Question 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

Reply to
goodnigh
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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.

Reply to
.boB

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.

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.

The timing is another issue as it seems really off if using a timing light But, the cars runs great.

mike

Reply to
goodnigh

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

Reply to
goodnigh

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.

Reply to
Ashton Crusher

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