Does anyone know what difference of the tuning of the 74 vs the 75 engine?

[collector]

I was looking at my Chilton book and in 74 the timing was 10 btdc with a spark plug gap at 35. Now the 75 engine is the exact same as I compared all the specs and the 75 uses a different spark plug with a gap of 60 and the timing is at 6 btdc. What would be the reasoning? Even the bhp is the exact same. Did it cause the car to run smoother, or use less gas, or put out less emissions? Im curious to find out why. I would love to know the logic behind the change. Please if any one is really mechanical please tell me. Thanks, John  one note, my car has the HEI ignition just like the 75

[Ted in Olympia WA]

The very early 74 had points and the later engines had HEI.  The HEI had the larger spark gap.  I have 74 cars with both types of distributors.  I think the timing was due to emissions.  The 74 did not have a Cat and the 75 did.

One more trivia fact for you.  The 1974 Eldorado was the only year that had a rear sway bar, even though the 75-78 lower control arms had the holes drilled in them for it.

TED

[EAM 17806]

Regular unleaded gas was required for 1975 on and I think you had a choice in 1974.  Since 74 was the first year for HEI it's possible the engineers fine tuned it a little bit more to meet certain conditions such as emissions, gas requirements, etc. The air flow to the carburetor was rearranged somewhat and I think the EGR valve was changed too.  I do know with the engines with carbs the carburetor was changed and even into 1976 there were more minor improvements.  Engines from 1974 to 1976 were basicaly the same except for these minor changes:  HEI, unleaded regular gas, timing differences to meet emission standards, spark plug change, carburetor change, not a major one but the specs did change somewhat along with a cat convertor for the first time for Cadillac in 1975.  All three years were good engines except for the gas mileage!  WOW!    EAM

[TJ Hopland]

I would guess that the cam would be the biggest hardware change along with things like the HEI and Cat others mentioned.  Im sure it was all due to emissions.  I think the CAFE standard came out around then also.

The extra 4* in timing should not hurt anything other than in theory emissions.  May even help performance.  Detonation / pinging is the main thing to listen for. If it does ping first thing I would expect is the vacuum advance. Most aftermarket vacuum advance cans have way too much range in them.  The shop manuals have specs for what the mechanical and vacuum are supposed to be.  If you are trying to go 'factory' they can easily be measured with a timing light and vacuum pump.   If you dont have a 'dial back' timing light that can show more advance than what the gauge on the engine shows you can mark off the balancer with chalk up to about 40*.  You can measure the distance of 10* on the existing mark and then mark of that distance on the balancer.   

[collector]

Ok, mine is an early production date 11/73. It has the every option except track master, including the HEI system. The timing is at 10 and the gap is at 35 and the spark plugs are NOS and the wires are original and dated 3Q73. I use regular cheap unleaded 87 octane and the car has No predetination at all. My only issue is the idle isn't as smooth as I thought it would be and was wondering if enlarging the gap slightly would improve the idle. Or were these cars just not totally smooth running except for the EFI versions from 75 up? I know no matter what I did on my 72 the idle was horrible and the same on my 73 so I got rid of them. I don't want to get rid of this one and I want to keep it original. I replaced the wires with new ones just to see the difference and there was non, so I put the original wires back on. I do notice to me the idle is high in park but when in drive it is steady at 600 and varies to 610 and back to 600 but never above or below. I just don't understand why it wont idle smooth. I think the car in idle runs at about 800 in park. Sounds kinda fast to me but wish I could get the idle down. I have found the lower the idle the rougher the engine. Since the car has almost no miles on it, shouldn't it be smooth or is this the nature of the beast?

[Barry]

My 76 also suffers from a less than smooth idle. Runs perfectly with acceptable gas mileage and no spark knock on the 87 octane gas. I was around them when new,and don't really recall a less than smooth idle.

[TJ Hopland]

A gap of 45 should be safe with a HEI.  Some HEI's were up to 80 (late 70's olds IIRC).

Have you confirmed that the mechanical and vacuum advance is working properly?

[collector]

Yes all is working fine. In fact all the cap and rotor are original and like new so I didnt play with them. I felt the weights move freely and the vacuum advance does work.

