vacuum canister swap
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
No. Unless there is some unknown problem inside the carb. Ported vacuum and manifold vacuum is created by the downward stroke of pistons on intake cycle. That is available to the carb and is used to generate air/fuel mixture. Ported (timed) vacuum is accessed from ports just above the primary throttle plates; so when plates are closed (idle condition), there is nearly ZERO vacuum at the ported vac sites. Once the throttle plates open a bit (off-idle), ported vacuum level approaches the same level as manifold vacuum. This ported vacuum was used to negate the vacuum advance at idle...making the engine retarded at idle, running the combustion chambers hotter so excess hydrocarbons would be burned instead of coming out the exhaust. Of course, having no vacuum on the advance can at idle made the engine run hot, the combustion chambers run lean, and the idle run like cr@p. But, the air was cleaner. 
In most cases, manifold vacuum run to the advance can is better than using ported (except for emissions).
In most cases, manifold vacuum run to the advance can is better than using ported (except for emissions).
Last edited by 7T1vette; Nov 28, 2018 at 10:46 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Make note of how much manifold vacuum you're pulling at idle with the ported setting. Once you switch to manifold vacuum, the idle speed will likely increase, so you'll need to lower the idle speed. Once you have lowered the idle speed to the previous idle speed, note the manifold vacuum again. If manifold vacuum is significantly higher than before, you will need to re-adjust idle mixture: Idle circuit fuel is pulled through the carb by manifold vacuum, and the volume of fuel through the circuit varies with vacuum. More vacuum will pull more fuel and will richen it up at idle.
Lars
Last edited by lars; Nov 28, 2018 at 10:49 PM.
