Relentless spark knock at warmup (1976 Fleetwood)

[Thejumpsuitman]

Good afternoon, folks!  This is my first post on this forum, though I have owned Cadillacs all my driving life. 

I am currently workin on a '76 Fleetwood Brougham with 66k mi.  I am resurrecting it after decades long carport storage.  I have replaced the following...  Spark plugs (AC Deco gapped at 0.060). New AC Delco wires, distributor cap, rotor button, ignition module, water pump, alternator, belts, hoses, vaccuum advance, vacuum lines.

I visually inspected the timing chain and set the timing at 6 degrees before TDC while idling at 600 rpm.  Fuel tank is clean, carburetor appears to have been worked on just before my acquisition and performs very well.  I did dial in the air/fuel mixture, but that's all I've done with that.

The car runs smoothly at idle but under load spark knocks like you wouldn't believe.  At first I thought maybe it was carbon build up so I hit the highway for about 100 miles with it, but the more I drive it, the worse it gets.  I am pretty much out of ideas...  The car is basically undriveable as is and I am afraid the engine will be damaged if I run it anymore the way it is.  It does not seem to lack for power, but I am wondering if I have some restriction somewhere causing higher engine temps.  Any help would be greatly appreciated. 

Thanks,
Marc

[35-709]

Where did that new vacuum advance come from?  Is it an aftermarket piece?  Some of the vacuum advances you get these days are generic and adjustable and need to be adjusted with a timing light.  Many will fit but are not correct for a 472/500.   You adjust them with a small screwdriver through the vacuum hose opening.  Sounds like your vacuum advance is coming in too soon and/or way too much.

[35-709]

You will need to refer to your factory shop manual to see at what RPMs you should have how much advance.  You will, of course, need to be handy with a timing light and have a tachometer you can hook up.

[Thejumpsuitman]

Thank you so much for the lead!  A mechanic friend of mine put it on, but I don't know what method he used to set it up.  I will call him and see. The spark knocking was there before as well, but it seems worse now and Maybe that's why.

[Thejumpsuitman]

Mr. Newcombe, you are my hero!  You were exactly right...  I disconnected the vaccuum advance and all spark knocking went away immediately!  Thank you, sir!

["Cadillac Kid" Greg Surfas 15364]

Wow Geoff.
Greg Surfas

[35-709]

Now cut that out, Greg.

You're welcome, Marc --- there's more work to do.   

[TC]

Just thinking, it couldt also be the centrifugal advance breaker plates in the distributor stuck in an advanced position? The early HEI distributors were famous for seizing.

["Cadillac Kid" Greg Surfas 15364]

Certainly one of the first things to check.
Greg Surfas

[bcroe]

I visually inspected the timing chain and set the timing at 6 degrees before TDC while idling at 600 rpm.  Fuel tank is clean, carburetor appears to have been worked on just before my acquisition and performs very well.  I did dial in the air/fuel mixture, but that's all I've done with that.

The car runs smoothly at idle but under load spark knocks like you wouldn't believe.  At first I thought maybe it was carbon build up so I hit the highway for about 100 miles with it, but the more I drive it, the worse it gets.  I am pretty much out of ideas...  The car is basically undriveable as is and I am afraid the engine will be damaged if I run it anymore the way it is.  It does not seem to lack for power, but I am wondering if I have some restriction somewhere causing higher engine temps.  Any help would be greatly appreciated.  Thanks, Marc 

Better check for slack in the timing chain.  If its more than a couple degrees,
your cam is probably bouncing back and forth, pinging every time it goes
ahead.  That is what happened to me.  good luck, Bruce Roe