! I would check that first. Without being insulting do you know how to adjust the pump cam? They will bleed off fuel if they are not right. good luck!!! M

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    Not insulting at all thats a valid question but yes i do know how to adjust the pump cam. I actualy think it is not getting enought fuel as i have to pump the throttle quite a few times to get it to start and stay running until it warms up a little. With the orange cam its one small pump turn the key and all is good.

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    gn7
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    If you are having a hesitation problem, why did you pick the pink cam to begin with? The pink cam moves less fuel per degree of throttle rotation than the orange cam. It ends up moving the same amount of fuel at the end of the rotation, but its below the orange cams curve all the way to the end.
    In fact, the pink cam moves less fuel per degree of rotation than any cam except the black cam, and its only slightly less over all, and actually delivers more fuel than the pink or orange can at the very beginning of the throttle rotation.
    Not surprised if it takes more a pump to get it started. Why it acts the way it does once it starts I can't tell you. But if you were looking for more fuel from the pink cam, you picked the wrong cam.



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    Last edited by gn7; 09-14-2013 at 11:45 PM.

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    Quote Originally Posted by gn7 View Post
    If you are having a hesitation problem, why did you pick the pink cam to begin with? The pink cam moves less fuel per degree of throttle rotation than the orange cam. It ends up moving the same amount of fuel at the end of the rotation, but its below the orange cams curve all the way to the end.
    In fact, the pink cam moves less fuel per degree of rotation than any cam except the black cam, and its only slightly less over all, and actually delivers more fuel than the pink or orange can at the very beginning of the throttle rotation.
    Not surprised if it takes more a pump to get it started. Why it acts the way it does once it starts I can't tell you. But if you were looking for more fuel from the pink cam, you picked the wrong cam.

    Bet ya can't repeat that word for word..lol
    I run the black cams..ground them down so the lever sits on the throttle shaft at idle..goes instant open from there and uses about 90% off the pump capacity and the entire circuit is drilled to .125 including the bolt for the squirts...but my stuff tends to be a little odd..


    Quote Originally Posted by 78hallett View Post
    Have a pair of holley 600's on a 460 with TR. Iam having some hesitation at full throttle and have been playing with pump cams. I changed the orange cam that came with the carbs to the pink cam on number 1 which got rid of the hesitaton but the engine is hard to start. I adjusted the cam arm tension so there is no slop but still has a hard time starting. If i put the orange cam back in, it starts right up but then my hesitation is back. Any ideas why the pump cam change would effect starting? Iam not much for carb tuning so i hope this isnt a dumb question. Thanks
    Quote Originally Posted by 78hallett View Post
    Not insulting at all thats a valid question but yes i do know how to adjust the pump cam. I actualy think it is not getting enought fuel as i have to pump the throttle quite a few times to get it to start and stay running until it warms up a little. With the orange cam its one small pump turn the key and all is good.
    You swapped cams and removed and adjusted it so all the slop was out of the linkage...

    Did you verify that it was pushing fuel at the very beginning of the pump stroke?

    Even a slight lag in delivery might cause a hesitation..it goes lean..then you pump it more to give it more fuel....a bunch of little pumps might be making it lean as opposed to one 1/4 throttle pump.

    And while i know GN7 will correct me if i am wrong i read somewhere that the acc pump linkage is adjusted by cranking the throttle all the way open and then push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever.
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    Quote Originally Posted by 78hallett View Post
    Not insulting at all thats a valid question but yes i do know how to adjust the pump cam. I actualy think it is not getting enought fuel as i have to pump the throttle quite a few times to get it to start and stay running until it warms up a little. With the orange cam its one small pump turn the key and all is good.
    hey hallett! I think that the squirter may a bandaide for a deeper problem. Are you running a fuel pressure gauge and how does the engine actually pull on the top end. Reason I ask these questions it could be that the drip of fuel may be getting you thu the stumble but if is real lazy on the top you may have other issues? Like what carbs are you running (yes I know 600 cfm) but which ones? Metering plates or blocks and power valves or no jet size or plate size?? M

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    Well took the boat out today and it ran great. Started up everytime and no hesitation. I double checked my pump adjustment and raised the fuel pressure from 5.5 to 6.5 and she pulled hard all the way to full throttle.

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    Ol guy, set up is holley 600 vac secondaries yellow springs, 68 main jets stock secondary metering plates which i believe are equivalent to 69 jets, 3.5 power valves, 35 squirters.

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    Quote Originally Posted by 78hallett View Post
    Ol guy, set up is holley 600 vac secondaries yellow springs, 68 main jets stock secondary metering plates which i believe are equivalent to 69 jets, 3.5 power valves, 35 squirters.
    If it ran good after the changes then treat it like a sore dick and dont screw with it. although I believe you may be lean on the secondaries. I think what made the biggest is the increase in fuel pressure which goes back to my last post regarding getting enough fuel at higher rpm. 6.5 is about the lightest I would run but the engine likes it--run it. M

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    yep I agree "if it aint broke don't fix it" I do think the fuel pressure was most likely my biggest problem too, the boat definitely had much better throttle response.
    Last edited by 78hallett; 09-16-2013 at 12:23 PM.

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