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1973 Eldorado Convertible
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Discussion Starter #1
A good while ago I posted a topic trying to figure out why the 500 in my '73 Eldo has had a 'hiccup', for lack of a better term. The best way to describe it is to say it had a popping coming from the exhaust at idle, and a little shake when the popping started.
Recently I decided to take a look at the compression readings, and found they were between 115-130 (dry test). I was scared of an overall dead cylinder, so I found those reading to be a real relief. I also checked to see if the EGR was leaking, possibly letting a vacuum leak take place. The EGR hold vacuum, and I used some gasket maker to seal it back up. That didn't fix it. All of the vacuum lines are either new or the ports sealed. I plugged the brake booster line, and that didn't help.

After all this fooling around, I remember one of my old Chevrolets had a similar popping sound......when I had the timing too advanced. I had advanced the timing on the 500 because I read with the lower octane fuel, it was supposed to help it out. After retarding the timing, the popping in the exhaust is minimal, and the car is running smoother. Am I crazy, or is something messed up that I'm just putting a band-aid on?
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Higher octane fuels allow advanced timing; lower octane fuels require less advance - higher octanes burn slower than the lower octanes so you can use (correct, calibrated, measured) advance to keep more of the fuel burn within the most advantageous arc of downward piston travel.

Your 500 probably uses a vacuum advance can, so initial mechanical setting might be around 6 - 10 degrees BTDC, and as the engine starts and vacuum rises the timing, at idle, advances to about 16 - 18 degrees. It will slowly add mechanical advance as rpm rises - but if you accelerate strongly or punch it, vacuum drops (normal) and some vacuum advance drops back to prevent ping. Normal. Tooling down the road at 65 you might see a total advance of 38 - 46 degrees - BUT if you punch it that figure drops way back to prevent ping.

Unless the engine and ignition vacuum systems are properly connected there's no telling just what is causing the stumble or slight backfire.

Quadrajet carburetor ???? Find the HP book on Rochester carburetors by Doug Roe. All sorts of hints and vacuum diagrams.
 

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1973 Eldorado Convertible
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15 Posts
Discussion Starter #3
Wow, thanks for all the information. I just always connected and disconnected the vacuum advance as needed and never put much thought into exactly what it was doing numbers wise.

The vacuum advance is connected to the front of the carburetor, so I would consider the timing vacuum systems in place. Everything else has been disconnected over time, however.

Another piece of advice I received was to lube the pivot points on the mechanical advance with dielectric grease. I've done that and the weights now move freely. Playing with the distributor this weekend I noticed you can pick up one of the weights about 3/8 in before the pivot stops it. I'm not talking about picking it up to remove it. I'm pulling on it opposite of where it is connected. Do these things wear out to the point where it can affect how the car runs?

Yes, it still has the "quadrajunk" on it lol. I may have to look into purchasing it.

Thanks again for all your help!
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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The QJ is arguably the best 4-barrel carburetor invented. It flat, hands-down beats any Holley on the market when properly tuned.

Get the Doug Roe book. Depending on the engine, car, transmission, model, year the vacuum advance can may be connected directly to manifold vacuum,or through a VDV (Vacuum Delay Valve) inline can, or to a ported vacuum source. If you don't have the correct setup the engine will never run correctly.

Distributor flyweights should move freely out to the stop pins - and when you get really good at it, those flyweight stops can be "tuned" with rubber sleeves to control the amount of mechanical advance available. An adjustable vacuum advance can from Crane Cams allows you to tune the vacuum advance curve to the best possible for THAT engine. But, it's a time-consuming, experienced labor of love on the engine - unless you have one of these ............ obsolete as hell, but necessary for a real muscle car tuner...................
 

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I could never get much more than a year out of a set of points/condensor. If it was in my shop, I wouldn't touch anything until I knew they were new.
 

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1973 Eldorado Convertible
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Discussion Starter #6
I've actually heard the Quadrajet was second to none in both fuel economy and performance when done right. I had one on my '67 Impala with a 327 when I was in high school and I poured more money and time into that thing and never got it right. I finally traded it for an Edlebrock. That was fun while it lasted....which wasn't long. I think I still have a bad taste in my mouth from it.

The points and condenser unit I have installed are about 8 months old. I bought a new set after I changed the cam, lifters, and timing chain set and it still wasn't running as smooth as it needed to. The plugs, wires, cap, and rotor were all replaced, too, thinking maybe it would help. I can't help but think the weights on my distributor are worn out....I just don't know if that is a plausible explanation.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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I could never get much more than a year out of a set of points/condensor. If it was in my shop, I wouldn't touch anything until I knew they were new.
The trick is to replace the point set with a Pertronix Ignitor - fast, hot spark and no more points.
 
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