BIG BLOCK ENGINE STUMBLES ONLY UNDER QUICK/HARD ACCELERATION???

[HAHNAGON]

Sounds like an accelerator pump problem but not sure exactly where. I see this Holley is a quasi-Carter AVS. I’m not sure how the accelerator pump bowl is fed, through a small check valve port from the bowl or a “V” notch in the side of the pump well wall or something else. I once had trouble with a Edelbrock AVS type with the V-notch in the AP well that the pump height setting had to be increased to get the pump skirt above the bottom of the notch to let gas in. Once I did that the stumble was gone. If it’s the former with the small check valve port then I would probably assume since it’s new it’s probably OK. I would try a bigger shooter nozzle and see if that helps. Keep the accelerator pump link on the #1 hole until you get it working right and then switch it back to #2 and see if it still works OK. Every time you change positions on that link, the pump height needs to be checked and adjusted to maintain its setting.

In case you don’t have this forreference.

https://documents.holley.com/lit701rev10.pdf

Ok. Thank you for the info and your time.

 

[HAHNAGON]

Holley manual says " larger engines, larger cams or air gap style manifolds" may need a larger discharge nozzle. I would say a mopar big block intake or torker intake would definitely be considered an air gap intake.

 

[R413]

NOt an air gap. It’s a Torker and it has a heat crossover. Did you plug the heat passage or leave it open? It is a single plane which still isn’t very good for a stock engine with a small cam. But the heat crossover will help.

if you don’t STAB the throttle how does it drive?

 

[Canadian1968]

I really don't think it's an intake problem. You said the car idles and revs fine. But all other driving conditions are fine? What about rolling into the throttle to WOT? From a stand still and rolling does it do the same thing when stable the throttle? I may have missed it but did u check fuel level as suggested ?

Honestly first thing that's comes to mind is the pump shot or whatever the equivalent is on that carb

 

[HAHNAGON]

I really don't think it's an intake problem. You said the car idles and revs fine. But all other driving conditions are fine? What about rolling into the throttle to WOT? From a stand still and rolling does it do the same thing when stable the throttle? I may have missed it but did u check fuel level as suggested ?

Honestly first thing that's comes to mind is the pump shot or whatever the equivalent is on that carb

Yes. I agree. I'm looking at discharge nozzles but haven't picked a size. I also have a couple edelbrock 750s and i think im going to pop one of those on it. I haven't had any problems running edelbrock 750s on my 440s. Fuel tank is 3/4 full. I added 5 or 6 more gallons of gas and recalibrated fuel gauge 2 days ago. I double checked with a factory fuel gauge i had as well. It is accurate. And I'm sorry I typed Victor. It is a torker intake.

 

[BeepBeepRR]

Id say give it a little more initial timing like around 15 degrees and see how that works for you. Easy to dial in and free to try. 12 initial is alright but I would think that would be a little low for what your asking that car to do.

 

[HAHNAGON]

Id say give it a little more initial timing like around 15 degrees and see how that works for you. Easy to dial in and free to try. 12 initial is alright but I would think that would be a little low for what your asking that car to do.

 

[HAHNAGON]

View attachment 1577949

View attachment 1577950

Discharge nozzle comes right out. I'm thinking about drilling out the nozzles a little bit.......

 

[HAHNAGON]

Hmmmm....Edelbrock nozzles are much smaller.

 

[HAHNAGON]

Sounds like an accelerator pump problem but not sure exactly where. I see this Holley is a quasi-Carter AVS. I’m not sure how the accelerator pump bowl is fed, through a small check valve port from the bowl or a “V” notch in the side of the pump well wall or something else. I once had trouble with a Edelbrock AVS type with the V-notch in the AP well that the pump height setting had to be increased to get the pump skirt above the bottom of the notch to let gas in. Once I did that the stumble was gone. If it’s the former with the small check valve port then I would probably assume since it’s new it’s probably OK. I would try a bigger shooter nozzle and see if that helps. Keep the accelerator pump link on the #1 hole until you get it working right and then switch it back to #2 and see if it still works OK. Every time you change positions on that link, the pump height needs to be checked and adjusted to maintain its setting.

In case you don’t have this forreference.

https://documents.holley.com/lit701rev10.pdf

 

[BSB67]

What you are looking is not the squirter hole size. Look closer

 

[HAHNAGON]

What you are looking is not the squirter hole size. Look closer

Yeah. I was wondering if it was the inside one. That one is much smaller.

 

[Canadian1968]

Look at the shape .....

The edelbrock has a venturi effect. The tube is larger then tapers down to smaller before discharge. Can't tell what the demon does.

Regardles of intake , torker , Victor , rpm ect I can't see it causing your full out bog/ stumble .

It's in the fuel delivery I belive.... you talk about your fuel tank being correct level but what people are referencing is your carburetor fuel BOWL level. There is a certain amount of fuel that had to be In the fuel bowl at all times to keep a reserve for the carburetor to perform properly at all RPM/ throttle. This is controlled by float level and fuel pressure .

