Vacuum advance and ignition questions

[dlb]

there's an arrow on the right side of the diagram that points toward the front of the car so use that for orientation.

[irowiki]

Haha, sometimes when trying to figure something out, it is easy to miss the obvious.

[irowiki]

Alright, now that I have a working advance, I'm about to hook it back up, but for now it'll only be the upper port (and capping the HAC port). I do not yet have a fixed carb for ported vacuum for the lower port. Could I get a quick summary of what the upper and lower ports do?

The upper port, connected to manifold vacuum advances timing by 8 degrees from what I've read, correct?

How much advance does the upper port do? Right now my messy setup has vacuum from the lower right carb being supplied at idle to the lower port advancing the timing past 20 degrees. Making the idle too high as well. I'm assuming I can just use the upper port and ignore the other for now.

[dlb]

the upper port port with the smaller hole goes to the HAC to advance timing at high atitudes.

you are correct that the upper port with the larger hole connects to manifold vac and advances the timing at both idle and hwy cruising.

the lower port connects to ported vac on the carb and advances timing at hwy cruising.

the upper port advances the timing by about 8* while the lower port advances timing by about 12* or 13*. this means that with the hoses connected properly and a base stock timing of 5* BTDC, timing at idle should around 13* BTDC. if your base timing (timing with hoses disconnected and plugged) is 5* but you are getting 20* when you hook the hose up to the lower port, you have the hose connected to the wrong source of ported vacuum. at idle, you should be able to disconnect the hose from the lower port with no effect to the timing or RPM.

once the timing is correct with the vac hoses hooked up correcty, you must then reset your idle to 800-900 RPM.

yes, you can use all the vac advance ports, just some of them, or none at all, but you will get the best running and fuel economy with all of them hooked up correctly.

hope that helps, good luck!

[irowiki]

It's hard, because with the hose disconnected and plugged, the car runs like crap and normally stalls!

[deejay1272]

Regarding routing, the upper port with a brass restriction on it (might be hard to see in place—use a small camera to take a pic of each port and compare) goes to the HAC. if you don't have HAC, just plug this port for now. the other upper port goes to manifold vacuum, and the lower port goes to ported vac on the carb. use this colour-coded diagram to help find where each thing is. the HAC line is grey, the manifold vac line is green, and the ported vac line is red.

DLB - Does this advice reflect your most up-to-date understanding of the proper way to hook up the vacuum advance - even for Tercs with Weber carbs? I recall going 'round and 'round on this topic a few years ago with you and wanted to make sure that this is in line with your current understanding.

Also - just to be fully clear: A good place to get "manifold vacuum" is to connect a vacuum line to the "gas filter" that is screwed directly into the top of the intake manifold. And a good place to get "ported vacuum" is to find a vacuum port somewhere above (or "before") the carb butterfly. Correct me if I'm wrong.

Thanks

[dlb]

haha, i remember you and i trying to figure it out a few years back too. yup, that quote is the correct and most recent conclusion i came to on the vac advance hose routing. also, you're correct about where manifold vac and ported vac are hooked up. you can see in this pic where the ported vac is on the weber.