Engine 304 Intake vacuum port identification

[Tim_Coble]

Here's the intake from my AMC 304 , can someone help identify these potential vacuum leaks? The tubing was already rotted away when I got the Jeep and I can only guess at a few of them. I assume either 1 or 5 went to the carb(?) and I can probably use one of them for my power brake upgrade. 2 probably went to the distributor. I have no idea about 3 and 4.

That loose hose off the EGR sensor takes a vacuum from the carb, right?

 

[TSB8C]

EGR is correct.
PCV is correct.
Exhaust back pressure sensor is correct.
1 = Full manifold vacuum port, usually used with a factory air pump and spark control switch. Cap it off if not used/needed.
2 = Temp gauge sensor.
3 & 4 = Choke heater (could have a cover plate if not used/needed).
5 = Another full manifold vacuum port (could be used for power brake booster if not using a lower port on the rear of your carb). Cap it off if not used/needed.

 

[Tim_Coble]

Awesome, thank you!

So, distributor gets it vacuum from the carb? Interesting. Wish my manual had a nifty diagram like that.

 

[TSB8C]

Yes - in the stock setup, the distributor gets ported vacuum from the carb, not full/constant vacuum from manifold. However, you should consider either as an option, depending on engine setup and desired outcomes. Here is the main rule-of-thumb:

Manifold Vacuum:
Better idling and low-speed performance
Improved fuel economy
Better throttle response

Ported Vacuum:
Reduced emissions
Improved high-speed performance
Minimized risk of engine overheating

I have usually chosen ported vacuum so that timing was adjusted based on the throttle position.

Here's a quick side-by-side chart for reference:

Parameter	Manifold Vacuum	Ported Vacuum
Vacuum at Idle	High	Low
Vacuum with Throttle Open	Increases	Increases
Timing Advancement	Steady	Throttle-dependent