My 75 supercab 460 with stock ignition is having issues with the vacuum adcance. The engine runs a bit rough with slight acceleration and slower speeds with the advance hooked up, but when it's capped off the engine seems to run much better. I have been just driving it with the advance capped off. Timing has been checked. Is my vac advance just messed up?

 

You could have a leak in the canister resulting in a vac leak when connected.

I assume you set timing with vac disconnected?

 

Yep, timing set when vac disconnected.

 

I had similar problem and it was the canister. If you're connected to manifold vac (below throttle plate) and not ported vac (above throttle plate) then you only have vac advance at idle and light load. If you have a handpump vac & gage (like a mighty vac), you could pull a vac on it and see if it holds.

 

As stated above, you can pull vacuum on the unit and see if it holds; If it does, it's a good unit but likely needs some adjustment. The adjustment probably wouldnt cause the problems youre talking about, though. The adjustments on your vacuum advance tends to be drivability adjustments under acceleration, not idle.

Has it always done this?

Unlike above, however, if you have it connected to manifold vacuum, you'll have vacuum any time the engine can provide vacuum, which is almost everything below full throttle...at least to varying degrees.

If youre hooked to ported vacuum, which you should be, you'll have no vacuum at a properly set idle; anything above idle and you'll have the same vacuum as manifold vacuum.

Having the advance hooked to manifold vacuum, with a stock adjustment, will cause the timing to be too advanced throughout the vacuum curve and could be your issue.

 

Something else to consider and check is how much advance are you getting out of the vacuum advance if it is working. Most Ford advance units can be adjusted with an allen wrench. It is inside the nipple where the hose connects. You need a timing light that you can measure timing advance with or put the distributor on a distrubutor machine to run it and I have not seen one of those that worked in a good 25 years or so. Good rule of thumb on a stock engine is a total of 40-45 degrees of advance at around 3000 rpm or so. That would be initial, centrifucal, and vacuum combined.