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Author Topic: Relentless spark knock at warmup (1976 Fleetwood)  (Read 800 times)

Offline Thejumpsuitman

  • Posts: 4
  • Name: Marc Woodward
Relentless spark knock at warmup (1976 Fleetwood)
« on: June 25, 2017, 04:38:47 PM »
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

Offline 35-709

  • Posts: 5365
  • The most valuable antique is an old friend.
  • CLC Number: 4719
  • Name: G. Newcombe
Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #1 on: June 25, 2017, 06:38:49 PM »
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.
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

Offline 35-709

  • Posts: 5365
  • The most valuable antique is an old friend.
  • CLC Number: 4719
  • Name: G. Newcombe
Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #2 on: June 25, 2017, 06:51:45 PM »
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.
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

Offline Thejumpsuitman

  • Posts: 4
  • Name: Marc Woodward
Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #3 on: June 25, 2017, 07:12:05 PM »
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.

Offline Thejumpsuitman

  • Posts: 4
  • Name: Marc Woodward
Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #4 on: June 25, 2017, 08:07:21 PM »
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!

Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #5 on: June 25, 2017, 09:19:59 PM »
Wow Geoff.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline 35-709

  • Posts: 5365
  • The most valuable antique is an old friend.
  • CLC Number: 4719
  • Name: G. Newcombe
Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #6 on: June 25, 2017, 09:48:47 PM »
Now cut that out, Greg.

You're welcome, Marc --- there's more work to do.   :)
« Last Edit: June 25, 2017, 09:52:54 PM by 35-709 »
1935 Cadillac Sedan resto-mod "Big Red"
1973 Cadillac Caribou - Sold - but still in the family
1950 Jaguar Mark V Saloon resto-mod - Sold
1942 Cadillac 6269 - Sold
1968 Pontiac Bonneville Convertible
1935 Glenn Pray - Auburn Boattail Speedster, Gen. 2

Offline TC

  • Posts: 68
  • Name: Tom Stephens
Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #7 on: July 01, 2017, 11:35:00 AM »
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.

Re: Relentless spark knock at warmup (1976 Fleetwood)
« Reply #8 on: July 01, 2017, 12:19:23 PM »
Certainly one of the first things to check.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline bcroe

  • Posts: 3898
Relentless spark knock at warmup (1976 Fleetwood)
« Reply #9 on: July 01, 2017, 04:47:08 PM »
Quote from: Thejumpsuitman
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

 

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