[Otto Skorzeny]

Cadillac learned in the 1950s that advancing the timing at idle helps to cool the engine. My 1956 advances a whopping 17 degrees at idle by vacuum according to the manual. I don't know what yours is supposed to be, if any,  but I would check your vacuum to make sure it is functioning to its fullest potential.

As for the 35 year old cap and rotor, just because they look good doesn't mean they are. The $15 cost for those parts would be worth it even if only to eliminate them as suspects in your problem.

[Ted in Olympia WA]

I would replace the cap, wires and rotor with good quality parts.

I would play with the timing a bit, move it one way or another and see if it helps.  If you are checking the timing at idle and it is at 800 you could have some advance built in there already, so actual may be only 3 or 4.  I would advance it to 12 or a little bit more.  The goal is to have it ping only when lugging up a big hill and a very short time.  This will give you your best idle, power and mileage, and this way it will almost never ping.  You don't have to keep it at factory stock.

When you advance it the idle will increase and you will have to bring it down a bit. 

Just play with it and you will get it right.

TED

[collector]

Thanks, Im not changing any of the factory parts. They are good. I only change parts when needed and then only NOS original. I dont monkey around with aftermarket as I am a purist to the enth degree. I will however move up the timing a bit but as for the car when on the dyno it burns clean with low emissions. It doesnt sputter just doesnt idle the best

[Otto Skorzeny]

I don't want to beat a dead horse here, but you can't always tell if a part is good just by looking at it. I appreciate your attitude toward factory OEM parts and wouldn't advocate that you throw away the originals. In fact it's nice to have them  to install for a show or to pass along to future owners.

I think it would behoove you to purchase new, top quality parts to use as a basis in pursuit of the cause of the idling problem. Something is clearly amiss with your car and the more things you can empirically rule out, the better. If there is no measurable change with the installation of a new cap and rotor, then re-install the old ones. At least you will have ruled out one of the possible contributors to your rough idle.

[Ted in Olympia WA]

This is especally true for the HEI cap and rotor.  I have seen great looking caps that were shorting out on the inside.  Buy original equipment Delco Remy if you want to keep it original. At the very least keep it black to look like it's original.  I don't anybody would be bothered if you use original equipment replacement parts.

If the timing is not stock it may not meet emissions but if it runs better who cares.  Don't have to worry about it till next year.  Here in Washington since the car is over 25 years old you would never have to worry about it.

TED

[collector]

I have tested the items in question and they meet factory specs. I have changed the parts out and no difference so I put the originals back. Im sure to most the idle might be exceptable but I dont think a Cadillac should have an idle that is noticable. So lets move on about changing out parts that dont need changing. The original question at hand was about tuning differences and what they ment not anything else. Thank you and Im sorry if I am sounding rude, but the subject matter always seems to get off kilter somewhere down the line.

["Cadillac Kid" Greg Surfas 15364]

Mr. Collector,
If this is the car that sat in the Museum for 28 years then it is POSSIBLE that not only have the valve seals hardened and deteriorated, but the oxidation in the valve guides of the valves open to the atmosphere (not fully closed), built up and acted as an abrasive when the car was first started and scored the valve guides.  Another problem with internal combustion engines sitting dormant is that the piston rings can scratch the cylinder walls when started dry.  Just things that happen to any long sitting engine.  Believe it or not even the cam gear plastic will deteriorate with age and not just use.  This might be to the point where the cam timing is off.
Or, on the other hand, the car may just need to be driven a few hundred miles on the highway to work the kinks out of it's machinery.
This also might be the issue with your AC.  You might try cycling the selector lever back and forward while the car is running and/or driving slowly.  This in conjunction with rotating the temperature selector dial back and forward slowly might "wake up" a hibernating device or two.
Just a thought
Greg

[collector]

I have thought about these issues but if this were so then I wouldn't be able to keep consistent timing, and also I would have soot on my tail pipe. No, the exhaust is original and there is absolutely NO soot on or in the pipe. It doesn't leak or burn oil so the seals are good. I drove the car 400 miles and the car runs well except at idle. Like I said in another post Im going to take the car to a proshop and have them do modifications to the idle circuit of the carb or adjust the timing accordingly. The emission are very low and California laws are very strick so for that the car is running as it is supposed to but I don't like how it idles. Ill let them decide what it needs. We got almost 400 miles out of one tank so the car is running well.

