Sluggish/failure to accelerate and hesitation at full throttle ('75 Eldorado)

Started by benji808, June 21, 2022, 11:50:00 PM

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'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 may 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!


It sounds like the carb was initially running good after the rebuild. Maybe a foolish question but did you clean the carb good with brake cleaner and blow it out with compressed air? Maybe some trash caught in the needle and seat? Did you replace accelerator pump cup? Another possibility is maybe some brake cleaner damaged the pump cup? Is the fuel filter mounted in the carb or external filter. Maybe clogged?

Also, another possibility is it is unrelated to the carb and it could be an ignition issue. A lot of low quality parts out there these days. Even if they are fairly new a part could be bad. Pull the plugs and either check or replace them as well as ignition wires, cap, coil, rotor and even ignition module etc. Always keep the old module as a spare for emergencies or if it was good when you changed it out, to test if that is the bad part at a time like this. Start with plugs and wires and maybe that fixes it. If not proceed with cap, rotor and then the coil and/or ignition module.

Possible exhaust restriction? Is the original catalytic converter on the car. Listen to exhaust and see if flow seems strong or choked off. Could be a restricted cat or a collapsed internally rusted muffler.

Good luck with it. Let us know how it turns out.


The exhaust catalyst above is very probable, been there done 1976 car finally slowed on the freeway to a stall. As I cranked it it would not restart but I could hear a hissing noise through the exhaust. The original catalyst was plugged solid. The newer catalysts are MUCH lighter and less restrictive, really woke the car performance wise.

Another possible is a timing issue? Maybe related to the problematic plastic timing gear failing/slipping?


No catalytic converter- looks like someone straight-piped it years ago.

Carb was cleaned pretty thoroughly and soaked in chem dip etc

I am wondering about the transmission lockdown, so may check that today...let me know any other thoughts. Thanks!

Matti R

Best regards,
CLC #33333
67 DeVille convertible Sudan beige
79 Seville
64 Sedan DeVille
66 Calais Coupe


Are you sure the secondaries are opening?
If you choke is only partially on, the choke pull-off prevents the secondaries from opening.  You need those open to go over 70 mph.  The choke pull off is vacuum operated and can fail and prevent full release of the choke linkage.
'61 Fleetwood 60- owned for 31 years
'61 Series 62 2-door - sold
'64 Series 62 2-door -sold
'67 Sedan De Ville - sold
'74 Eldorado Convertible - sold


Transmission seems to be shifting fine, and fluid level is good.

The Choke idea is an interesting one... what would be the easiest way to disable it to test?

Dave Shepherd

Tough to diagnose this from here of course.  The is a small filter in the carb inlet fitting, make sure it is in the in the right direction and clean. Btw, under normal cruising throttle the secondaries would not open unless were you down pretty hard the pedal.


The transmission kick down is also a common fail, the plastic breaks and it no longer functions. This would cause bogging upon a jab of the accelerator and labored acceleration however would not effect higher speed travel in general


Transmission kickdown at carb is good (voltage transfers when depressed), and it seems to be be downshifting appropriately when I floor the pedal.

Replaced the fuel filter. It was a bit dirty...we will see if that makes any difference. Fuel line at carb was producing an appropriate 5psi pressure.

Going to try to get my head around how to test the choke.


Backfire in the exhaust?

If you are getting unburned fuel/air mixture in the exhaust that would indicate a misfire from an ignition problem.
1970 Cadillac Deville Convertible
Detroit, MI


Not really a full on backfire - just a pop or two very rarely. But it wasn't happening before.

Daryl Chesterman

A couple of things to check:
     1.  Is the vacuum advance working.  You can check it with a handheld vacuum device, or you can (if you
         have a long enough hose to the vacuum advance) pull the vacuum-source end off and see if you can suck
         vacuum with your mouth, sticking your tongue on the end of the hose to see if it will hold vacuum, or
         you can use a timing light and disconnect and reconnect the vacuum advance hose to see if the timing
         mark on the harmonic balancer moves in relation to the fixed pointer.  If you can't hold a vacuum to
         the vacuum advance, that indicates the diaphragm in the advance is ruptured and the unit need
     2.  Look directly down into the carburetor and with the choke butterfly fully open, actuate the throttle
         linkage, and you should see two strong jets of gas spray into the venturi of the front two barrels of
         the carburetor.  If you see the jets of gas, that indicates the accelerator pump of the carburetor is
         working.  If not, further investigation of the accelerator pump is warranted.

Daryl Chesterman


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!


A restriction between the tank and the carb is a possibility.  Lot of things, plugged pickup sock, hoses, rusty line, filters.  One way to keep a handle on this, is a fuel presure gauge at the carb.  Not the clumsy mechanical ones from the last century, but the new electronic gauges with solid state sender.  Bruce Roe


Thanks, Bruce! Do you have a recommendation on a fuel pressure gauge? Most of the ones I'm seeing I feel like it would be hard to discern between say 5psi and 3psi because they are made for fuel injection.

Daryl Chesterman

NAPA has a low pressure fuel pressure test gauge that is a combination vacuum/fuel pressure gauge.  It is part
# 2521.  See the screenshot from their tool catalog below.

Edit:  I should have read Bruce's post first—the NAPA pressure gauge is not electronic!


haha! Yes - I gauge tested pressure and it's right at 5 psi...she seems to be accelerating fine now that I have good gas in her, but I was interested if there was something I could use to monitor pressure while driving (under heavy acceleration, etc.).

TJ Hopland

They do make fuel pressure gauges that are electric so you are just running a wire into the cab but they are not cheap and a vast majority of them are intended for EFI so won't have the resolution to tell us anything in the carb pressure range. 

For just quick test drives I have just taped the gauge to the hood and or windshield.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason