66 425 mystery knock

Leo66

Member
Help!
I have a 66 Riviera with a 425. 29,000 original miles. When I accelerate slowly there is a knocking in the motor. More of a knock than a ping. I put premium through it still there. Replaced the lifters they were fine. Just happened to leave a vacuum line for the heater controls off after the start up from replacing the lifters. No knocking. Put the vacuum line back on..it knocks. Thought the vacuum advance was stuck or bad. It all checked out ok.Reset the dwell and timing to 30 degrees(dwell) 7 1/2 degrees advance timing with vacuum disconnected per my manual. Hook everything back up still knocking. The car runs great except for the knocking. The slower I accelerate the more pronounced it is.Just a touch frustrating to say the least. Any ideas would be very appreciated
 

Leo66

Member
It doesn’t sound like detonation to me. It sounds mechanical. I don’t understand why leaving a vac hose off would stop anything mechanical from knocking
 
OK, first thing I would check is the torque converter to flex plate bolts. Converters can make a knocking noise when those bolts are loose. It can make the same noise when the flex plate is cracked. The converter moves back and forth from either cause and makes an irregular knock. It can be worse the rougher the engine runs. Pretty easy to check once you have the car in the air.
 

Leo66

Member
Only thing is why would leaving a vacuum hose off stop the knocking if it’s a loose bolt or a flex plate?
 
Leaving the vacuum hose off led to a large vacuum Ieak so the vacuum advance was getting zero vacuum.
I MUST ASK. IS this a 1x4 or a 2x4 engine. Reason I ask is a 1x4 initial timing is 2 1/2*, per the '66 chassis manual, at a slow idle with the vacuum advance hose dis-connected. IF the timing is set at 7 1/2* it is 5* too far advanced. Also the rubber limiting bushing on the vacuum advance & on the mechanical advance, even though it only has 29K miles, have after more than 50 years, has fallen off because they have deteriorated MANY years ago & fell into the distributor housing. Since they are rubber you will NEVER know they have fallen off as they will cause NO damage.
So after checking the above advice this is the next step.

Tom T.
 

Leo66

Member
It’s a 1x4. My manual shows to set it at 7 1/2. I will give that a try. Never checked the limit bushing. Will do all of the above this week. Thanks for the info!!
 
Leo - there is something else I would suggest after reading the above thread.

Recently after rebuilding a 401 in my 61 Electra (same basic engine) I had a bad "knocking" and for a moment thought the engine was shot
Seemed worse running slower.

The solution was so simple it almost makes one cry.

In troubleshooting one of the first things I did was remove the LHS valve cover and run the engine without it (because the nailheads are designed with a flat mating surface you can do this for a short time no mess) (try that on a small block chev). The noise disappeared. Strange.

Looking inside the removed valve pan we noticed a couple of shiny marks - obviously something touching/rubbing.

What we discovered is that a couple of the valve spring retainers were touching the inboard vertical face of the valve cover. Made an awful noise.
On a nailhead there is not a lot of clearance on the inboard vertical wall of the valve cover to the springs, and turns out a small offset caused a noise. Solution - well there is some adjustment in the 4 mounting bolt holes, and the downward bent tang that locates the valve cover needed a bit of straightening; a little pressure to the middle direction when snugging up the valve cover bolts and - voila - engine runs quiet again.

Give this a try - it would seem, all things being normal, that your low mile nailhead should otherwise still be healthy.

Dave
 

Leo66

Member
Still haven’t had time to look at all suggestions except the rubber bushings. Mine doesn’t have a stud anywhere that bushings could be put in… will check out the covers thanks Dave!
 
The vacuum advance has a rod that bends up on the end. This is where one rubber bushing is installed preventing the arm from going all the way in one direction or both depending on the function of the original vacuum advance. It's highly unlikely the vacuum advance is the original because the rubber diaphragm years ago ruptured because of age/deterioration. Replacements have no where near the original specs. of the originals.
The second "Pin" is located under the upper advance plate & attached to the upper shaft. It's only .1875" in diameter & rides in a slot that limits it's fore & aft movement is where the rubber advance limiting bushing would go.
Look closely as ALL distributors have them.
I have these rubber bushings in stock.

Tom T.
 
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