Rpm drop

Started by Sourmug, June 23, 2022, 07:53:08 PM

Previous topic - Next topic


Long time lurker first time question.  First, this is an amazing forum and i have learned so much from all of you.
Anyway,  i have a 75 Eldorado i got from the Original family 18 months ago. I wanted to get the carb rebuilt due to some ruff idle and small backfires but when it's warm it cruises wonderful.  Today coming through town the RPMs dropped and it stalled while going 15mph. It restarted but had to keep my foot on the accelerator to keep it from stalling.  Got home and put the timing light with RPM on and it was 160-200 rpm. Adjusted up to 700 and when i put it in drive i heard the rpm drop and it stalled. Immediately put it back in the garage.   It has Original Cat, carb. 88k miles. Original owner past and his widow had no paperwork on maintenance history.  Suggestions? Thoughts? 
Thank you


Welcome, and congratulations on your new vehicle! Please post some pictures!

My recommendation is to check a lot of the basics first. I've listed some of the things I recommend starting with below. I'll let others weigh in since I haven't quite figured out timing yet, but I would not recommend touching timing until you check the fuel, electrical, and vacuum systems first. If the timing is off, it's possible someone did it to compensate for issues in these other systems, and I think you want a healthy baseline.
Fuel stuff (I'm not sure from your note - did the carb already get rebuilt? If so, I would hope point 1 and 3 are already taken care of, although might be worthwhile to double-check):

1) Check the fuel filter (it's in the front of the carb, and you'll need a 1" socket/wrench to get it out of there. They're like $2 at Autozone, so might as well replace. Pay attention - if it's really dirty, it may be a sign of junk elsewhere in the fuel system.

2) Check the fuel lines, particularly right at the front of the tank - the fuel sending unit transitions from metal to rubber back to metal here, and the rubber can deteriorate.

3) If the carb hasn't been rebuilt for a while, that's likely a good thing to do. Cliff's performance carburetors has great quality rebuild kits, good prices, and excellent support if you want to do it yourself (https://cliffshighperformance.com/)

4) Fuel pump is a possibility, although if it runs fine when hot under load, it's a less likely culprit. They are pretty cheap, although can be a pain to get at if it has the original metal line and hasn't been touched for a while. It should deliver about 5psi to the carb...if you want you can pull the fuel line off the pump on the carb end, disconnect the distributer, run a clear fuel line off the carb to a gas tank/clear glass and crank the engine - watching for full, rhythmic pumps of clean fuel. If it's irregular, dirty, or otherwise constrained there may be an issue with the pump, or back towards or inside the fuel tank.

Other things:
1) Distributer/sparks/wires...if you're mechanically inclined it will likely cost less than $100 and at most a couple hours of labor to replace plugs and wires, and rebuild the distributer. Again, if it's been a while, probably good to take care of at some point...old electrical things can get hot and behave strangely (or so I hear  ;D ).

2) Vacuum lines...look for dry rubber, rotting rubber, unplugged rubber. HIGHLY recommend getting a vacuum gauge. You can get a nice OEM brand for like $50. Vacuum diagrams are available online...I might be able to send you one if you need it. Daunting at first, but not terribly difficult once you know what you're looking at.

Once you've verified working condition of fuel and electrical systems, you could set timing (I believe factory is 6 above dead center, check with vacuum unplugged). May also need to do some adjustments on the mix screws (there are two on your quadrajet...standard setting is about 3.5 turns out), as well as the idle screw. If it's trouble starting, then the electric choke can be adjusted as well. I do recommend buying a copy of the Shop Manual (I think the 1976 Cadillac Shop manual is the way to go - I haven't found a stand-alone version for '75, and the '75/'76 Eldorado's were close to identical).

I'd start with the basics, and see how it goes. Feel free to post on the forum as much as needed if questions arise...I know these guys and ladies have really helped me!


Thank you for the response. The fuel filter was replaced a couple months ago along with the plugs, wires, cap rotor. Also replaced all vacuum lines that i could trace. A Brand NEW carb was put on around the same time. It ran a bit better but still had some hesitation in the low rpm range and occasional small backfires. 
I have to diclose that until a few months ago i have not had a timing light in my hand for 25 years. Also tried to ajust air/fuel by vacuum gage but it was a heck of a challenge due to where the AC sits.  I'm trying to find a shop in central ohio to take it to but keep getting turned down because it's older than 1996 or it is a 2 month plus wait. I have not had any experience with a mechanical fuel pump. Can they SLOWLY crap out?
Thanks again for you help.


Fuel pump can become less effective over time...internal parts wearing out, or even just build up of junk and stuff. Yeah - finding shops that are willing to work on them can be tough!

Dave Shepherd

Good advice here, maybe check that no vacuum hoses popped off due to a backfire, as stated, from our desks it is hard to pin down your problems.


One other thing...and I hate to say this because it's not fun to think about...do you know if the carburetor number on the new carb matches the number on the old carb? There are several different quadrajets that look very similar, but have slightly different characteristics, and if you got the wrong number (not hard to do), things like float height, etc. might not be set up properly for the Eldorado. If everything else looks good it might be worthwhile to do some research on that.

These guys have a pretty solid series of 3 videos on taking apart and rebuilding a quadrajet if you ever want to try it. The Eldorado float height is different than what he recommends (the shop manual has the correct height), but other than that it covered about everything I needed to learn how to rebuild (other than dealing with all the vacuum lines, fuel line, etc, and one or two other things I learned elsewhere).

First part: https://www.youtube.com/watch?v=sml0Ur9Csf8

TJ Hopland

The 'brand new carb' raised my eyebrows. Everything about that statement sounds bad to me.  Tell us everything you know about this unit. Maybe even some pictures?  Hopefully you still have the original so IF we determine this new one may be a problem you can get it rebuilt.   Many of the 75's has a very unique 6 jet version of the Quadrajet.  This means that the rest of the 'tune' on the engine is expecting that carb.

I don't suspect the cat.  Plugged cats will usually idle just fine they just won't let the engine make any power and tend to raise under hood temps so you have vapor lock issues.

Since this seemed to come on suddenly I too would be looking for something that suddenly changed. Checking vacuum lines are a good idea.  If you can keep it running try pinching off every line you can access to see if you find one that seems to be causing it.  Brake booster could have just failed or the level ride compressor.  Checking/replacing those 3 fuel lines back over the rear axle is also a really good tip.  Lots of people forget about those.

If the distributor is original they 75's had a different design in the weights than was used later and it seemed really prone to getting stuck, likely why they changed the design.  IF something just got stuck in the timing that could have caused that sort of an rpm drop.  I think 75 was using the thermal vacuum switch for the vacuum advance so if that suddenly changed its mind how it was operating that could effect the idle speed too.

Also don't forget about the really basic stuff like did end up with some bad gas?  I know I have wasted a lot of time and money trying to make an engine run on half water.   
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


Thanks again to all, i got it in to a shop in Worthington Ohio that has done work fo a friend of mine who has several 66-74 classic cars including a 70 Deville Conv and was pleased with the results.  I will keep all updated and i now have a list of things i will be replacing a bit at a time thanks to you all.


And do not forget the timing chain set.  If a quick check
of slack between the crank and the distributor shows less
than 2 degrees crank rotation, that is not your problem
today.  Much more and it probably means it is still the
original, a time bomb.  good luck, Bruce Roe