Mechanical Advance Question
Thread Starter
Pro
Joined: Nov 2002
Posts: 501
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From: Ventura Highway in the sunshine
Mechanical Advance Question
Thanks for all this forum does for the Corvette community. I have learned a great deal through your postings. 
I've run into a strange problem (to me) with the original distributor on my 69 L-46 Corvette.
The mechanical advance seems to be limited to about 9 degrees, yielding 25 degrees with the vacuum advance disconnected at 2500 rpm. Beyond 2500 it doesn't advance any further.
To accomplsh this I installed two silver springs from a #928 Mr Gasket kit, and re-used the orignal weights (which were in good shape). The heavy original springs were on it and it was hardly advancing at 2500 rpm before the kit.
A rubber advance limit bushing was in place on the main shaft cam, but I replaced that with the brass metal bushing in the #928 kit. The two bushings were the same outer diameter, measured with a caliper.
I found that there is about 3/8 inch up/down play in the main shaft of the distributor with only a single thin shim. I intend to shim to the play out of the shaft, and am curious whether this would be causing my mechanical advance limit problem? Or, won't this have any effect on mechanical advance, and should I remove the limit bushing on this distributor?
I've run into a strange problem (to me) with the original distributor on my 69 L-46 Corvette.
The mechanical advance seems to be limited to about 9 degrees, yielding 25 degrees with the vacuum advance disconnected at 2500 rpm. Beyond 2500 it doesn't advance any further.
To accomplsh this I installed two silver springs from a #928 Mr Gasket kit, and re-used the orignal weights (which were in good shape). The heavy original springs were on it and it was hardly advancing at 2500 rpm before the kit.
A rubber advance limit bushing was in place on the main shaft cam, but I replaced that with the brass metal bushing in the #928 kit. The two bushings were the same outer diameter, measured with a caliper.
I found that there is about 3/8 inch up/down play in the main shaft of the distributor with only a single thin shim. I intend to shim to the play out of the shaft, and am curious whether this would be causing my mechanical advance limit problem? Or, won't this have any effect on mechanical advance, and should I remove the limit bushing on this distributor?
Thread Starter
Pro
Joined: Nov 2002
Posts: 501
Likes: 1
From: Ventura Highway in the sunshine
Shimmed the main shaft to .007". Still only getting 9 degrees mechanical advance from initial.
Going to remove the advance shim next. Hopefully that will get me to 36.
Thanks in advance for any pointers.
Going to remove the advance shim next. Hopefully that will get me to 36.
Thanks in advance for any pointers.
Tech Contributor
Joined: Aug 1999
Posts: 14,377
Likes: 6,377
From: At my Bar drinking and wrenching in Lafayette Colorado
There are no Delco distributors that only produce 9 degrees centrifugal timing with the stock weights. Chances are that you're getting some of the initial advance in right at your idle speed, and this is giving you a false indication of a short curve. You might want to measure the length of you advance limit slot and see if it has been altered...
Also, if you had 3/8" up-and-down movement of the shaft, you might want to make sure the upper shaft bushing is installed correctly - I've never seen a shaft with that much play in it.
Also, if you had 3/8" up-and-down movement of the shaft, you might want to make sure the upper shaft bushing is installed correctly - I've never seen a shaft with that much play in it.
Thread Starter
Pro
Joined: Nov 2002
Posts: 501
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From: Ventura Highway in the sunshine
Thanks lars. I used rubber-bands to hold the weights closed while measuring initial advance, and took the rubber-bands off to measure mechanical advance.
The advance limit slot doesn't look modified, but it's possible it was was carefully closed a little to avoid pinging by a previous owner. It's an original 11:1 L-46, so pinging would likely be a problem on straight 93 octane premium.
I'm going to take the limit bushing off this week, to see whether it gets me up to 34-36 degrees. I've been doping the 93 octane with Max Lead 2000 for as long as I've owned the car (5 years), so hopefully I can avoid pinging when I get advance problems solved.
I found my #928 spring/bushing kit at a local hot rod shop. Coolest auto parts store I've ever been in. The guy behind the counter at Napa directed me there for parts.
The advance limit slot doesn't look modified, but it's possible it was was carefully closed a little to avoid pinging by a previous owner. It's an original 11:1 L-46, so pinging would likely be a problem on straight 93 octane premium.
I'm going to take the limit bushing off this week, to see whether it gets me up to 34-36 degrees. I've been doping the 93 octane with Max Lead 2000 for as long as I've owned the car (5 years), so hopefully I can avoid pinging when I get advance problems solved.
I found my #928 spring/bushing kit at a local hot rod shop. Coolest auto parts store I've ever been in. The guy behind the counter at Napa directed me there for parts.
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Thread Starter
Pro
Joined: Nov 2002
Posts: 501
Likes: 1
From: Ventura Highway in the sunshine
Removing the advance limit bushing seems to have solved the problem. Without it, I'm set at 15 initial and 35 total w/o vacuum.
The car seems much more powerful than it did before I started. Thanks for all the help!
The car seems much more powerful than it did before I started. Thanks for all the help!
