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Issues with smooth running. 72 350 stock with mild cam stock Q-jet. No accessories or emissions, so the only thing needing vacuum, other than the power brakes, is the distributor advance. The carb has 3 outlets, none of which seem to produce any vacuum. Have a port on the manifold that when hooked to the advance runs the idle too high. Is there a diagram I can work off of? Here is a picture. Arrows show the carb ports and the manifold is circled. Also 2 ports on the backside. As always, my thanks,
Connect the distributor to manifold vacuum and adjust the idle speed as needed. The reason why you don't get vacuum at the carb ports is because the original setup used ported vacuum, which is zero until the throttle starts to open. Since you no longer have the original intake, distributor, or TCS system, there's no need to use ported vac.
The picture below shows the various intake manifold ports. As Joe said, connect the distributor vacuum advance to manifold vacuum and cap all of the unused carburetor ports.
The picture below shows the various intake manifold ports. As Joe said, connect the distributor vacuum advance to manifold vacuum and cap all of the unused carburetor ports.
That's a helpful photo, but the reality is that any manifold vacuum port is exactly the same as any other one.
Yep. This is just a picture posted a while back in response to someone asking about the factory intake manifold ports compared to an aftermarket Edelbrock aluminum intake manifold.
To directly answer your question about the carb fittings:
The two on the top of the air horn on the back are fresh air sources - they just provide air from inside the air filter. One was typically used to provide air for the hot air choke, I've always seen the other capped.
You may have a manifold vacuum port on the base plate on the back - some qjets did, some didn't.
On the front there's a range of ports. Looks like you have a 73 carb, which I'm somewhat familiar with. Note that the port configuration changed a lot so knowing the specific carb number can help.
On the baseplate, the port on the far left (looking at the front of the carb), is.... shoot, I forget what that one is. That was used for a 73 and up emissions thing, but can't remember if it's manifold or ported.
On the baseplate, the port on the middle/left, is manifold vacuum and is for PCV, as you have it hooked up.
On the baseplate, the port on the right, is a metered ported vacuum port for the charcoal canister.
On the main body, the one on the left is connected to the vacuum diaphragm. Good, don't do anything else with it.
On the main body, the one on the right between the two screw standoffs, is ported vacuum for distributor advance.
So you can use manifold or ported, whichever the car prefers.
It's quick to tell if a port is manifold or ported. Fire up the car and attach a vacuum gauge. If you have vacuum, then it's manifold. If there's nothing at idle, but shows vacuum when you rev it, then it's ported. Note that the qjet had a lot of provisions for various emissions controls.
April 19th, 2020, 06:08 AM
