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Old 04-08-2013, 02:28 PM
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Holley accelerator pump pullover!

I'm posting this to help some of you that may have this problem.

Accelerator pump pull over is when the accelerator pumps flow gas at certain RPMs when the pump isn't even actuated by the pump cam. usually caused by vacuum through the venturi across the pump nozzle pulling fuel out of the nozzles!

Ive worked on 2 new Holley double pumpers in the last month on engines that were running rich that had Accelerator pump pull over at speed!

Even on the secondary's when the secondary throttle plates were closed ( no Vac. through the venturis) That really threw me because there was nothing to cause the fuel flow through the pump nozzles? I changed nozzles to no avail and tried other normal remedies no help! And pulled the pumps to find a rubber check instead of a check ball on the fuel bowls! I tried changing fuel bowls to the ones with check balls and it cured the problem!

I called Holley and talked to a tech about it and was told the rubber checks can vibrate at certain engine RPMs and actually pump fuel out the nozzles that's why the secondary pump nozzle was pumping fuel even though the secondary throttle plates were closed!!!!

This morning I did a search and found this Video!!!!




I hope this helps some of you that may not even now you have this problem at higher RPMS with the rubber pump checks

Jester

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  #2 (permalink)  
Old 04-08-2013, 03:39 PM
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Good post.
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Old 04-08-2013, 04:53 PM
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It looks like they left out the check below the squirter.

This happens on every double pump QF/demon/bg/etc ?

Hard to believe this was overlooked in testing.
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Old 04-08-2013, 06:05 PM
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Quote:
Originally Posted by spinn View Post
This happens on every double pump QF/demon/bg/etc ?
Hard to believe this was overlooked in testing.
This really happens on all those carbs you work on In 40 yrs Ive only had maybe a few that had accelerator pump pullover!!!! And it was very odd for me to have 2 in one month with the same problem with the same remedy of changing the bowls to the ball check type instead of the rubber check Type


[QUOTE=spinn;"It looks like they left out the check below the squirter." /QUOTE]
Over looked in testing??????

The check is there!! Its the (not the ball) rubber diaphragm check for the pump on the bottom of the bowl that's causing the problem!!!!

Anyone that knows carbs, Knows what I'm talking about by "pump pullover!" and its not the bullet check under the nozzles missing that caused this problem!! Its like you didn't read the thread or watch the Video




Jester

Last edited by painted jester; 04-08-2013 at 06:17 PM.
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Old 04-08-2013, 06:14 PM
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What's wrong with drilling a small hole in the pump circuit like everyone else does to cure this problem? It's not a perfect solution but it seems to work just fine.
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Old 04-08-2013, 06:33 PM
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Originally Posted by ap72 View Post
What's wrong with drilling a small hole in the pump circuit like everyone else does to cure this problem? It's not a perfect solution but it seems to work just fine.

AP: Its the rubber check that actually is acting like a MINI pump pushing fuel up and out the nozzles from the resonance of the engines rpm at certain vibration points on some engines!!! The check ball doesn't react to resonance like the rubber check does!

Its not caused from the vacuum draw down the veturis going by the nozzles!!!

Jester
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Old 04-08-2013, 06:39 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
Good post.
F-Bird: Its like you are the only one that watched and paid attention to the video!

Jester
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Old 04-08-2013, 06:41 PM
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Quote:
Originally Posted by painted jester View Post
AP: Its the rubber check that actually is acting like a MINI pump pushing fuel up and out the nozzles from the resonance of the engines rpm at certain vibration points on some engines!!! The check ball doesn't react to resonance like the rubber check does!

Its not caused from the vacuum draw down the veturis going by the nozzles!!!

Jester
What's wrong with drilling the hole though? I mean it still works. Saves you from having to swap fuel bowls.
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Old 04-08-2013, 07:10 PM
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AP: at 4:50 of the video pause it and read the bulletin and pause it at each bulletin after that, and read them it explains how the rubber check acts like a pump and drilling doesn't work! This problem is rare on most engines! It only happens when a precise resonance (vibration) reacts on the rubber and it oscillates and pushes fuel like a mini pump LOL! I tried everything to correct it and it continued till I changed the bowls and it stopped! It only happened at 2800 RPM on that particular engine, faster and it stopped, slower and it stopped, then I found this video and it explained why drilling, changing shooters and other cures wouldn't work! I have bowls all over my shop so its no problem for me to swap bowls! I posted this because most builders don't know about this odd problem. And could be losing a lot of power and never know the problem is there. It could be happening at any RPM that creates the right resonance to get that rubber umbrella check to osculate and pump fuel!

