Diagnosing running issue - '75 Eldorado

[benji808]

Hello!

So, over the past 6 weeks I've done a lot or work on my new-to-me '75 Eldorado Convertible, and thought I was doing pretty well... Took it to check timing today as that was not something I had experience/tools to do. Turns out the timing was set at 16 degrees (vs factory rec of 6), which isn't necessarily terrible IMO, but when it's turned down to 6 degrees, it runs horribly - idles OK (not great), but won't accelerate correctly, popping, backfiring, etc. At 16 degrees, with the idle set a bit higher than it should have to be, it runs nice - smooth idle, decent acceleration, almost never backfires or has issues. So that seems bizarre that timing would have that much impact.

So I'm trying to figure out some directions for research to figure out what the underlying issues are...I might still run the timing a bit forward, but it seems like something is quite wrong if it doesn't run at factory settings.

What I've done so far that I think is relevant:
-Spark plugs/wires/cap/distributer
-Rebuilt carb (it was my first time, so possible I screwed something up, but it seemed to come apart and go back together nicely)
-Fuel pump and filter, plus visually confirmed good gas flow from pump (so no junk in the tank fouling things up AFAIK)

I have a mechanic in town who's making some time for me hopefully later this week, and we're going to do some checks, but in the meantime do y'all have any ideas on what I could be checking? It seems like it should be getting good fuel, air, and spark, so I'm somewhat at a loss as to what to check next.

Appreciate any ideas!

[1776-1976-Eldo]

Might be time to check the valve timing. You might have an issue with the timing chain throwing the cam timing off.

[Scot Minesinger]

Your Cadillac suffers vacuum leaks.  Replace each vacuum line one by one so you do not mix them up.  For the thru - firewall lines, attach a role of new hose to a straight coupling and pull it thru from inside the car.  Also, don't forget the vacuum line that connects to trans THM on transmission (short section about 3").  Turning up idle and advancing timing are frequent quick fixes for bad vacuum lines.  They can look bad, but they will not be good. 

Make sure the snorkle damper is open to draw air from engine compartment, not across exhaust manifold.

Also, I usually deactivate the EGR by blocking vacuum line to it.

[The Tassie Devil(le)]

G'day Benjamin,

Are you checking the timing with the Vacuum still connected to the Distributor Vacuum Advance?

The Timing needs to be adjusted with the Vacuum off and plugged.

Bruce.

[dochawk]

Replace each vacuum line one by one so you do not mix them up.

It seems to me that when I was working on this a few years ago, there was a set of vacuum lines available with color coding that followed the factory coding (but would never be mistaken for original).

Also, I found that just plugging one of the many leaks with my thumb made a *huge* difference in the engine running.

And I don't think it came from the factory with a screw terminating a vacuum line . . .

[Scot Minesinger]

The vacuum lines that have color coding, are available in white, blue, red, green and yellow.  The factory had many other colors, like brown, purple, light green and etc.  They are OK, but like you wrote not resembling factory.

[TJ Hopland]

If you have not got a look at the timing chain/ sprocket I would for sure make that high on the list of things to inspect.  You can see it enough to tell if its plastic by pulling the distributor and looking down the hole.   You can also see it from the fuel pump hole if you have a bore scope.   If its shot you need to deal with it and that could explain why the timing seems off.  If its fine than you can take that concern off the to do list and move on to other things.

If the chain/sprocket is good then I would just put it back at 16 and call it good.

[dochawk]

I *want* to say that this kit had other colors, too, and came precut.

But thee were so many higher priority things at the time (including getting the front left wheel off so it could go in for new tires and roll in and out of the garage), that it all runs together.

[benji808]

Thanks everyone! The vacuum lines seem like they are in decent shape - no obvious cracking or brittleness, and I've replaced the few that seemed to have an issue, but I'll plan to be more thorough with some new rubber.

Also plan to run an ignition scope later this week to try to get a baseline understanding of what is happening...hopefully will have a better idea after that!

[fishnjim]

One could spend a lot of time and money chasing this.   If it runs satisfactorily at 16, run it there.   46 year old car, in unknown condition, probably won't run much better than it does now if you get it to factory spec.   Learn its quirks and enjoy it.

Without a complete maintenance history, mileage, etc. it's not easy to tell what modifications or substitutions, etc. have been done to a car to explain 10 degrees. 

Timing is a relative measure.   If the reference has changed, then the value must.
It might be a simple move the distributor gear one tooth, and things will line up on the mark with a bit of adjustment.   Sometimes the harmonic is bad/slipped moved mark, indicator bent, etc.   If he's a sharp mechanic he should figure it out, but if he a diagnostic code guy, open your wallet.