hi all, this concerns my 77 f150 with 400 engine ,if you care to check my old posts (there aint alot)you can see i have been trying to get this truck dialed in right for some time...

With a new distributor i was able to eliminate the ignition problem,but i still have erratic idle and acceleration,advancing timing from 12degrees to 20 helped out plenty but am sure is not a good thing.

what i did notice was when i plug the vacuum on distributor while timing engine,there seemed to be very little vacuum ,didnt matter if i gave it throttle or if was hot or cold there was very little vacuum(only tested with finger over hose)... So i removed one of the vacuum lines on top of aircleaner that has plenty of good vacuum ,hose was long enough to reach distributor, and WOW IDLE PICKED UP and engine seemed to smooth out alot.

ok,so i see that dizzy vac line heads into the thermostat with two other lines and following lines around i notice they connect to the same line that the aircleaner line does,....So how can i have good vacuum on one line and not the other,Is there some kind of vacuum switch on thermostat housing that controls vacuum to dizzy??? can it be bad??

I ran truck around town today and it performed very well with the hose from the aircleaner and as soon as i hook up the origonal hose (line that connects dizzy vacuum to thermostat housing spaggetti central) it starts to run like is on 6 cylinders

anyone shed some light on this for me...
thanks

 

The reason the idle picked up when you swapped the vacuum hoses was because the vacuum signal running to the air cleaner (for the warm-air pickup system) is sourced from manifold vacuum, which is high at idle. When you connected it to the distributor, the timing advanced by several degrees and the idle speed increased.

You probably have a three-port ported vacuum switch which swaps in manifold vacuum to the vacuum advance under one temperature condition, and ported vacuum under another. These valves are prone to failiure.

My advice would be to set the timing to the factory specification. Connect the vacuum hoses to the PVS per the correct vacuum diagram (you'll have to do some searching). Test the PVS and replace if necessary.

Based on your description, your PVS is most likely bad and not switching the vacuum signals properly. To simplify things, you could try setting the timing between 8 and 12 degrees, and then connect the distributor vacuum advance directly to ported vacuum on the carburetor (bypassing the bad PVS). Then dial in your timing as required. This would be the most basic setup.

 

To the OP.

Vacuum to the distributor is not needed until the Revs go up from idle,that's why the vacuum advance is either connected to a Ported nipple at the carb or to the VCV (Heat actuated) switch on the T-stat housing.

If you post more info about your truck I can direct you to the vacuum diagram you require.

 

if you read my post you will see even while revving engine there was very minmal vacuum at the line from dizzy to the PVS
dont know what else i can tell you bout the truck besides what i initially posted ...truck is 100% origonal unmolested condition. im certain carb is the 2150 two barrel

Maybe you could tell me where on the carb is the ported nipple, there are so many vacuum lines it blows my mind...