Jester

Last edited by painted jester; 04-08-2013 at 07:27 PM.
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Old 04-08-2013, 07:28 PM
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Was this occurring with the engine under load or just free rev?
The Ford L4 vibrators would pull over due to the buzzing of the engine at 7500RPM. The spring trick was the only solution to that issue that I'm aware of.
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Old 04-08-2013, 07:49 PM
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Quote:
Originally Posted by painted jester View Post

Anyone that knows carbs, Knows what I'm talking about by "pump pullover!" and its not the bullet check under the nozzles missing that caused this problem!! Its like you didn't read the thread or watch the Video




Jester
Whatever. I watched the video. It looked like they left the check out. They explain differently, but that is how it appeared to me. it is called an observation.

Also didn't realize if I responded, any question I asked would need defending. Perhaps the bg/demon/qf had corrected the design somehow.

No I m not using A holley DP on a 4 cylinder at 8000 . Is that some new requirement , in order to know carburetion.

The video is alarming, and I cant see how holley let this updates failure slip through. I enjoy the video, thank you.
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Old 04-08-2013, 08:20 PM
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Quote:
Originally Posted by lmsport View Post
Was this occurring with the engine under load or just free rev?
The Ford L4 vibrators would pull over due to the buzzing of the engine at 7500RPM. The spring trick was the only solution to that issue that I'm aware of.
It was happening on a RW dino under load on a 440 Mopar! It would still happen in park on a free rev but at a different RPM! The secondary squirters were pushing fuel not the primary and the secondary throttle plates were closed the pump cam never moved the pump arm and there was no air flow down the secondary venturis to cause a pump pullover (I never saw that before! I tried a few things and finally I changed the rear bowl and the problem was resolved . The only difference was the rubber umbrella check and ball check of the 2 bowls! I stopped the problem but I didn't really know how I did it with a bowl change so I googled Pump pull over and finally found that video on UTUBE that covered the problem I was having, Then I talked to a Holley tech with a good explanation! It really isn't a vacuum pull over, but I guess they haven't named it yet LOL Maybe they could call it a ???? What would you name it?? I never heard of this happening and worked on and blue printed thousands of Holley and copy cat carbs over the years and never had the problem with the rubber umbrella accelerator pump checks! It really threw me! So I posted this thread!

But precision Drilling and adding a tiny spring of the right pressure seems harder then just changing the bowl LOL!

Maybe those umbrella checks oscillate at different vibrations at different RPMs and you have to build an engine that is just right to produce the right vibration LOL I don't really understand how its happening or how that little umbrella can do that? it would be trick if someone could film it happening in the little pump cavity! I for one would be very interested in seeing it

Jester
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Old 04-08-2013, 08:36 PM
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They have these cheap 18 dollar USB scopes. You can fish them through the air cleaner. Watch the laptop as you see the pump shot.
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Old 04-08-2013, 09:02 PM
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Quote:
Originally Posted by spinn View Post
They have these cheap 18 dollar USB scopes. You can fish them through the air cleaner. Watch the laptop as you see the pump shot.

In the video of the inline 6 engine vibration caused the little red rubber pump check valve to flutter and act as an accelerator pump itself. Watch the video again and notice that most of the time the throttle is fixed on the primary but closed on the secondary yet the secondary shooters are squirting even though the throttle plates are closed! What I want to see filmed is the red rubber umbrella check vibrating and pumping fuel inside the accelerator pump housing under the bowl pretty small spot LOL But I would really like to see it happening Its like "BASIC HYDRAULIC THEORY" in engineering We know how it works and is happening but cant prove it LOL I'd like to see that little bugger do what they think its doing LOL

Jester
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Old 04-08-2013, 09:36 PM
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I don't buy his theory but he makes a nice video and has fixed the problem.
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