 

[HAHNAGON]

Look at the shape .....

The edelbrock has a venturi effect. The tube is larger then tapers down to smaller before discharge. Can't tell what the demon does.

Regardles of intake , torker , Victor , rpm ect I can't see it causing your full out bog/ stumble .

It's in the fuel delivery I belive.... you talk about your fuel tank being correct level but what people are referencing is your carburetor fuel BOWL level. There is a certain amount of fuel that had to be In the fuel bowl at all times to keep a reserve for the carburetor to perform properly at all RPM/ throttle. This is controlled by float level and fuel pressure .

I was wondering that. I'm definitely not an expert on carburetors.Thank you for explaining it nicely. Ok. I'll research that and how to diagnose it. But still, I feel like I'm missing something simple. I think I'll try an edelbrock I've used before on a built 440, without issues and then at least I can maybe narrow it down.

 

[Geoff 2]

A mate put one of these on a 400 Pontiac & it always bogged when the secs opened. Rang Holley tech [ expensive ] & they had no idea. Carb came off, QJ went back on, no more bogging....
Is this bogging on the primaries...or when the secs open?
If just on the primaries, bigger acc pump shot may help. In my opinion, not using an air valve dashpot [ saving $$$? ] like the TQ & QJ, is the main reason why these bog when the secs open.

 

[Car #4]

I have a 750 Edelbrock on a 440 with Edelbrock heads, air gap intake and big cam. I have the same problem. If I punch it hard it stumbles bad. If I feather the rpm up a bit and then punch it it’s usually ok. Watching this thread closely.

 

[Roscoethug]

Man this sounds like deja vu. Bought a '69 Roadrunner. Popped, stumbled, stalled. I did the same troubleshooting as you. Plugs, timing, Demon carb tweaking. (Different model carb). Was flummoxed. Turns out it was old gas. Fresh gas solved the problem. Just sayin'

 

[AR67GTX]

The Carter-style carbs with the plunger/piston type accelerator pump can have an assortment of bogging problems when quickly matting the accelerator. The two springs - the strong upper delay spring and the weaker lower return spring require close coordination. If the upper spring is too weak, or the lower spring is too strong, then the piston will initially hang in the bore and not shoot fuel until the upper spring finally overcomes the lower spring to move the piston. Both springs should always be replaced at the same time and beware that I have purchased rebuild kits where delay springs was mistakenly provided for the return springs. Since the upper spring is usually already installed on the pump it may not be readily apparent unless you happen to think about it or compare it to the spring removed. I missed it and it and had a big bog as result.

Some Edelbrock carbs (an 800 cfm AFB style as my example) have a V-notch opening in the side of the pump well to replenish the well with fuel as the pump rises back up. I found on mine, with a new, Edelbrock sold, correct pump set at Edelbrock height specs, the piston did not rise above the bottom of the notch so the well would not get filled. Big time bog. It makes me wonder what is happening after cycling the pump and releasing the throttle, the check weight under the shooter seals and basically the piston has to draw a vacuum on return unless the piston skirt seal is overcome by the vacuum and lets fuel pass. I had to make some careful measurements and set the pump height far above the Edelbrock specs to get it working and get rid of the bog. I’m not sure why Edelbrock specs are so far off. The Carter system with a port at the bottom of the well with a ball check valve is far superior IMO. That the ball check valve is sealing can be checked by filling the bowl and the pump well with fuel with the top and shooter removed. Then cover the shooter port with finger pressure and slowly depress the pump by hand watching the fuel in the bowl near the valve to see if there are any eddies in the bowl or signs of leak back that would decrease a fuel shot.

If the check weight or ball under the shooter does not seal tight fuel in the passage, fuel will be pulled back to the well as the pump returns, slowing and decreasing the next fuel shot. This could probably be checked by putting a vacuum pump and rubber hose section down in the well to seal over the port and see if a vacuum could be pulled indicating the check weight or ball is sealing. If installing a new check weight it’s a good idea to take a piece of wood dowel and very lightly tap the weight to conform it to the seat.

Over the years, an older carb’s pump well bore can get rough from wear and deposits. It can be lightly polished with 400 sandpaper or fine crocus cloth to smooth it up and reduce pump drag and wear.

And leather skirt accelerator pumps are still the preferred accelerator pump material with ethanol fuels.

 

[BSB67]

To the OP, have you simply tightened the air-door?

 

[mopar 3 B]

Yeah, I hadn't had that happen before. I didn't think so. I was thinking about removing it and then take it out for another drive because that shouldn't happen. Carb came new from holley and says "it is calibrated from the factory and should NOT need adjustment"...? but I definitely had to adjust idle mixture and I have tried adjusting the choke.

Forget the Holly calibration crap and learn how to do it yourself if you don't already know how to.
I am not sure if that carb has a power valve or not. If it does I would start there.
This day and age I don't know why anyone would want to mess with the carb crap.