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Discussion Starter · #1 ·
What would cause this?
Its a new holley 600, with vacuum secondaries and electric choke.

The first pump of accelerator and no gas comes from the accelerator pump.
The second and each pump afterwards it does.

Let it sit for a few minutes and its back to a first pump situation, and no gas.
This is causing the car to lean out and just die if I hit the gas.

The carb is about a year old, but has only been on the car a few months, been doing this since I put it on, but finally diagnosed what is actually happening today

Need thoughts on what to do to fix it.
Could it have a defective check valve or something?
I have a rebuilt kit I bought for the old carb, but didn't end up rebuilding it, so I have some extra parts.

Thoughts?
 

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Is the little drop-in needle check valve in place right below the pump nozzle(shooter)?? You'll have to pull the nozzle screw and nozzle off to see it. 3/8" or so long, about .100" in diameter, pointed on the bottom end.

Could also be a problem with the orange rubber umbrella check valve in the accelerator pump cavity in the bottom of the fuel bowl(have to remove pump cover and pump diaphragm to see it).....if they sit to long on the shelf they can dry up and curl, then they don't seal well.
 

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How about the float, fuel level in the bowl ...check the simple things, could also adjust the time of the squirt, make is squirt sooner.

Would not rebuild as a first option, carbs are not always good out of the box. Sometime they need have the basics checked.

Just adding to the thread.

Pep
 

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Accelerator pump problems must be going around I had an issue with my AVS this morning. In your case my first suspect would be the discharge check valve leaks air allowing the feed passage inside the carb to drain back to the pump. The first pump refills the passage the second pump and on ejects fuel. When you stop pumping the fuel again drains out of the passages. My secondary is it could also be the fill valve sticks open to where the first pump shot goes back into the float bowl, then once cycled it starts to operate.

Mine this morning was weird, get in pump the throttle (recently removed the EFI and installed an AVS carb) crank the engine got one weak fire then nothing but cranking. Check the ignition just fine, checked the electric fuel pump that had proper pressure, checked the carb little to no pump shot. Removed top of carb to find the pump cup seal rolled a side off its cross shaped retainer. I modified a thin brass washer to fit just shy of the bore diameter and fit over the hub so it is trapped against the bottom of the pump cup by its return spring. Put it all together and continued on with my day. Hopefully this will provide better support under the pump cup than does the plastic cross. Root cause may be a burr in the pump bore, I'll peek at it this weekend to see if a "why" sticks out at me; I'll lean out the primary jetting a little while I'm in there as well as it's running a hair rich on cruise.



Bogie
 

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The accelerator pump should fill with fuel when the fuel bowl fills. Sorta assuming here that the bowl is full and stays full. There should be fuel coming through the accelerator nozzle with any movement of the throttle. The check needle that is located under the nozzle is only to keep fuel from being pulled through the nozzle by the venturi effect. Pull the accelerator pump and verify that the umbrella check valve is in good shape. IF you need to replace it, put a touch of Vaseline on the stem, use needle nose pliers to pull it in place on the fuel bowl. The Vaseline will help keep from ripping the stem off as you pull it in position. You said the carb was new but also make sure that the accelerator pump gasket and spring are in their proper position and that the orifice for the fuel pump is not being blocked by anything. Again, any movement of the throttle on the accelerator pump arm should have a discharge of fuel coming from the nozzle.
 

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Holley flow tests there carbs with some type of liquid that is supposed to flow like fuel and there has been times that when I bought a brand new holley that some of that liquid stuff was still wet on the carb in the accelerator area and on the gaskets and it does not hurt anything but sometimes it can sit long enough on the accelerator diaphragm and it can be on the stiff side and I have had seen them rip before which could cause pump issues as well.

The accelerator check valves normally last for quite a long time and I have yet to have one go bad on me even on carbs that was used and sat for a while but with quality control nowadays that is a lack there of like stated above I would take the carb off and remove the shooter and check to make sure the gaskets are sealing and the needle is there and also check the diaphragm and spring and check valve and the check valve should be pretty much flat against the surface of the bowl.

The diaphragm should not be overly stiff and should have some softness to it but not a lot. If in doubt get a kit and put new stuff on it and it should fix your problem unless you have something of a defect in the carb itself which can happen. I don't buy the holley brand carbs new anymore as the last several I got I had to pull out the boosters and replace them as I could wiggle them with my finger no problem.

I now take the whole carb apart and check the metering blocks and spray them with carb cleaner and test everything before I even bolt them on anymore to try and avoid issues like your having. Hopefully its something simple.
 

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Discussion Starter · #7 ·
Looks like one of the problems was it was way out of adjustment.
It was only pumping on the last 1/2 of the travel of the accelerator pump.
That caused me to think it wasn't pumping on the first pump cause I was always giving it about 1/3 throttle on the first pump, and then pumping more on the 2nd pump.

I adjusted it and moved the cam up a notch to the other hole, and now it starts squirting as soon as you touch it. This keeps it from dying out, but it still coughs, so I guess its not quite getting a big enough shot of gas?

Time to go up on the nozzle Size maybe?
 

