Sluggish acceleration and hesitation at WOT

[Benji808]

Sorry for the double-post: I thought I had fixed the issue, but it is not resolved.

'75 Eldorado, carb rebuilt with Cliff's kit a few weeks ago. Has been running very nicely. This week I'm experiencing acceleration issues. Driving on the highway up a hill around 70mph, the car will lose speed, even at WOT. Not normal for the car - they accelerate slow at that range, but they should accelerate. Also getting a slight hesitation at slower speeds (accelerating from a stop sign, for example).

New symptom tonight when I took it for a drive in the cooler temperature: driving 20-30mph and flooring the accelerator results in the car failing to accelerate - it sort of "grinds" (not really a loud noise, but that's the best I can think of to describe it) and does not speed up as it should, and will even do what sounds like a backfire.

I would appreciate any help in diagnosing...I'm considering if it could be a fuel delivery issue, although tank is pretty clean, and fuel pump is new.

A couple questions:
1) Any carburetor parts or settings I should check?
2) what should fuel pressure be at the carb (I main run a gauge to check that, and replace the fuel filter to be safe)?
3) What is the backfire telling me, and are there any further diagnostic steps I should take?

Thank you in advance!

[Kenth]

I would start with check ignition for proper function.
Does the choke system work properly?

[quadrajam]

If it's backfiring out the exhaust I agree with Kenth, check ignition.
If your referring to popping back through the carb look for lean
fuel delivery.

[Mudsport96]

You said sounds like it is grinding.... that to me seems like it may be detonation. You need to check timing before driving it anymore. Detonation will kill an engine quickly.
Your other post said the engine was rebuilt. Do you have a list of parts and things that were done to it during the rebuild?

[Benji808]

Ok, I’ve checked a bunch of stuff and so far it checks out ok.

I’m wondering about the choke- in theory if it wasn’t releasing it could prevent the secondaries from fully opening, right? What is the best way to test the choke- should I just remove the pull off and see how she drives?

[Benji808]

Well, after all that, it appears to have been bad gas. Not sure what was in it, but after filling up I went through about 20 gallons in 80 miles, refilled, and she seems alright now.

Thank you all for the ideas- I was able to check out a number of things and learn some new ones!