Vacuum can not working
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Pro
Joined: May 2013
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Vacuum can not working
Rwcently set the timing on my 69 427. I set the timing advanced 36 degrees when at 3k rpms and it is around 12 at idle. The timing was set with the vacuum advance disconnected. When I reconnect the vacuum canister there is no change to the timing. When i remove the hose from the nipple there does not seem to be any vacuum suction at idle. I don’t have a vacuum gauge but should I be able to feel vacuum suction with my finger?
Drifting
Joined: Aug 2013
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From: perth Aus w.a
2020 Corvette of the Year Finalist (stock)
C3 of Year Winner (stock) 2019
Is it hooked up to ported or manifold vacuum .My l68 was ported and changed it to full manifold vacuum and advanced the timing ang runs great now.
Drifting
Joined: Aug 2013
Posts: 1,263
Likes: 512
From: perth Aus w.a
2020 Corvette of the Year Finalist (stock)
C3 of Year Winner (stock) 2019
That is the correct factory location for emissions , your car will run better with full vacuum . Check you vacuum tank is holding vacuum .It is possible your tank is split in one of the indents on the tank. I unhooked all the vacuum related bits ,wipers and lights so I could concentrate on the engine
Team Owner
Joined: Jan 2006
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From: Crossville TN
That is correct. Ported vacuum has little or no vacuum at idle. After the throttle plate begins to open, ported vacuum attains the same vacuum level as manifold vacuum. It's an emissions "thing", intended to make the [retarded] combustion chamber temps hotter so waste hydrocarbons would be burned rather than spewed into the air via the exhaust system.
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Thread Starter
Pro
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Thanks for the responses. Another question. Will changing to manifold vacuum help in setting the idle speed. Currently the car is performing great with the timing settings I have but while driving and coming to a stop the engine wants to die. I have to feather the throttle to keep it running. If I adjust the idle up on the carb I get dieseling (run on) when I shut the engine off. Also I noticed the engine is running hotter than normal with the current timing settings.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Most likely it will help your idle problem. It should raise idle speed when you add in the vacuum advance. That will allow you to adjust idle speed down and thereby close the throttle plates a bit more and lean-out the idle mixture. Ideally, you would want to set up your distributor advance curve to 'performance' mode, as Lars Grimsrud advises in the instructional paper he provides. (request it by e-mailing him at V8FastCars@msn.com) If you do that, your advance at idle (when vacuum advance is blocked) should be in the range of 12-16 BTDC.
When you connect the vacuum advance and set idle speed to spec, it should idle well, have good off-idle operation, and improve performance significantly.
When you connect the vacuum advance and set idle speed to spec, it should idle well, have good off-idle operation, and improve performance significantly.
Thread Starter
Pro
Joined: May 2013
Posts: 570
Likes: 22
Most likely it will help your idle problem. It should raise idle speed when you add in the vacuum advance. That will allow you to adjust idle speed down and thereby close the throttle plates a bit more and lean-out the idle mixture. Ideally, you would want to set up your distributor advance curve to 'performance' mode, as Lars Grimsrud advises in the instructional paper he provides. (request it by e-mailing him at V8FastCars@msn.com) If you do that, your advance at idle (when vacuum advance is blocked) should be in the range of 12-16 BTDC.
When you connect the vacuum advance and set idle speed to spec, it should idle well, have good off-idle operation, and improve performance significantly.
When you connect the vacuum advance and set idle speed to spec, it should idle well, have good off-idle operation, and improve performance significantly.
Manifold vacuum connection
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
If that is the line which feeds the power brake booster, it is not your best alternative for connecting to the advance can. The brake booster feed is not normally used to feed another vacuum source. Can you use it? Sure. But it would be best to find a less "important" manifold vacuum source to which you can connect. The issue is a low-probability safety/reliability concern...because it involves your braking system. It would just be "better" to connect it elsewhere.
If that line is NOT to the brake booster, put a "T" in it.
If that line is NOT to the brake booster, put a "T" in it.
Last edited by 7T1vette; Mar 24, 2019 at 11:24 PM.