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As long as you feel you've got the accelerator pump linkage set right(there should be no clearance anywhere in the system at idle) and the idle mixture set correctly, a pump nozzle change would be the next tuning step.

Make sure you haven't got the idle mixture screws set too lean...you want to tune for smoothest idle or highest idle speed those screws will give, then back them out richer 1/16 to 1/8 turn. If you've still got a stumble when hitting the accelerator, then larger pump nozzle.
 

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Discussion Starter · #9 ·
still frustrating

After adjusting the accelerator pump and swapping the cam with a more aggressive cam it is much better.
It used to die if I gave it gas anytime it was idling.
Now, as long as the car is in park and not moving I can jam the gas and it revs right up.

The problem is, under load it still stumbles as soon as you crack the throttle open. If I don't quickly feather it, it will just die out.\

I have a bigger nozzle on order, but does this make sense that it would work fine in park, but not under load?
 

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Yes, under load it takes more fuel to do the work(accelerate).

Is this by chance an Holley "Avenger" carb?? If so, you need to read this....
https://www.hotrodders.com/forum/holley-670-vs-street-avenger-hesitation-fix-507033.html
Some of it may apply to you.

You may find you need to increase the size of the idle feed restriction in the metering block to fix this if the pump nozzle change doesn't help enough.

You may also find with the pump nozzle change the stock pump cam now will be too small again. Holley sells a cam assortment.
Pump cam and pump nozzle walk hand-in-hand to produce both a volume and a duration to the pump shot.

I would recommend you get the vacuum secondary spring assortment also, Holley sends there carbs out with a secondary spring stiffer than almost everyone needs, so that the one guy who needs a super stiff spring to avoid a bog or stumble doesn't have a problem with his brand new carb....but that leaves everyone else with a secondary that opens too slow and won't even go to fully open on most engines. Common is plain(silver) medium strong spring, or the stiffest(Black)….purple is probably the most common one I find I use in most performance applications.
 

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Discussion Starter · #11 ·
more progress

I swapped the 31 spray nozzle with a 40.
Dramatic improvement.
I can romp on it now and it won't die.
It still gives a minor hesitation right off idle, actually does better now if I stomp on it.
At least it seems to be going in the right direction.
Really appreciate everyone's input.

My buddy keeps telling me to toss the Holley and buy an edlebrock carb.
Seems like I'd just be starting over with the tuning.
Anyone have good results with edelbrock?
 

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From 31 to a 40 is a HUGE jump, you jumped right over the sweet spot and now it's creating a rich bog if it isn't cranked wide open!!.

I've only just got a 40 nozzle on my 950 cfm race carb on my 500+ hp 406 SBC with the 50cc "REO" accelerator pump

You skipped right over sizes 32, 35, and 37.

From a 31, the next logical choice would have been a 35, along with a pump cam change possibly to go along with it.

Holley also says the 50 cc larger capacity pump and the hollow hold down screw are needed when you go to 40 or bigger nozzle size to avoid the shot running out of being weak even though the nozzle is big.

No Edelbrock advice from me, I don't do "Edelbroke"
 

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Discussion Starter · #13 ·
From 31 to a 40 is a HUGE jump, you jumped right over the sweet spot and now it's creating a rich bog if it isn't cranked wide open!!.

I've only just got a 40 nozzle on my 950 cfm race carb on my 500+ hp 406 SBC with the 50cc "REO" accelerator pump

You skipped right over sizes 32, 35, and 37.

From a 31, the next logical choice would have been a 35, along with a pump cam change possibly to go along with it.

Holley also says the 50 cc larger capacity pump and the hollow hold down screw are needed when you go to 40 or bigger nozzle size to avoid the shot running out of being weak even though the nozzle is big.

No Edelbrock advice from me, I don't do "Edelbroke"
Yea I ordered 32, 35, 37 and 40
And only the 40 showed up, so figured I'd try it out.
 

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If you have access to pin drills you can take your old shooter and drill out the nozzles by hand. It works just fine and could save you some money. You might be a bit oversized now with the 40. I wouldn't make any other changes until you try going down in size.
 

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Discussion Starter · #15 ·
tried the 35

The other nozzles finally got here, so I tried the 35.
I can't really tell any difference between the 35 and the 40
Car accelerates better than the 31, but there is still a dead spot right off idle.
When you first give it gas its flat, acts like its about to die, then catches up.

Also went to the hollow nozzle screw, on the initial change to 40 nozzle, still using hollow screw with the 35.

Timing?
I have it set at about 10 BTDC, with vacuum advance disconnected and plugged. Then verified that the vacuum advance does continue to advance it when connected. Not 100% sure I have the right timing tab for this engine, but it does line up at 0 with #1 at TDC. The balancer is 10 degree off keyway.

The HEI distributor is new, its from procomp electronics, model 6000. 75K volts, plugs and wires are new too. Plugs are gapped at 35.
 

