Bad vacuum advance?

[JohnnyP]

I have a '76 280z. It starts up and runs, but I've been having a problem I can't narrow down. Wen the car is running and I press the accelerator, it lags for just a quick second. By lag, I mean, when the gas pedal is pressed, the engine looses a little power and slows down, than resumes as normal. The RPMs go down a little, and you can hear it in the way the engine runs, and feel it. This takes place in about 1-2 seconds. Also, at the same time, but randomly, there is a sort of a low popping sound. But it isn't very loud, it doesn't sound like backfiring. Bad gasket somewhere?

This is what I've done to try and diagnose the problem:

Oil change (Castrol 10W-30) new oil/air/fuel filter

New fuel pump (it was bad anyways)

New air/fuel hoses under the hood

New NGK plugs, new wires, cap and rotor

Correct timing per '76 FSM specs.

Followed the EFI Troubleshooting in the '76 FSM (through the 35 pin connector)

I tested the vacuum advance by removing the hose to the vacuum advance (…not the end connected to it) and pulled air through, but could not see the plate move. Do these symptoms fight for a bad vacuum advance?

Anything to be offered is always very appreciated. Thanks everyone!

John

Edited September 18, 2010 by JohnnyP

[EScanlon]

If you sucked (or got fancy and used a vacuum pump) on the hose leading to the vacuum advance pot and the points plate did NOT rotate at all, then YES, it is a good indication that the vac adv plate is shot. It rides below the points plate and will /can lose the ball bearings that allow it to move freely.

Replace it and then test.

FWIW

E

[Walter Moore]

The part is called a "breaker plate" I think. If you buy a "Vacuum advance" you will the the diaphragm housing that bolts to the side of the distributor.

But you may be chasing a different problem. A lot of older cars have broken or defective vacuum advances and work reasonably well. (Racers tend to disconnect them entirely) The Vacuum advance only actually does anything at part throttle, like running in a high gear at a constant speed then coming to a slight upgrade. The vacuum advance kicks in then to boost the spark timing to get just a little extra horse power without having to down shift or stand on the gas.

At anything approaching full throttle (or anytime that you put your foot into it for that matter) the Manifold vacuum drops off to the point where the VA doesn't work.

[Gary in NJ]

The symptoms you describe are also indicative of a lean condition. A vacuum leak is a possibility and that would also explain the poor performance of the vacuum advance.

[doradox]

The Vacuum advance only actually does anything at part throttle, like running in a high gear at a constant speed then coming to a slight upgrade. The vacuum advance kicks in then to boost the spark timing to get just a little extra horse power without having to down shift or stand on the gas.

At anything approaching full throttle (or anytime that you put your foot into it for that matter) the Manifold vacuum drops off to the point where the VA doesn't work.

Walter,

Running at cruising speed on a level road the vacuum advance should be all in, or close to it. Coming to a grade the vacuum advance retards, or advances less if you prefer, the timing so the engine doesn't death rattle all the way up the hill. Same thing for when you stand on it. The vacuum advance increases the efficiency of your engine under low load conditions such as cruising by allowing maximum spark advance to be used.

At any given RPM

More throttle = less vacuum.

Less vacuum = less advance.

Therefore: more throttle = less advance

So going up your hypothetical upgrade to maintain the same RPM, and therefore speed, you need a little more throttle so....

Racers don't use vacuum advance because they don't cruise, it doesn't add power, and it's one more thing to destabilize the timing.

Steve

[Walter Moore]

Either way, the vacuum advance is a part throttle effect. It doesn't have anything to do with his problem.

[JohnnyP]

Thanks for all the info guys. Ok so I'm getting that its not a vacuum advance issue. What else could be causing it this problem?

John

[Gary in NJ]

Have you ruled-out my suggestion in #4 above. Do the easy things before looking for hard things.

[doradox]

This..

http://www.xenonz31.com/s130/files/280zfuelinjectionbook.pdf

Is handy if you don't already have one.

Steve

[FastWoman]

About the vacuum advance: If the breaker plate is frozen, it could be frozen in almost any position, and it could even decide to shift around and stick in different positions from time to time. That might make it difficult to know the engine's timing at any given moment, unless you're aiming a timing light at the crank pulley at the time.

I agree with others that it probably wouldn't cause your hesitation, but it's still a problem. FAIW, AutoZone has lifetime warranted rebuilt 280Z distributors for about $100. I'm told the rebuild can sometimes be a bit ham-fisted, though. Check to see that there isn't any wobble in the shaft. I figure the part is a pretty good deal because (1) it's very easy to replace and (2) Datsun's breaker plate mechanism is so prone to failure. So far my rebuild is working just fine.