When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
For some reason (can't remember why) I bought a 68 roadster a few months ago. Stock, restored 20 years ago, with an L79. Shiny shark.
The previous owner did not drive it much at all, but I drive it whenever possible and as a result have been short-circuiting the drive/repair cycle. So, now I have a few things to do to the car that should have been accomplished long ago. One of the first things I did was set the timing, since it was a little bit (a lot) sluggish. The mech was maxing out at 28 and advancing it to 36 made it much snappier. However, the total mech advance available is only 16, so at idle we have 20, which I know is excessive. Yes, I disconnected the vac can. I have had the cap off many times scratching my head. I took some photos and did some analysis. It looks like the weights prevent the cam from retarding all the way. I measured the possible advance with the weights installed = 8 degrees. Full retard with weights Full advance with weights
The possible advance with the weights removed is 14 degrees, which seems normalish. Full advance no weights Full retard no weights
The weights and diz cam look typical to me, but I am not an expert. Help?
"Normal" springs, I guess. One is a little stronger than the other. It is all in by 2500. It doesn't seem to be a spring issue though- the weights interfere with achieving full retard.
From: At my Bar drinking and wrenching in Lafayette Colorado
Those are aftermarket weights, and the distributor mainshaft "football" is also aftermarket with a non-linear shape - it looks like a commercially "rebuilt" distributor. This will cause the symptom you describe. It can be corrected by re-shaping the "football" and installing original weights to create a more linear advance curve allowing full retard at low rpm.
However, there is nothing really wrong with running 20 at idle. Just limit the vacuum advance to no more than 10-12 degrees and you'll have a great running car. I would play with the springs to delay the full centrifugal advance to 2800 - 3000 rather then the current 2500. Installing slightly tighter springs to delay the full advance to 3000 rpm will probably pull your initial timing down a little lower at idle and you'll have a near-perfect setup
Lars
That’s what I was gonna suggest, reshaping center piece. I would grind a little off the weights too to get full retard, if you don’t want to buy new ones. Make it work like you want it to, nothing lost in doing so. If you mess it up you can always buy another aftermarket set and try again.
i ended up making my own center piece for my HEI cause I couldn’t find specs on different center pieces and I knew roughly how I wanted the timing to advance. Ended up with something that works very well after a couple tries.
Last edited by REELAV8R; Jan 13, 2019 at 10:49 AM.
Those are aftermarket weights, and the distributor mainshaft "football" is also aftermarket with a non-linear shape - it looks like a commercially "rebuilt" distributor. This will cause the symptom you describe. It can be corrected by re-shaping the "football" and installing original weights to create a more linear advance curve allowing full retard at low rpm.
However, there is nothing really wrong with running 20 at idle. Just limit the vacuum advance to no more than 10-12 degrees and you'll have a great running car. I would play with the springs to delay the full centrifugal advance to 2800 - 3000 rather then the current 2500.
Lars
^^^This^^^
I made a jig to verify total advance without having to reassemble and test it. I have rotor/pointer for HEI and points.
Hey Lars, are those weights up-side-down? Too cold here or I would go look under my dizzy cap. So I Googled a picture. It shows the tail of the wt going the other direction, indicting the O.P. wts need to be flipped.
From: At my Bar drinking and wrenching in Lafayette Colorado
For a Chevy, they are installed correctly. If it were a Pontiac, they's be backwards. Also, the HEI weights go the other way. Here is a photo of a stock setup with stock weights. You can see the correct shape of the "football" in the right weight interface - notice that it has a smooth, curved transition, as opposed to the aftermarket one that the OP has: notice the sharp "kink" or mid-point "ridge" that the OP aftermarket distributor has. That sharp edge really screws up being able to achieve a smooth advance curve.
You could notch the leading edge to pull them in some more......but as Lars says....just leave it and limit your vac advance.
This setup is perfect for large cam!
For a Chevy, they are installed correctly. If it were a Pontiac, they's be backwards. Also, the HEI weights go the other way. Here is a photo of a stock setup with stock weights. You can see the correct shape of the "football" in the right weight interface - notice that it has a smooth, curved transition, as opposed to the aftermarket one that the OP has: notice the sharp "kink" or mid-point "ridge" that the OP aftermarket distributor has. That sharp edge really screws up being able to achieve a smooth advance curve.
Hard to understand why incorrect parts were incorporated into the rebuild, but they obviously don't fit, so- I think I'll hog the weights out a little and see if I can ****** back a couple of degrees. I'll attempt to recurve the advance once I destroy the weights. Pity I don't have a rig like Big2Bird's, but I wouldn't know what to do with it anyways.
Many thanks to all.
68 Dizzy making me dizzy
