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Diagnose a Quadrajet carburetor problem / Quadrajet runs better without accelerator pump.
« Last post by NathansCorvette on November 20, 2024, 03:42:29 PM »
Hello all,

1974 Corvette L-82 stock quadrajet. I had been struggling with tracking down the cause of an issue for a long time until recently. The issue was this: Car started right up when cold, idled and sounded great. If I was to rev the engine it was snappy and responsive. Once the engine got up to temp, the throttle response greatly diminished. This resulted in a bogging sensation that could nearly kill the car if repeatedly rev'd. It made 1st gear starts awkward.

I wrote an exhaustive list of every suggestion that was made on CorvetteForum and went through checking/re-adjusting each item. This included timing, idle mixture, re-torquing carb/intake bolts, isolating all vacuum lines, verifying the vac advance works, verifying choke works, checking for vapor lock/heat soak, etc... No change to the issue at all.

Any time there is a post about throttle hesitation the first thing everybody says is "Your accelerator pump is weak and not giving enough gas! There's a momentary lean condition!" Back when I rebuilt this carb, I found the old accelerator pump completely broken and replaced it with the new one in the Cliff's High Performance rebuild kit. Keep this in mind that before the issue started, the accelerator pump was giving no fuel shot.

At the very end of my list was to see if messing with the accelerator pump changed anything. First, I moved the pump rod to the outer hole on the pump arm. Immediately, I noticed that the issue improved, but was still present. Eagerly, I removed the pump arm to completely prevent the accelerator pump from actuating...

The throttle hesitation disappeared! Perfect snappy revs even at full temp now!

But this is weird. There must be some super rich condition where the pump was making it even more rich.

Yes. The idle mixture screws are set correctly. If I move them in any more, it will drop in idle speed and idle poorly. And yes the timing is spot on... I set it according to Lars' paper. And yes I am fully confident in my carb rebuild. I was very methodical and followed a guide by Lars and used Cliff's kit.

My two theories:
Too strong of a power piston spring, causing the power piston to not be fully shut at idle and/or snap wide open when throttle is touched. This should be a very simply fix, if the case. How can I test for this?
The rebuild kit from Cliff's included what is listed on the website as the "HP Accelerator Pump" which "...will deliver more fuel to the engine than a standard pump.". Also available on the site, but not included in the rebuild kits is the "Standard Accelerator Pump". Which should deliver less gas. But remember even with the pump arm in the outer hole (less pump shot) the issue was still half-there.

Linked below is a video I made that shows the issue, shows the acceptor pump fully hooked up sending a full shot of gas at idle, and showing the revs with the pump disconnected.

https://youtu.be/l7fC5txC7a8

Any advice would help, thanks!

Nathan
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Quadrajet Carb Talk and Tips / Re: Fuel pressure reg.
« Last post by 73ss on November 20, 2024, 01:10:44 PM »
Thanks for the replies.

I measured the pressure at the carb inlet. I temporarily installed a gauge at the cowl. I didn't pay attention to it at the launch, I wanted to see the pressure on the big end and it held a steady 5 psi. The car is currently doing mid~high 1.7's on the sixty foot. My old engine with a mechanical pump would steadily loose pressure through a run and then typically nose over right at the end. A/F gauge also reads a steady 12.2 ratio during the run which would indicate the carb is staying full. I like the idea of keeping the reg in the back. I just need to do a more permanent install.
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Quadrajet Carb Talk and Tips / Re: Fuel pressure reg.
« Last post by 77cruiser on November 20, 2024, 10:36:33 AM »
If you only have 6 lbs. pressure from the rear fuel will stall in the line at  1 G of less.
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Quadrajet Carb Talk and Tips / Re: Fuel pressure reg.
« Last post by lightning boy on November 20, 2024, 05:24:46 AM »
No, 73 you have it plumbed just like it should be. The reg. after the carb. It doesnt matter where the regulator is mounted. The point is to have 6lbs or whatever available at the carb no matter how much fuel the carb is pulling, all the extra goes back to the tank. This is the best set up for a carb and the performance of your car proves it.
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Quadrajet Carb Talk and Tips / Re: Fuel pressure reg.
« Last post by 77cruiser on November 19, 2024, 03:57:44 PM »
Don't know how hard your car leaves, but if it hooks & leaves hard you need to extra pressure to keep the fuel from stalling in the line. I don't know what kind of 60 ft. it takes to stall 6 lbs.
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Quadrajet Carb Talk and Tips / Fuel pressure reg.
« Last post by 73ss on November 19, 2024, 02:00:57 PM »
Question: Are there any negatives to running a fuel pressure reg at the rear of the car by the tank?

