Distributor vacuum advance

[PineClone]

'78 B210 auto...

 

Have been trying to narrow down my VERY poor fuel economy issue. Had a local mechanic spend a little time on it and he discovered that the vacuum advance on the distributor is not working correctly. 

 

He called the local foreign auto parts guy who said they no longer manufacture these...and then said that getting a new (rebuilt) distributor is the next option.

 

A couple questions for the experts:

 

1. I see replacement vacuum advance available online at Ebay and other parts stores. What am i missing?

 

2. What would be the effect of the vacuum advance not working properly? I suspect there are more issues than this.

 

Thanks!

[CWhip]

Rockauto still has them available.  Under ignition, Distributor vacuum advance.

[datzenmike]

Some background. There are three types of advance, static... set with a timing light and remains constant, mechanical... increases and decreases in direct response to engine RPM, and vacuum... increases and decreases in direct  response to manifold vacuum or another way to think of it... engine load. Vacuum advance on our Datsuns is ported from the carb and is designed to have NO vacuum at idle but anything above idle there is. So looking for vacuum at idle is a waste of time.

 

To get the most power out of the explosion of gas and air, the hot expanding gasses must reach peak pressure about 17 degrees (give or take) after top dead center to push down on the descending piston. Two things screw this up. As the engine speeds up it has less time to burn and hit this sweet spot so it has to be ignited sooner. Mechanical advance will take care of this as it it RPM dependent. The other is part throttle. A heavy throttle with a full cylinder of gas and air take less time to burn than a part throttle nearly empty cylinder. Vacuum advance is load dependent and will take care of any part throttle advance.

 

So if you drive at full throttle all the time the mechanical advance will tailor the advance to get the most out of your gas. But if you drive in town or light throttle cruise on the highway your cylinders are only partly filled and need more time to burn (advance) Without vacuum advance they are being lit way too late with the hot expanding gasses chasing a fast descending piston and not applying full force.You are loosing efficiency and will have to give it more throttle to make up for the loss.

 

The above is what happens without vacuum advance. If it is stuck part way it is adding unwanted advance at full throttle and may ping.

 

Take the distributor cap off and the vacuum hose for the distributor off the carb. Suck on the end and you should see the rotor turn clockwise about 1/2 to an inch? Hold tongue over end to hold the vacuum. The rotor should stay advanced until you release it. If it doesn't, try a new hose. All else fails get an E Bay new vacuum advance can.