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Old 05-13-2011, 09:00 AM
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Join Date: Apr 2011
Location: Toronto, Canada
Posts: 181
What is the purpose of the vacuum retard on ignition?

73 280 with the 110.921 motor has a vacuum retard unit on the distributor. According to the manual, it reduces idle advance from 7 BTDC (installation spec) to about 4 ATDC.

I'm preparing to install a new Weber carb and was wondering if I can do away with the vacuum retard and set the timing as close to 10 BTDC as possible. That's where the Weber likes it.

What's the purpose of 13 degrees of ignition retard at idle? What would I affect if I removed it?

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Old 05-13-2011, 09:22 AM
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Join Date: Jan 2006
Location: Tucson, AZ
Posts: 5,358
Emissions control. Retarding the timing creates a later "burn" resulting in a more complete combustion of exhaust gases at an idle.

Shouldn't be removed on states which require original "smog" equipment for emissions on older cars.

No problem with removing it otherwise. If your advance mechanism is working properly you should set it at 30 degrees BTDC at 3000 rpms for maximum benefit. Your initial at idle timing reading will vary depending on component wear.
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Old 05-13-2011, 12:21 PM
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Join Date: Apr 2011
Location: Toronto, Canada
Posts: 181
Great, that's really helpful.

Up here in Ontario cars pre-88 don't get smog checked, just need to retain cats if originally equipped.
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Old 05-14-2011, 01:37 AM
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Join Date: Dec 2010
Posts: 3,236
Some vacuum cells have two ports - one for retard and one for vacuum advance.
Even though it says vacuum retard it's really an advance control. VR pulls the distributor to a zero setting and as you throttle up a spring in the vacuum cell moves the distrbutor towards advance. If you remove this portion you will only get mechanical advance. The more VR you have generally the less mechanical advance is in the distributor. If the distributor has 20 degrees of advance and you can run the engine at 10 degrees BTDC you will get the needed 30 degrees to make it pull hard.

Initial timing settings are mostly concerned with idle fuel ratios and emission controls. The different carb will give you a different mixture at idle ( likely better ) so you will need to play around with timing to see where the engine runs best.

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