Need advice with timing advance
Thread Starter
Instructor
Joined: Jun 2001
Posts: 174
Likes: 3
From: Ann Arbor MI
Need advice with timing advance
I just installed a recurve kit for my HEI distributor ('76 L82). Before starting the project I read three papers on the subject - including the one by Lars, so I thought I knew what I was doing.
What I found was opposit of what should be happening with the mechanical advance. With the vacuum advance disconnected and plugged, my mechanical advance actually retards about 4 degrees as I increase the RPM's. This happens between 700 RPM's and 3000 RPM's. This is the first time I have ever checked the timing with a timing light so I don't know how it behaved before I replaced the weights and springs.
Is there anything I can do short of replacing the distributor? Any ideas on what causes this to happen in the first place?
Thanks for any insights you can provide - I'm pretty confused about this.
What I found was opposit of what should be happening with the mechanical advance. With the vacuum advance disconnected and plugged, my mechanical advance actually retards about 4 degrees as I increase the RPM's. This happens between 700 RPM's and 3000 RPM's. This is the first time I have ever checked the timing with a timing light so I don't know how it behaved before I replaced the weights and springs.
Is there anything I can do short of replacing the distributor? Any ideas on what causes this to happen in the first place?
Thanks for any insights you can provide - I'm pretty confused about this.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
I have seen a lot of worn out HEI dist. Take off the rotor and look at all the parts and verify they are moving. I'll bet you will find something frozen or the bushings worn or I have even seen the pins wear very badly. You might be in for a rebuild or replacement.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
do your weights look like this? the weights for HEI are installed differently then the weights for points distributors.
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Thread Starter
Instructor
Joined: Jun 2001
Posts: 174
Likes: 3
From: Ann Arbor MI
Thanks for all the responses. I installed the weights exactly like the ones I removed. The old ones could have been installed backwards, but I'm guessing that they were original. What parts besides the weights themselves are supposed to move around underneath the rotor? Nothing else seems to move much and I'm not sure what I should be loking for.
When I connect the vacuum advance, the total advance jumps as you would expect. Are the mechanisms for the mechanical and vacuum advance related? In orther words, if one is frozen, would you expect that they both be frozen?
Lastly, if it is determined that my distributor needs to be replaced, is it tricky to install a new one? Where is a good place to get a replacement HEI? Are there better products on the market that would install exactly like a HEI? I just bought an Accel distributor cap and super coil. Would they fit the same?
Sorry for all the questions, but I have no knowledge base here.
Thanks!
When I connect the vacuum advance, the total advance jumps as you would expect. Are the mechanisms for the mechanical and vacuum advance related? In orther words, if one is frozen, would you expect that they both be frozen?
Lastly, if it is determined that my distributor needs to be replaced, is it tricky to install a new one? Where is a good place to get a replacement HEI? Are there better products on the market that would install exactly like a HEI? I just bought an Accel distributor cap and super coil. Would they fit the same?
Sorry for all the questions, but I have no knowledge base here.
Thanks!
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Pro
Joined: Sep 2002
Posts: 690
Likes: 1
From: near Memphis TN
Maybe the springs are so light that the weights are not retracting? Was the timing working before you added the recurve kit?
To answer your question the weights move under centrifugal force as the distributor turns. The weights then move the center mechanism by pulling on the springs and that advances your ignition timing.
The vacuum advance is somewhat related to the same mechanism that moves the mechanical advance, but it has nothing to do with the weights and springs.
To answer your question the weights move under centrifugal force as the distributor turns. The weights then move the center mechanism by pulling on the springs and that advances your ignition timing.
The vacuum advance is somewhat related to the same mechanism that moves the mechanical advance, but it has nothing to do with the weights and springs.
Thread Starter
Instructor
Joined: Jun 2001
Posts: 174
Likes: 3
From: Ann Arbor MI
I don't know if the mechanical advance was working before I installed the recurve kit.
The center mechanism does not rotate at all. Is it supposed to move back and forth? Is this my problem?
The center mechanism does not rotate at all. Is it supposed to move back and forth? Is this my problem?
Pro
Joined: Sep 2002
Posts: 690
Likes: 1
From: near Memphis TN
Yes the center mechanism should rotate about 45 degrees or so. That's your problem - probably too much gunk from age.
You can either clean up your old distributor (probably requires taking it out) or buy a new one. A plentiful variety of aftermarket chevy HEI distributors are available from just about any automotive parts stores. THe accel cap and supercoil should fit them with no problem.
Taking out the distributor is easy, putting it back in in the same direction is a bit tricky. Do a search in this forum for proper directions.
You can either clean up your old distributor (probably requires taking it out) or buy a new one. A plentiful variety of aftermarket chevy HEI distributors are available from just about any automotive parts stores. THe accel cap and supercoil should fit them with no problem.
Taking out the distributor is easy, putting it back in in the same direction is a bit tricky. Do a search in this forum for proper directions.
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,402
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by A2Shark
The center mechanism does not rotate at all. Is it supposed to move back and forth? Is this my problem?
It's very common for the HEI mechanical advance system to be seized. Yours is seized. You need to pop the mainshaft out of the distributor body and disassemble the shaft and advance system. Clean it up, lube it, and stick it back together. This will make a huge difference in how your car runs. Just knock the roll pin out of the gear and the shaft will come out. Note that the dimple on the gear points the same direction as the firing tab on the rotor..
Thread Starter
Instructor
Joined: Jun 2001
Posts: 174
Likes: 3
From: Ann Arbor MI
I just want to say thanks for all the useful comments to help me figure this out. It was a great learning experience, and I think I know something about distributors now.
Lars was right on the money. I took the distributor apart last night and found that the mechanical advance plate was frozen solid to the distributor shaft. After soaking it in penetrating oil and wiggling it back and forth, it finally freed itself.
I'm guessing it was like this for a long time - can't wait to see how the car performs.
Lars was right on the money. I took the distributor apart last night and found that the mechanical advance plate was frozen solid to the distributor shaft. After soaking it in penetrating oil and wiggling it back and forth, it finally freed itself.
I'm guessing it was like this for a long time - can't wait to see how the car performs.