[mario]

15.3 mpg is excellant.
i am having trouble with the your concern of kalifornia's strictness smog laws. yes they are strict, but if your cad is a 74 or 75, you need not worry, you are exempt from testing.
ciao,
mario

[Philippe M. Ruel]

I assume you made all the tuning on a warm engine, with choke valve fully opened.

800 rpm on Neutral is way too high ! it should be 600 rpm, and drop to 550 or so on Drive. It may be lower and still smooth (I had my 331 run smooth as low as 250 rpm) but water pump would run too slowly for an efficient cooling.

Air leaks : have you checked all around intake manifold gaskets, vacuum hoses, carb base for leaks ? Air entering intake manifold other than through carb throttle is the first cause for rough idle.

Ignition : initial timing you may read in books is adapted to fuel available at the time book was printed. Today fuel has a different octane rating, is somehow ethanol-mixed, so set initial timing "just before it pings under load". A "too-early" timing leads to pinging and engine destruction. A "too-late" timing may give a smoother idle but decreases mileage and may overheat engine.
Centrifugal and vacuum advances are respectively effective at high revs, and at slightly-opened throttle. They won't cause a rough idle.
Spark plug gap is not wider on HEI engines ; it is narrower on non-HEIs because voltage would be too low to fire a wide gap. Plus, misfiring generally occurs at high revs, not at idle (unless components are bad). So keep it to specs.

Carburetor : 1975 manual supplement (page 6-45) suggests the following procedure for idle mixture screws adjustment : uncsrew 5 whole turns. Set idle to 620-650 rpm.Turn alternatively each screw inward 1/4 turn at a time until the 600 rpm speed is reached.

Valve system : if the car has been sitting for a long time, there may be oxidation on some valve heads or valve seats, causing slight leaks. It causes trouble at low engine speeds and/or heavy loads. Have you checked cylinders compression pressure ?

["Cadillac Kid" Greg Surfas 15364]

Mr.Collector,
Philppe's response is as accurate and realistic as you are going to get. In the words of Don Ameche in the role of the shoemaker,"Tings change".  Your car, regardless of it's certifiable infancy is 30+ years old, and its joints, nerves, etc have the potential of developing the problems associated with old age.  The food available, especially in California certainly ain't what it was when unleaded regular (as differentiated from 100+ Octane Premium) was 100% gasoline (not 15% Oxygenate) and had an octane rating well above (I believe) 90. Automobiles are not and can never be both Museum pieces and dependable transportation.  A 70's era Cadillac was designed to go cross country trouble free and the components and systems had the resiliency and the redundancy to allow that.
I certainly would not consider driving 1000 miles at 70 miles per hour with tires, belts, hoses and electrical and electronics equipment designed for usage cycles, not years.
The 1974 eldo engine, I believe uses several components such as camshafts and pistons that were used that year only.  My father in law , who at the time used to get a new Cadillac every 6 months was so displeased with his 74 Eldo that he never bought another cadillac. Coincidince? I don't know.
Go someplace and get a tank of real gasoline or perhaps some 100 LL AvGas.  Your car will have trouble starting on it but once the engine is warm it should represent the fuel the engine was designed to operate on and perhaps the 10 RPM "burble" that troubles you so might disappear.
Just a thought.
Greg

[collector]

Thanks Im going to  actually when I get back home at the end of Aug get some top engine cleaner. Ford is about the only one that sells it anymore and pour that into the carb and let it sit then start it up and let it blow out any and all carbon and clean the valves. Its the best stuff ever. That should take care my situation. If not I will take it to my mechanic and say, "HERE, fix this thing" Im hoping this will work. Ive done everything within my power to make it smooth. Im going to leave it as the factory said in 74 with the smaller gap and hope I running the way I think it should.