Mechanical advance timing trouble.
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Joined: May 2006
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From: Minnedosa MB
Mechanical advance timing trouble.
I just finished building up a 76' L48 with new heads, cam, and intake. The motor runs great but I can't get the timing right. I've read all the Lars papers that are around on this. I understand what I want but I can't get my distributor to do it.
I'm trying to get 14 initial, that I can get and 20 mechanical for a total of 34 before 3000rpm. I do plan to up it to 36 later after some test runs.
What is happening, I'm getting too much advance with any spring combo that gets to 20 mechanical before 3000rpm. So I get my total of 34 at say 2700rpm but if I keep reving the motor to 3700 it adds about 6 more and I'm at about 40. This is with stock weights, vacuum advance disconnected on heavy spring, one medium. I have a digital dial back timing light that is working correct. I have a summit mechanical advance spring and weight kit also Moroso kit. I have tried many combos but either the springs are so strong I dont get my 20 until 3500 and up or they are too weak and allow it to go over. What am I doing wrong here?
I'm trying to get 14 initial, that I can get and 20 mechanical for a total of 34 before 3000rpm. I do plan to up it to 36 later after some test runs.
What is happening, I'm getting too much advance with any spring combo that gets to 20 mechanical before 3000rpm. So I get my total of 34 at say 2700rpm but if I keep reving the motor to 3700 it adds about 6 more and I'm at about 40. This is with stock weights, vacuum advance disconnected on heavy spring, one medium. I have a digital dial back timing light that is working correct. I have a summit mechanical advance spring and weight kit also Moroso kit. I have tried many combos but either the springs are so strong I dont get my 20 until 3500 and up or they are too weak and allow it to go over. What am I doing wrong here?
Le Mans Master
Joined: Apr 2005
Posts: 6,350
Likes: 423
From: Seattle Area WA
Personally, I would only be concerned with the 34-36 degrees in by 2,700-3,000 RPMs and let the initial be what it is. Other than that option, take it to a shop with a Sun machine or send it out to Lars.
Instructor
Joined: Jun 2005
Posts: 140
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From: Bovingdon Hertfordshire
Pretty sure your timing mark has moved.
The harmonic dampers on these cars are know for slipping, the rubber gives up and the outer slips causing a false reading.
One way is to check and mark a new TDC pointer on the balancer.
Or buy and fit a new balancer (worth checking it is that first).
So although it 'LOOKS' like full advance is in at 2700 - it isn't.
Jeremy.
Try this article from LARS :
http://www.corvetteclub.org.uk/files...deadcentre.pdf
Alternatively - the stop bushing in the distributor is missing.
The harmonic dampers on these cars are know for slipping, the rubber gives up and the outer slips causing a false reading.
One way is to check and mark a new TDC pointer on the balancer.
Or buy and fit a new balancer (worth checking it is that first).
So although it 'LOOKS' like full advance is in at 2700 - it isn't.
Jeremy.
Try this article from LARS :
http://www.corvetteclub.org.uk/files...deadcentre.pdf
Alternatively - the stop bushing in the distributor is missing.
Last edited by langlj00; Sep 26, 2007 at 12:45 PM.
Instructor
Joined: Mar 2002
Posts: 140
Likes: 1
From: east hampton ct
If you are using a stock distributor I believe they allow for more than 20 degrees mechanical advance. You either need an aftermarket distributor with 20 degrees travel or reduce the length of the advance slots in the distributor.
Tech Contributor
Joined: Aug 1999
Posts: 14,377
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From: At my Bar drinking and wrenching in Lafayette Colorado
If I understand correctly, your distributor is pulling in a nice straight-line advance curve from idle to 2700, at which point you are getting your desired total advance. If you rev it up, the timing stays constant for a while from 2700 through 3500, then you suddenly get another 6 degrees slamming in at 3700.
If this is the case, the problem is not in the springs - it's in the shape of your weights. Take a look at your stock weights: Some of the stock weights had a long "tail" on the skinny part of the weight (the part of the weight that reacts with the distributor shaft cam). When this end of the weight is long, it will create a "flat spot" in the advance curve, and the last little bit of advance will suddenly slam in at a higher rpm. To fix it, you need to find a set of stock weights with the short arm on the small end of the weight. This will allow you a smooth advance curve that pegs out and stops firmly at the 2700 rpm point.
(Post a photo of your weights if you want me to verify this for you)
If this is the case, the problem is not in the springs - it's in the shape of your weights. Take a look at your stock weights: Some of the stock weights had a long "tail" on the skinny part of the weight (the part of the weight that reacts with the distributor shaft cam). When this end of the weight is long, it will create a "flat spot" in the advance curve, and the last little bit of advance will suddenly slam in at a higher rpm. To fix it, you need to find a set of stock weights with the short arm on the small end of the weight. This will allow you a smooth advance curve that pegs out and stops firmly at the 2700 rpm point.
(Post a photo of your weights if you want me to verify this for you)
Thread Starter
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Joined: May 2006
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From: Minnedosa MB
I am still having trouble with the timing. I have the proper 76' L-48 stock weights. I think the bushings being worn out on the weights may be the cause of this over the limit mechanical timing issue.
I think you would have to go to an auto wrecker to find weights and bushing because the GM dealer here in Canada don't seem to have any I spent the whole day looking even the internet doesn't come up with much.
I think you would have to go to an auto wrecker to find weights and bushing because the GM dealer here in Canada don't seem to have any I spent the whole day looking even the internet doesn't come up with much.
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Joined: May 2006
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From: Minnedosa MB
I just found a good site for HEI parts.
http://www.kendrick-auto.com/ignition.htm
they don't list weights but you can get the pins and bushings.
http://www.kendrick-auto.com/ignition.htm
they don't list weights but you can get the pins and bushings.
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Thread Starter
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Joined: May 2006
Posts: 71
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From: Minnedosa MB
I think I just figured out my problem but I have not tested it yet because I need distributor bushings to adjust end play on the shaft. I found the info here http://www.rustpuppy.org/ignition2/Ignition%202b.htm
It seems every .010" of end play on the distributor can give you a variation of one degree in your timing. I do have .067 end play so this just might be my problem.
It seems every .010" of end play on the distributor can give you a variation of one degree in your timing. I do have .067 end play so this just might be my problem.
