Vacuum Advance or Timing Issue

[c3po]

I went over a very bumpy road the other night, and the next day the El Camino was running rough, never going on this road until they get it fixed. So, the next day it is running rough like there is a vacuum leak. I get it home and I tighten down the carb bolts and check everything else, header bolts, change all of the vacuum caps and it is still running rough, so I plug the vaccum advance at the distributor and now the car is running smooth but if I go to pass someone the power is down a little bit, could the timing be off, I do not think there is a problem with the distributor because it is running smooth, could those 7 miles of bumpy road have knocked my timing off, this is a small block chevy with a Davis Unified Distributor that is 3 years old, the spark plug wires are only 2 years old, and the spark plugs were changed a year ago, could the diaphram in the vaccum advance on the side of the distributor be bad, why is it running smoother with the vacuum advance plugged, before with the vacuum advance on it was making a popping sound from the exhaust and the engine was shaking like a leaf, what could have happened going over that stupid bummpy road, I plan on talking to my mechanic buddy at chevrolet tommorow. The only other thing I can check is the accelerator pump on the carb, I disconnected the choke and there was no difference, could my timing be off from going over that crazy road.

 

[tom slick]

have you checked the timing? what is base idle timing? what is max advance? does the vacuum advance hold vacuum when you test it?

 

[c3po]

I have not checked the timing, that is going to be my 1st thing to check.

 

[mechtech2]

The vacuum advance could have a leak internally.
There is a rubber diaphram that often ruptures.

 

[c3po]

If this rubber diaphram ruptures or has a small leak can this cause driveability problems, and how long can I drive with the vaccum advance plugged, and if there is a tear do I have to buy a new vacuum advance, this is kind of new too me as before this distributor was put on I had a computer controlled distributor that did not have a vacuum advance.

 

[tom slick]

vacuum advance is for fuel economy and operates when driving at a constant speed. when you are at idle or accelerating it usually isn't functioning (depends on the system).

The easiest way to check to see if the diaphragm is bad is to hook a hose to it and suck on the end of the tube, if it is good it will hold the suction, if it is bad it will not.

these links might help
http://www.classicchevytrucks.20m.com/vacuum.htm
http://www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
http://www.tr6web.com/Documents/tr6/vacuumadvance.html
http://www.highperformancepontiac.com/tech/vacuum_advance_tech/index.html

 

[c3po]

I checked the diaphragm and it is bad, also when I came home tonite I took a temp reading on all of my header tubes, and the header tube on the passenger side front was down 100 degress from the header tube on the opposite side, could this be because the vaccum advance is not hooked up and the engine is lazy, it idles fine, starts fine, does not stall, I do not think this could be a dead cylinder. I ordered another vacuum advance piece as well as a cap and rotor. I also noticed that the coolant gauge was down a little on temperature, any thoughts would be appreciated.

 

[mechtech2]

Running with retarded timing may have fouled a plug, and thus the dead cylinder. Also, mixture distribution is a problem with carburetors.
But you need that vacuum advance for cruising economy and a cleaner engine. It is a huge hit MPG wise without it.

 

[c3po]

I am driving my Marauder today, and I guess I will drive the car on Friday when I get the other parts, I have noticed the gas mileage is terrible.