1975 Cadillac 500 cu in engine troubles-rough running

[Richard Scot Minesinger]

Working on a 1975 Cadillac with a 500 cu in engine with 4 barrel carb.  The car is dependable, but does not run evenly, idle is real rough, symptomatic of a vacuum leak.  The car had been worked on by previous parties and I got a lot of things fixed or discovered mistakes, here is what I have done, and maybe you can offer additional ideas:

1.  Carburetor has been re-built, but there was no gasket between air cleaner housing and carb-fixed that (it just indicates sloppy work).
2.  All of the Carburetor to vacuum hose fittings have been checked and are good.
3.  Every vacuum hose has been replaced including the one at the turbo hydramatic modulator ( i mean everyone for sure)
4.  Every grommet has been replaced, the PVC, and breather (both of them)
5.  Every rubber actuator has been checked or replaced, including EGR, heater control, choke pull-off, snorkel actuator, and etc.
6.  Timing correct
7.  New air filter, plugs, plug wires, distributor cap, and rotor
8.  Compression has been check, and it is fairly even, did not have a great tester though.
9.  Fuel pump and sender have been replaced.
10.  Gas tank has been cleaned and re-sealed
11.  Cooling system fluid has been replaced
12.  Transmission has been serviced
13.  Drive shaft has been balanced
14.  Rear end has had fluid changed

I know 11 through 14 have nothing to do with it, just listing all the work.

I'm leaning towards checking to see if the gasket between the carb and intake manifold has been replaced.

Please help.

Thanks,

Scot

[EAM 17806]

I suggest you begin as follows:  1.) Use a vacuum gage to determine the status of your vacuum. 2.) Make certain the two carburetor idle screws are properly adjusted for idling. 3.) There better be a gasket between the carburetor and the intake manifold. 4.) Make sure the timing is on target and what is required for your car. 5.) If none of these, then get a good mechanic.  EAM

[TJ Hopland]

Remember GM had some soft cams in the 70's.   I have had at least 2 Cads with a couple of lobes that were almost gone.  Sometimes you can pull the valve covers and see that by watching the rockers.

These engines had plastic covered cam sprockets, age and mileage are not at all kind to these things.

There were several variants of carb base gaskets and some of them do indeed leave little gaps with some combos of intake and carb so that is for sure a good place to look.

If you or whoever did the carb is not a true expert on Q jets find someone that is.  I have had more than one that I just could not get to run right and finally found a good shop.  Took him about 10 minutes to find that someone had put the wrong internal gasket in.  I never caught that I just replaced the wrong gasket I found with another wrong gasket.

You mention checking vacuum stuff but did you confirm that the EGR valve is actually sealing on the exhaust side of things?  A little leak there will sure mess up the idle but not effect running.

One of the screws that held the AC compressor goes into the intake, seen more than one car where that hole was left open after the AC was removed.

I dont remember how the vacuum advance works on the 75's but the ones where things go through a thermal vacuum they usually are not working correctly.  I also find that the idle tends to be smoother if you run the distributor off manifold rather than ported vac.   I know my 78 was like that and my 73's got through a switch.  Cant remember how the 75 is supposed to be, I know mine is manifold but it may not have been that way from the start.

Are you sure the weights are working properly in the distributor?   And you dont specifically mention the vacuum advance, is that working?

[R Schroeder]

Have you checked the plug wires that they are in the right order ?
Roy

[Jim Sparrow]

You mentioned "timing". I assume you're talking about ignition timing. What about valve timing? If this is a high mileage engine the chain may be sloppy or jumped a tooth. Other symptoms of this are late shifting of the transmission and boiling over (heat soaking) after shut down especally in the warmer weather. This is because when a timing set gets sloppy or jumps both your ignition and valve timing become retarded. Most mechanics will adjust the ignition timing to compensate but once the valve timing is out more than about 5 degrees or so symptoms develop. To check this remove your spark plugs, then remove your distributor cap. Set the timing mark on the harmonic balancer at about 10. Then, while watching the rotor on the distributor, turn the crankshaft pulley counterclockwise (opposite the direction the engine normally spins). When the rotor on the distributor begins to move STOP turning and look at where your timing mark is now. If the difference is less than 2 or 3 degrees you're ok. Any more then that the timing chain & sprockets should be replaced. EGR valve is another common cause, as mentioned earlier. What about vacuum operated accessories such as the climate control system, auto. parking brake release, power brake booster & booster hose assembly, etc.? Hope this helps.    -Jim

["Cadillac Kid" Greg Surfas 15364]

Scot,
I believe you had no complaints about performance other than the rough idle. If this is the case, and all the vacuum lines have either been checked or replaced, I would look to the carb itself as the culprit.  In 1975 (and that year only I believe) Cadillac Engineering in an effort to get just a little more performance out of the detuned (for emissions control) 500, used an "altitude compensating" circuit in the carb.  What this means to you is that perhaps whoever "rebuilt" your carb was not familiar with this particular feature and the carb itself is not set up correctly.
I chased a rough idle on my '76 for weeks, and even though I "knew" the carb was okay, I swapped it out for another one "just to see" and that was the answer.
It could be as simple as an idle screw adjustment, but I would concentrate on the carburetor.
Greg

[TJ Hopland]

It does not take only mileage to kill those plastic cam sprockets,  age will do it also.   Recently saw one supposedly out of a 30K motor that was worse than the one I took out of mine with 150K+ on it.

I forgot about most of the 75's having unique carbs.   They had 6 metering rods and jets on them vs the usual 4.     One was some sort of a part throttle variable rod and the other was an aneroid for ambient pressure compensation.   Supposedly the aneroid is a very fragile factory calibrated device.
 

[Barry Norman]

Do 75's and 76's share the same carb ? Mine has the California Emissions on it and my idle is less than smooth .

[Richard Scot Minesinger]

Wow,

Thanks this is some great places to start!

Scot

[EAM 17806]

BARRY!  The 76 carb was slightly improved over the 75.  They are not the same carb.  EAM