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Old 10-16-2013, 07:46 PM
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Possible Mechanical Advance issue?

New to forum. Played with cars a few lifetimes ago, now have enough time and hopefully $ to enjoy and learn how to maintain and update recent purchase.

1969 Beaumont...... 350, TH350........ will flesh out the dog issue with car in another thread.

Recently experienced huge detonation at cruise speed and eventual stall and subsequent tow. Car surged twice and exploded. Is this a result of timing, Mech adv sticking open, lean carb condition? The distrib was stock and pertronix ignitor upgrade, the weights didn't seem to work smoothly when I took dsitrib apart ( have new billet edition with full voltage, seems OK so far) I have 1406 Edelbrock, apparently very lean in primaries and horribly lean in secondary, I understand this can cause heating, detonation issues, have not jetted the carb to a richer calibration, that may be part of next thread about moving frwd with this car. Thanks in advance for any and all comments. I have also found that I had a broken motor mount, can this cause a detonation prob at wot or cruise, has been fixed.

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Old 10-16-2013, 09:43 PM
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Originally Posted by 6T7EH View Post
New to forum. Played with cars a few lifetimes ago, now have enough time and hopefully $ to enjoy and learn how to maintain and update recent purchase.

1969 Beaumont...... 350, TH350........ will flesh out the dog issue with car in another thread.

Recently experienced huge detonation at cruise speed and eventual stall and subsequent tow. Car surged twice and exploded. Is this a result of timing, Mech adv sticking open, lean carb condition? The distrib was stock and pertronix ignitor upgrade, the weights didn't seem to work smoothly when I took dsitrib apart ( have new billet edition with full voltage, seems OK so far) I have 1406 Edelbrock, apparently very lean in primaries and horribly lean in secondary, I understand this can cause heating, detonation issues, have not jetted the carb to a richer calibration, that may be part of next thread about moving frwd with this car. Thanks in advance for any and all comments. I have also found that I had a broken motor mount, can this cause a detonation prob at wot or cruise, has been fixed.
Sounds like a timing issue but when you say "exploded", just exactly how bad is that? A broken motor mount will not cause detonation. Poor spark, cam and spark timing, and lean conditions will. You need to get the timing set and the carb jetted, check for good spark. I would lockout the vacuum advance.
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Old 10-16-2013, 11:08 PM
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The explosion was just that, I thought the event went thru the carb or headers, shut down etc. Continued briefly and it happened again, decided to have car towed.
This is a new vehicle for me, finding that many people had hands on various systems etc. Balancer was changed , installed 7" with new double roller chain, I believe original balancer was 6.75 unit, as well when chain was installed it was set 2 degrees advanced, haven't checked for accuracy of timing marks but I'm guessing this is part of the reason it has been difficult to time properly. In order to take car on any extended cruises it will have to be jetted or metered richer by trial to keep cylinder heat down.

thanks for reply, car is now covered for winter here in Canada, soon to head to Az for winter, will have threads re: 67 Camaro that needs attention.
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Old 10-17-2013, 12:04 PM
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Welcome to Hotrodders!

When you get it where it can be worked on, start by verifying TDC as seen at the damper/timing tab is actually TDC. More on the different damper and timing tab combinations here.

The mechanical advance may be sticking, the same for the vacuum advance. You can check the distributor over for proper function, or replace it w/an HEI distributor if it's a points-type. I personally would recommend swapping in an HEI distributor, or use a conversion setup to get rid of the points if that's what you still have. The timing curve link also has info on wiring an HEI. Once you know the damper TDC line and timing tab is correct and the mechanical and vacuum advance is working right, set it up w/a performance timing curve.

If the carb is stock and in good condition, it's not too lean. It may need fine tuning, but the way it came from the factory would not cause the problems you're having.

Do a vacuum test to see what that shows you. You may have a severe vacuum leak, the carb could be faulty if it's been dickered with by a PO (float too low, jets too small, etc.), or may be in need of a good cleaning/rebuilding. Checking for vacuum leaks should be one of the first chores on your list.

Change out the fuel filters. Check the secondary side of the ignition (cap, wires, plugs, etc.) The color of the plugs can give you insight into the carb tune. Be sure the plug heat range isn't too hot.

Last edited by cobalt327; 10-17-2013 at 12:12 PM.
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Old 10-17-2013, 09:14 PM
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Thanks Cobalt for taking the time to respond, can see from wiki editor, to your experience and helpful replies to other threads that I have come to the right place to learn what it takes to keep a classic car on the road.
I replaced the distrib and had 6v resistor wiring eliminated full voltage to ignition, but haven't played with curving spring changes, will get an accurate timing mark first. Vacuum test showed 12-15 " hg and unsteady, I think you're right I'm guessing there is a vac leak, this would interfere with steady idle and overall performance..yes? The cam is mild in the 2224\224 @.050 range with 4* built in and advanced 2* when timing chain was replaced ( some of the history from previous owner, shouldn't impact vac reading too much. Fuel filter and pump are new but don't have regulator to tell what psi is going to carb, ( no gas smell) checked 1 plug when I first had issues, it went from blackened to tan so that's an improvement. Thanks again for your insight, will tackle rear axle to engine upgrades in spriing, will need more of the hot rodders expertise. Have 67 Camaro in Mesa Az. to further my learning curve............ LOL
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Old 10-17-2013, 10:04 PM
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Glad to help. You'll find many folks willing to help here so like you said, you've come to the right place.

The cam you have calls for more initial advance than stock. It wouldn't surprise me if you found 14-16 degrees BTDC at idle worked well. Just be sure the total advance (initial plus mechanical) doesn't go too high when you add that much initial timing to it. The HEI link has info on limiting the mechanical advance.
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