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Old 09-14-2019, 08:15 PM   #1
fordguy40
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Question Possible Quadrajet issue....surging at WOT...vacuum leak?

Distributor, wires, plugs, and fuel pump are new. Timing set to 16* initial, distributor provides 20* mechanical, vac. advance disconnected to eliminate a variable. Goodwrench 350 with around 5k miles on it. Carb has less than 100 miles on complete rebuild by me. Starts, idles, and runs at part throttle perfectly. Just started driving it a little harder and it has started bucking and surging under load, such as merging onto the freeway and holding the throttle open to maintain acceleration. When I left off the gas, it returns to normal. No backfiring, no smoke, no stalling.

Noticed a few things about the carburetor that maybe you guys can help me with. Really beginning to think I've got a serious vacuum leak, even though the propane test showed nothing. First, the airhorn bolts were a bit loose and the idle came down a little as I torqued them. Going to pull the carb and tighten up everything that comes up through the bottom.

Second, even though the throttle shafts have been bushed, there was noticeable play in the primary shaft when I took it apart. Is there any play allowable here?

Also, can't get it to idle below 750 rpm, even with the curb idle screw backed out all the way. The lever that the screw rides against also does not touch the forward stop on the carburetor body. Need to check the throttle butterfly alignment, but assuming they're not jammed, should the throttle shaft lever touch the carb body with the curb idle screw backed out and should I be able to kill the engine with the screw backed out all the way?
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Old 09-14-2019, 08:26 PM   #2
geezer#99
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Re: Possible Quadrajet issue....surging at WOT...vacuum leak?

Sounds more like a fuel pump dieing or a plugged filter.
On the high idle check the high idle screw.
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Old 09-14-2019, 08:31 PM   #3
DoubleDubya
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Re: Possible Quadrajet issue....surging at WOT...vacuum leak?

The idle issue could be due to over fueling caused by too much fuel pressure or the float not being set correctly.
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Old 09-14-2019, 10:43 PM   #4
Dead Parrot
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Re: Possible Quadrajet issue....surging at WOT...vacuum leak?

Might double check the intake bolts for tightness.
A brake booster can leak enough to mess with the idle and still be effective at braking.
Check the suction side of the fuel lines. Bad rubber lines tank to frame and frame to pump can let air into the system. A small leak can cause your WOT symptoms.

2nd on checking the high idle screw(opposite side from the normal screw).
Also possible that the linkage isn't letting it fall to the low idle side. Lots of variations on how that works depending on the type of choke.

Yes, if everything is working, backing the main idle screw out enough should kill the engine.
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Old 09-15-2019, 06:13 PM   #5
fordguy40
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Thumbs up Re: Possible Quadrajet issue....surging at WOT...vacuum leak?

Thanks everyone for the suggestions....you guys were pretty much right on. All the screws holding the carb together could have been tighter, but there were a few other issues.

The filter looks okay and everything, except the chassis-mounted hard lines and tanks, is new so I'm tentatively ruling out a fuel system issue. Retorqued the intake, and it's got hydroboost. Vacuum gauge was pretty steady at 19 when I was setting the idle mixture a while ago, but I didn't check it before I pulled the carb off.

The high idle screw was definitely holding the throttle blades open. Probably the result of my advancing the timing and not readjusting the fast idle.

Float level was also set too high. Misread the instructions that came with the rebuild kit and set it to 3/16" when it should be 3/8". Funny thing is that the float level was similar to the float I pulled from the carb when I rebuilt it, so the last guy also biffed the setting.

Swapped the throttle shaft from a carb I picked up from ebay so I could reinstall the throttle lever actuator to complete the emissions system, and it turns out to be a tighter fit than the one it replaced, so I'm hoping this reduces the unmetered air coming in. Waiting for new air horn and throttle body gaskets to put everything back together, but will update the thread once it's back together.
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Last edited by fordguy40; 09-15-2019 at 06:28 PM.
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Old 10-12-2019, 01:06 PM   #6
fordguy40
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Re: Possible Quadrajet issue....surging at WOT...vacuum leak?

Thanks again to everyone for the help, especially Geezer. It's definitely the fuel filter; it looked okay, but I should've actually looked inside it before I said that, as it's completely plugged with rust. Looks like it's time to replace the tanks.
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