Distributor vacuum advance?

[bucks77ford]

I have had this issue for quite some time. When my distributor vac advance is hooked up, the truck sputters a bit from a stop to acceration and seems to run fine at higher speeds. With the vac advance plugged off, truck runs much better. I played with timing a bit, but never messed with the distributor. I have just kept the vac advance unhooked. Could it be that the vac advance in the distributor is out od adjustment? It's on my 75 supercab 460. All stock ignition.

 

[Fellro]

I would be checking where it is hooked to. It should always be hooked to the base of the carb on the "ported" hookup. The vacuum needs to be relative to the throttle plate position. If you hook it to the wrong port, you could have full vacuum, which then advances it at the wrong times and could actually retard it under low vacuum conditions, instead of advance as you want it to be.

 

[bucks77ford]

Yep, I checked to make sure it was hooked to the correct vacuum port on the carb.

 

[cumminsb]

Fellro - does the larger engine have a PVS to advance the timing to cool the engine if hot like the 351?
Bucks77Ford - is it a CA emission?

 

[Fellro]

Actually, I believe the vacuum switch was more to prevent advance when cold. I don't think it was size specific, but more model year for emissions specific. You can't really cool an engine by changing timing, you can only retard timing to prevent pinging.

The vac switch is more of an emissions thing really, an engine will perform just fine without it, just when cold will be less clean emissions, which you will have anyway really.

 

[Mil1ion]

The heat activated vacuum switch changes the vacuum according to how warm the coolant heats the switch to alter vacuum.

I would check the vacuum advance to see if the diaphragm is busted or seized.

Make sure it passes the WORKING test.