ctrain22

Should the vacuum advance on a 68 400 automatic get manifold or ported vacuum? Should it have vacuum at idle?

Yellowstatue

The dist. hose should go to a heat switch on the front left of the manifold, top port is dist. middle port is to carb and bottom port goes to manifold....if engine over heats then the switch puts full manifold vacuum on the dist...

svnt442

The source for that is the ported source on the carb.

joe_padavano

The thermal vacuum switch has three ports, one labeled "D" that goes to the vac advance can on the distributor, one labeled "C" that goes to ported vacuum on the carb, and one labeled "M" that goes to manifold vacuum on the manifold. The TVS connects the D port to the C port under normal conditions, but switches and connects the D port to the M port when the engine starts to overheat.

svnt442

Thanks Joe. Is there anyway we can get your brain on a cd?

MN71W30

I was wondering what that thing did.

ctrain22

Thank you all for the info. I don't have the thermal switch. Guess I should get one. Where does the temp switch go if not in the hole on the left side of manifold? The factory wire harness's wire for the temp switch doesn't seem to be long enough to go to the pass side of manifold if that's where the switch goes. Did all 400's have the thermal switch? MIne is an automatic trans.

MX442

I've been screwing around with this for a couple years now... I found that with a fixed 20 degree can, it worked better with ported. Here's why. With say, oh about 16" of vacuum at idle, the can was full advanced at idle. Not good. Where as ported vacuum, it was only adding a few degrees advance at idle. This summer, I'm running an adjustable can. I haven't fine tuned it yet, but still running ported vacuum and the least amount of advance. I haven't put the pump on it yet to find out exactly how much. Hope this makes sense.

I didn't know about the temp controlled switch adding full vacuum to advance timing when hot. (mine is long gone.) What a cool concept! Makes sense, I might have to figure out how to put one back in.