This is an easy one here. I replaced the carb on my 302. When it came time to connect the two vacuum hoses from the vacuum advance, I couldn't for the life of me remember which went to bottom front and which went to the left side of the Holley carb. Where does the vertical connection out of the advance go to and where does the horizontal one connect to(the one pointing towards the radiator)? As you can tell, I have no idea! One last thing. In the horizontal connection coming out of the vacuum advance, it is possible to adjust the spring inside it. What does that need to be adjusted to? (I don't have a vacuum gauge) I know I can slip an allen wrench in there, but what do I do next? Thanks for any replies.

Joseph Osborn
70 F-100 Flareside
302 V-8 3spd stk

 

On the vacuum advance/retard dual diaphragm unit, the vertical port is the retard side of the diaphragm and should be connected to full time manifold vacuum, which is the bottom front fitting on the carburetor. The horizontal port is the advance side of the diaphragm and connects to the timed vacuum fitting on the side of the carburetor. The vacuum advance diaphragm preload spring can be adjusted on some of these units, turning the allen screw clockwise puts more preload on the spring and delays the advance. Be careful with this adjustment as you can easily rupture the diaphragm by going past the end of its travel. You should really set up the advance on a distributor machine, first dialing in the centrifugal advance and then finishing up with the vacuum advance. If you must tinker, pick a hot, dry day and drive up a grade carrying a load with the engine hot and running on the cheapest grade of gas you will ever use. If it does not ping steadily at part throttle, go ahead and turn the allen screw in one turn at a time and repeat the test until it barely starts to ping. I don’t like doing this by itself because the vacuum advance is now optimized with a centrifugal advance that could be way out in left field.

Barry

 

What I meant to say was, turning the allen screw counter-clockwise puts more preload on the spring and delays the advance. In other words, clockwise = more ping, counter-clockwise = less ping. What I described on the road test is correct. It’s really best to do this on a chassis dyno so just carry the allen wrench in your glove box and adjust it when the need arises. Just a note, a 302 in a truck would be considered a low power to weight ratio vehicle and should not have too much vacuum advance set up. Continuous pinging over the long haul will eventually crack the rings. Sorry for the CW/CCW mix-up, it’s this darn digital world!