Acceleration issue (Quadrajet on a ‘75 Cadillac Eldorado)

[Benji808]

I’ve recently encountered an issue where 1) if I floor the accelerator from a standstill or 2) floor accelerator to increase speed above ~70mph, the car staggers a bit. In the second scenario at 70mph, I can feel the jerks, but it holds speed. In the first scenario from a stop it will quickly get to ~30mph, but then can fail to accelerate further unless I let off the pedal a bit and “nurse” it up to speed slowly. Also seems worse under load, like going up a hill.

Starts fine, idles great, accelerates at WOT to ~30mph with no issue, and can cruise at 65mph all day as long as no major hills.

Quadrajet is fully rebuilt with one of Cliff’s kits. Only change I made recently was to swap out the power piston spring- I had lost the original while it was apart, and replaced it with one of Cliff’s black springs (I believe it is strongest spring). I replaced the fuel filter maybe a week ago when I started having issues, and idle, timing, and vacuum arrangements are at factory spec.

My two theories are as follows:
1) maybe the power piston spring is too strong, and when vacuum drops under acceleration, it is dumping too much fuel into the engine.

2) fuel pump failure.

Any other ideas, or thoughts on which of these is more likely? I’m not sure how to test other than just replace them one at a time and see what happens. Appreciate any advice.

Thank you!

[Kenth]

When troubleshooting carburetors with similar symptoms, I have found that the owner put the fuel filter in backwards.
Filters with a faulty check valve also restricted fuel flow.

[Benji808]

Well, I’ll be. Replaced my (new) fuel filter with one that does not have a check valve, and so far the issue has not recurred, and acceleration feels smooth. I have a longer drive tomorrow so can report back if I encounter any issues. Thank you!!

Follow-up question: is it important to have a check valve? I’ve seen filters come with and without them.

[Kenth]

Check valve in fuel filter was implemented 1975 as a part of the "crash roll over safety policy" so the fuel tank would not be emptied via the carb if car is going off the road and ends up on roof.

[Cliff Ruggles]

I would replace the stock type filter with a Marine high flow micro-screen filter.  I have them here and supply a longer spring with them for the later side inlet carburetors.  They are a LOT less restrictive than the paper type filters......

[Benji808]

Was encountering the same issue just a little bit today…~70mph going up a hill. Doesn’t seem nearly as bad or as constant but was noticeable. Any other ideas, fuel pump, or carb adjustment?

[Kenth]

Hesitation/surge at that speed, mostly due to the ethanol in todays fuels, can be cured with .002" larger main jets.
You can also raise the APT under the brass plug in front of the air horn 1-2 turns and see if that helps some.

[Benji808]

Thanks, Kenth! That could make sense, since I've been filling up with ethanol-free fluid, but today had to put some of the newer stuff in in. I also think there is an issue with my kickdown switch which may be impacting acceleration.

Thank you for the insight! Hopefully I can resolve soon.