I installed a in tank fuel pump assy that was supposed to be regulated for carb use. One fuel line to the front of the car and the excess fuel would be dumped back in the tank. It didn't work well at all. Barely could keep the carb full, Had about 4 psi and dropped under load. They (holley) sent me a new internal regulator and pump with the same result. I ended up blocking the internal reg (Very poor design) and temporarily installed a Edelbrock external regulator. I plumbed the return port back to the tank and used the factory steel line to the front of the car. As it turns out, It works great. I haven't touched it in a year. I have it set to 7 PSI and it drops and holds 5 under hard runs. The car runs great with no sign of bowl going empty. Actually ran my best quarter mile ever in 25 years.

I would like to leave the reg at the rear of car instead of moving it to the front and then plumbing a return. Space is limited under the hood, Big block with A/C. Any negatives to leaving the reg in the back? I mounted it to the crossmember between the tank and rear end. It's probably safer in the rear, cooler. Would like to hear others thoughts. Thanks.
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Quadrajet Carb Talk and Tips / Re: th350 rebuild quick question
« Last post by stevef on November 19, 2024, 10:32:41 AM »
Also, I have another BOP th350 steady-rest pump that's missing the plastic steady-rest ring. What do you do when it's missing? How important is it to have it? Thanks.
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Quadrajet Carb Talk and Tips / th350 rebuild quick question
« Last post by stevef on November 13, 2024, 04:31:32 PM »
I have Cliff's th350 book, but didn't see this addressed. For my cruiser street car, I'm trying to rebuild a th350, BOP style, probably about a 77 model. It has the torrington bearing, and has the "steady-rest" style stator. My rebuild kit comes with the sealing rings for the stator, and gives me a choice of "scarfed" teflon seals, or metal interlocking ones. So, that's no problem. It had teflon, so I'll just go back with them. So that takes care of the 5 oil sealing rings, but I also have that 6th bottom ring slot in the stator for the steady-rest ring. But I see no steady-rest rings in my rebuild kit. It's a very complete kit otherwise, so kind of surprised that it's not included. I still have the old steady-rest ring that I took off it, which actually still looks pretty good. It's a whitish color. It has square cut ends, so just butts up to itself. Don't know if it's teflon also, or maybel some kind of urethane. I read in the Ron Sessions book that they vary by color, like purple, green, white, yellow, depending on ring depth size, and he gave measurements. So, mine matches up with the white one. I could re-use the old one since it's in good shape, but wondering if new ones are available somewhere? Thanks.
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Diagnose a Quadrajet carburetor problem / Re: Quadrajet choke affecting timing?
« Last post by Cliff Ruggles on November 12, 2024, 05:24:41 AM »
Did you rule out bad fuel here?

Looking at your posts it appears that the car has been sitting around for quite some time with little use. 

This new fuel goes bad quickly.

Since you've narrowed it down to the carb at a minimum I'd pull it apart and clean it out, and see if anything found it's way into the N/S assembly causing it to flood....
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Diagnose a Quadrajet carburetor problem / Re: Quadrajet choke affecting timing?
« Last post by BenNoakes on November 12, 2024, 03:08:21 AM »
Ignition is new and good. Turns out qjet started dumping fuel into the intake and fouling up the plugs instantly so I've now taken it off to see what's going on or if leaking. That will sure make a difference in trying to tune...hopefully.
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