1970s 1974 Eldorado stumbles at acceleration

[danjarrett1]

It's been warm here for the  past few days, I've noticed that accelerating from a stop causes the engine to stumble....tapping the accelerator seems to sort it out some but not acceptable....any ideas? Accelerator pump? Vacuum leak.....

[TJ Hopland]

Both are possible.  Accelerator pump is easy to check except for the car being huge and how far you have to lean to see what you need to see.  Pull the lid off the air cleaner, remove the lower housing too if you don't have cruise or an assistant. 

Get yourself positioned so you can see down the throat of the carb (easier said than done). Engine not running have your assistant operate the gas pedal or grab the cruise control dodad to operate the throttle or if you have the lower housing off grab the arm and move it.  You should see a nice stream coming out from fairly close to the top of the 2 main bores.  They kinda shoot across and splash off the wall and dribble down over all the stuff in there.  If the engine is cold you will first stick your finger in to hold the choke blade open since that will try and close as soon as you move the throttle.  Warm engine it should stay open on its own.

Being a 74 its probably got points ignition? Or does it have something like a petronix or does it have an HEI?   All those options have different things that could cause your issue so we need to know what you got to go there.   

[danjarrett1]

Thank you TJ, it does have HEI. I read somewhere that the  4MV Quadrajet does have a small bowl and can be depleted under hard acceleration....which I do not do. The @uadrajets were also plagued with heat dissipation and vapor lock of sorts. Going to confirm throttle position as you indicated, probably need to get the vacuum gauge out and re-check idle circuit setting. I'm leaning towards this being a heat related issue, but who knows. I know GM had an issue with this early on and the problem was solved with changing fuel chemistry that defeated vapor issues...ran some non ethanol fuel through it the other day, maybe I just need to go back to 93 octane ethanol enhanced fuel. I understand the compression ratio is relatively low for these 500's, I presume they call for premium fuel....by todays standard, I think the fuel that was available in 1974  might have been better that's what's being offered today....who knows.. thanks TJ for your response

[bcroe]

By now the original timing chain set should have
been replaced with an all metal set.  Check slack.
Bruce Roe

[35-709]

Both my 472s ('71 & '73) were built to run on regular gas --- '71 being the first year for regular gas Cadillac 472/500 engines --- and that is what mine get today, no ping, no stumble, no ethanol.  Ethanol laced fuel was non-existent back then.  Only time I have any problem with vapor lock is when I am forced to use ethanol laced fuel and my electric fuel pump switch usually solves that. 
My guess it is the accelerator pump, and Bruce Roe has a good suggestion also.

[danjarrett1]

Bruce....I'm a novice at this, but are your telling me that the 500 engine does not have a metal timing chain?

[V63]

The originals were metal...but capped with nylon for 'noise' control. The nylon degrades and shred the bits into your oil. , then the timing is lost.

Bruce has an illustration of this.

[The Tassie Devil(le)]

G'day Dan,

Bruce is referring to the Cam Gear which has a Nylon tooth covering.

This is what happens to them.  And the nylon bits end up filling up the Oil Pickup screen.

Bruce.

[TJ Hopland]

A common problem with an HEI is some internal wires crack so when the vacuum advance operates it looses the connection which causes the vacuum to change which moves it again so it makes contact again.  Easy way to check for that is disconnect and plug the vacuum hose for the advance to the distributor.  Assuming it was all working to start with this will make the performance kinda flat but if the hesitation goes away you know you are in the right area. 

Yes it could be the timing chain and a lot of other things.  You can get an idea what sort of shape the timing chain is in by rocking the crank back and fourth while observing the distributor rotor to see how long it takes to respond to direction changes in the crank.  One annoying thing is these didn't come with a bolt in the end of the crank so its not so easy to rock the crank.

These guys have a bolt and washer kit.  Its a little bit of an odd size so its not like you can get it at a typical hardware store.  Maybe look through their catalog and you can find some other goodies to make it worth the shipping cost?
https://cad500parts.com/product/pb11122/

[danjarrett1]

Thank you gentlemen! All very good recommendations.