A few of you may remember a couple of weeks ago I posted a question about the symptoms and diagnosis of late valve timing due to a cam/head installation on my 90 L98. Here were my symptoms:

Low vacuum (10"/hg) as measured by my pod pillar vacuum/boost gage
Exhaust popping during deceleration
Very poor low end performance

Here is what I did:

Hung a second vacuum gage on the engine -- it read 14"/hg
Checked the base timing -- read "0" with the ESC disconnected
Then found #1 TDC and checked the position of the rotor -- ok
Tried to move the distributor -- couldn't turn it -- assumed bolt tight
Adjusted base timing to 10 degress BTDC -- timing mark steady
Tightened distributor clamp -- noticed bolt did not feel quite right
Went for test hop

WOW, what a difference! Car had low end punch, did not "pop" during deceleration and the pod pillar vacuum gage now read 14"/hg. Meaning my other vacuum gage will be reading around 18"/hg. Will check to verify though.

Here is what I think happened. In tightening the distributor hold down bolt I noticed I had to really put some torque on it to keep the distributor from being able to be turned. I am using the stock bolt in an aftermarket manifold. I theorize the bolt will put just enough pressure on the hold down clamp to give the illusion the clamp is tight enough. In reality the distributor is slowly turning (retarding timing) while the engine is operating. I will put some miles on the car when the rain stops and recheck the timing to see if my theory holds water. At least I feel confident that my poor performance was not caused by late valve timing and I won't have to pull the timing cover. Hooray!

I guess what I overlooked was starting with the simple things. Instead of jumping to one of the worse possible conclusions, I should have stepped back and started with the basics. So far, good outcome, lesson learned, happy shade tree vette mech.

90Indy

 

I have the same problem. I hope that somebody here can tell me how to solve this problem with an LT1.

 

90Indy, the same exact thing occurred to me last summer. i discovered that the dist. bolt worked it was loose enough for the dist. to "walk" all the to 4 degrees ATDC instead fo the stock position 6 degrees BTDC, so thats a total of 10 degrees retarded. i really tightened it down the second time, and its been fine ever since. the main symtpom was excessive exhaust popping during decel.

congratulations on the easy fix

 

Nice easy fix !!

 

I believe I'm having the same problem. Does the holding plate forks curve down onto the distributor, or curve up ? Mine is up, but I put it on that way after an intake removal. It seemed to idle fine but under a load it runs rough.

 

Gotta love the easy ones!

 

um, stock bolt is metric, is the new manifold metric also?

 

It seems to me that there are 2 fixes for this
1) Get a shorter bolt, It sounds like the bolt is bottoming out in the hole before clamping down on the hold down clamp.
2) Place a washer under the bolt head, same as a shorter bolt.