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Car Forum / Ford / Ford Mustang / October 2006Question vacuum advance
The distributor has two vacuum lines connected to the advance mechanism on my '71 351C 4V. Why two lines and do I plug both for timing purposes? mike > The distributor has two vacuum lines connected to the advance > mechanism on my '71 351C 4V. > Why two lines and do I plug both for timing purposes? > > mike It's a smog thing. One of them is connected to a temp/vacuum switch located on intake manifold. It's open when cold, closed when hot. Lets see if I can remember this. The front port is full vacuum advance, connected to ported vacuum. The rear one is connected to the above mentioned switch. When the engine is cold, the two vacuum signals cancel each other out, and you get no vacuum advance. As the engine warms up, the switch slowly closes and you get full ported vacuum advance. Unless the thermal switch is fairly new, I would disconnect it or replace it. You must block both lines to set base timing.  Signature.boB Arrived: 2006 FXDI, Red. 1997 HD FXDWG - Turbocharged Stolen 11/26/05 in Denver 1HD1GEL10VY3200010 CO License J5822Z 2001 Dodge Dakota QC 5.9/4x4/3.92 1966 Mustang Coupe - Daily Driver 1965 FFR Cobra - 427W EFI, Damn Fast. >> The distributor has two vacuum lines connected to the advance >> mechanism on my '71 351C 4V. [quoted text clipped - 10 lines] > advance. As the engine warms up, the switch slowly closes and you get > full ported vacuum advance. The port nearest the distributor is connected to manifold vacuum and has a T-connect going to the TVS. The distributor port at right angle to the distributor and farthest away is connected to the TVS. The TVS has three ports. (1)One connected to the closest distributor port through a T-connect and to manifold vacuum. (2) The second connected to the distributor's farthest port (right angle to distributor). (3) The third TVS port is connected to the base of the carb. It does not make any sense. > Unless the thermal switch is fairly new, I would disconnect it or > replace it. Don't know the condition or age of the TVS. But if it is ok to plug the distributor port nearest to the distributor and connect the right angle port directly to vacuum, so be it. > You must block both lines to set base timing. The timing is another issue as it seems really off if using a timing light But, the cars runs great. mike >>> The distributor has two vacuum lines connected to the advance >>> mechanism on my '71 351C 4V. [quoted text clipped - 33 lines] > >mike My memory is that when I had a 69 Firebird the purpose of the valve was to provide an bump in "baseline" advance whenever the engine was on the verge of overheating. That caused an increase in idle speed to better turn the fan.. The normal setting was better for emissions but worse for efficent running. >> The distributor has two vacuum lines connected to the advance >> mechanism on my '71 351C 4V. [quoted text clipped - 13 lines] > replace it. > You must block both lines to set base timing. I checked for vacuum at the port furthest from the distributor, the line connected to the TVS which is mounted on the block when the car was cold and there was vacuum. Checked it when engine was hot and there was no vacuum. Checked the vacuum at the port nearest the distributor which is connected directly to the intake manifold. Vacuum runs at 17 inches at 700 RPM which I think is about right for a cam. Pump the throttle and the vacuum drops to near zero then to 21 inches and back to 17. Sounds about right. mike |
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