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What is the specs of your engine such as cam size and what heads your using and what vehicle and transmission etc as everything works hand in hand. If you have anything out of a stock motor with anything bigger then a rv cam I bet your idle circuit is to lean and it part of all of your stumble issues. This is a ongoing problem with the holley street avenger carbs most normally the 670 which has the exact same carb as the 600 your talking about and the only difference is the 670 has center hung bowls and a secondary metering block and a quick change vacuum pod on top and most newer aluminum ones are 4 corner idle.

That carb is very lean on the idle circuit and many folks have fought the off idle stumble with pump cam size changes and squirter size change and just could not narrow it down. i went through the same things and eventually after making sure timing was correct and everything else was correct I went and upgraded my very first 600 holley I had and got a secondary metering block for using jets as the secondary metering plate is equal to about a 68 jet which is to lean on the secondary side which still has no effect on your issue but stating one of the carbs problems out of the box tune from holley and also the idle circuit on the carb is very lean and is meant for mostly stock motors and nothing performance wise with how they have it setup.

I went through all the different combos and stuff of squirters and pump cams and nothing got rid of it. I finally did some research on carb idle circuit tuning and learned about air bleeds and also idle feed restrictor size and what function they have on a carb and on the transition circuit. That carb has a primary idle feed restrictor size of .028 and the rear metering plate is .031 and when I swapped out my rear metering plate with a metering block it still had the same .031 idle feed restrictor size and still kept my idle circuit the same calibration so after ripping my hair out I went and took my pin drill gauge drill bit set and opened up my primary idle feed restrictor to .031 and all of my issues of off idle stumbles and hesitations went away.

I am not saying you need to do that but that is an option if all else fails after you have exhausted everything else. The carb is very lean and is picky about settings and stuff and even on stock small blocks there has been times I still had to open up the primary idle feed restrictor to .031 and leave all else the same and it ran fine with no issues any more. The thing you have to be careful though is once you go bigger you can't go back smaller unless you drill out the pressed in restrictor and then tap it for 6/32 brass allen screws which are pretty cheap from mcmaster.com for a box of 25 and it quite an easy mod to do.

But before doing anything like that post your specs so we can have a better idea on what your working on. I know that carb like the back of my hand and am running a customized one I build from the ground up myself with all of my own modifications to even swapping out my own boosters for higher flower ones and recalibrating my whole fuel curve to work with my dart shp 377 small block chevy.
 

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Discussion Starter · #17 ·
specs

I bought the car, and PO did not have any specs on the engine.
If I were to guess, I would say other than intake, and headers the engine is a stock GM 350.

I had the pan off to replace the 2 piece rear seal, and it is 2 bolt main.
The casting numbers on the engine itself are missing, but the heads are 882
Everything about the valve train looks stock GM from what I can tell.
Has a generic aluminum intake that resembles an edelbrock performer, but has no markings.
It has timing gears in it instead of a timing chain, and it possibly has a mild street cam, but if so it is very mild.

I have had the front timing cover off and checked the cam timing marks are correct.

Engine is definitely an older build, haven't run compression or leak down tests, don't have the equipment.
 

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What color pump cam are you using currently? The cam that carb comes with out of box is orange and on the number one position most of the time. The number one spot is mostly used for engines idles below 1000 rpm as if you use the number 2 spot it will delay the pump shot just a hair and give a slightly longer overall throttle duration travel. If you have a pink cam on it then that could be a big problem. Every carb that I have used that had a pink cam on it had to have it taken off and replaced with a pump cam that gave a more bigger instant less duration shot or it would just lean and bog all the time.

If your timing is good and ignition is good and everything else is good and your carb is adjusted good with enough idle mixture screw so its not to lean and also you have proper transfer slot exposure then you might maybe think of making your idle circuit just a hair richer by going to a .031 size idle feed restrictor on the primary metering block. Before you do all that so I don't have to write a book read the post about the holley 670 street avenger sticky and it will have a load of information on the issue and i have a lot of commentary in there as well as several others.
 

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Discussion Starter · #19 ·
What color pump cam are you using currently? The cam that carb comes with out of box is orange and on the number one position most of the time. The number one spot is mostly used for engines idles below 1000 rpm as if you use the number 2 spot it will delay the pump shot just a hair and give a slightly longer overall throttle duration travel. If you have a pink cam on it then that could be a big problem. Every carb that I have used that had a pink cam on it had to have it taken off and replaced with a pump cam that gave a more bigger instant less duration shot or it would just lean and bog all the time.

If your timing is good and ignition is good and everything else is good and your carb is adjusted good with enough idle mixture screw so its not to lean and also you have proper transfer slot exposure then you might maybe think of making your idle circuit just a hair richer by going to a .031 size idle feed restrictor on the primary metering block. Before you do all that so I don't have to write a book read the post about the holley 670 street avenger sticky and it will have a load of information on the issue and i have a lot of commentary in there as well as several others.

The carb came with a red cam, I have tried an orange cam, and now have the blue on on there.
I adjusted the idle screws using a vacuum gauge, starting them at 1 turn out then adjusting out until I got highest vacuum. I believe they are about 1.75 turns out.
I'll read the thread you were talking about.
Thanks for input.
 

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Back to the very beginning you said it didn't squirt on the first pump and only half heartedly after that,,,, then suddenly your changing nozzles????

Bogie
 
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