Symptoms of a Vacuum Advance Failure?
 

Greetings All,

My '84 F250 351W (2bbl) suddenly started running funny yesterday. It developed a nasty off-idle stumble that would sometimes lead to bucking. Overall, power was down, too. Acceleration was much slower than normal. At higher speeds, the engine seemed to run smoothly enough, but I had trouble with moderate grades that previously were no problem. I should mention the truck was fully loaded with a slide-in camper and pulling our Jeep, but normally the truck had plenty of power for that. We covered only ten miles and decided to turn around. For all I know, it had been like this for a while but wasn't noticeable while running empty.

While troubleshooting today, I found a severe vacuum leak at the vacuum advance actuator on the side of the distributor. No movement of the actuator with my MityVac connected, and I could not build up any vacuum. Would such a leak cause these symptoms? I plan to replace the actuator, but it's not easy to find. I had always been under the impression that a loss of vacuum advance was not always noticeable, and would mostly affect fuel economy and emissions. Perhaps the vacuum leak itself was bad enough to be the root cause, and not necessarily the lack of vacuum advance.

Any thoughts? I've got some unhappy campers waiting for me to get the truck back on the road, but the timeframe isn't looking good.

FWIW, I've also ran a compression test, changed plugs, wires, cap, and rotor, but have not tested done a test run yet. Fuel pressure at the carb inlet was good the entire time.

You are correct, the advance missing would only cause economy problems. But you are also correct, lots of air entering a vacuum line or faulty component can cause other problems because of the too lean mixture at idle. Vacuum leaks are not as noticeable at higher speeds because there is a lot more air coming into the engine, but has more affect during idle when there is little air going into the engine.

Vacuum advance adds quite a lot of ignition timing for just this reason. Mechanical advance is strictly RPM based and consequently provides nowhere near enough lead for many driving situations and weight/gearing combinations, vacuum advance was the only way (before computers) to get enough ignition advance on top of the mechanical advance, it isn't just for fuel economy, it improves overall driveability. Engine will run quite a bit cooler as well. If you drive around with a vacuum gauge and observe it you'll get a better sense of this. The vacuum is constantly moving up down very fast in typical city driving. That's why it "improves" fuel economy, the engine will always get the best possible economy with an optimal timing advance. Basically this means as much timing as it will stand at all times.

I would check to make sure the mechanical advance is operating correctly as well, though. Could be problems with both. Vacuum leaks don't help anything either.

We will have to disagree on the above. If you are accelerating, there is little vacuum so there is not much if any vacuum advance. Same with pulling a load, when the pedal is pressed down and there is a pull on the engine, there is little to no vacuum, so the vacuum advance has little affect when the going gets tough on cooling. Part throttle cruising is the vacuum advance's claim to fame.

I think this is where theory gets shot down by real life.

Connect a vacuum gauge and plumb it in to the cabin and just watch, the manifold vacuum and thus the vacuum advance is constantly moving to every input of the accelerator, pulling in and taking out advance based on engine load. The advance plate gets a real workout!

The engine vacuum does drop instantly on acceleration, but notice that it rebounds back in just a split second too. It is true it won't rebound to the vacuum level of steady level ground cruising when pulling a hill, but as the load levels off so does the vacuum to some degree. And degrees count when it comes to timing. That's why it's so important to use vacuum advance in a street driven motor. There is simply not enough ignition advance available from an RPM based mechanism for street driving.

Update: A new vacuum advance actuator seems to have done the trick. Went for a 30 minute test drive and power/drivability was back to normal. Phew! Glad I looked at it.

Of course the fridge decided to act up. Had to bypass the 12v auto shutoff feature. I will operate it manually for this trip and worry about the auto feature when we return.

Glad you got that sorted! FWIW when I replaced my dizzy the brand new vac advance lasted maybe 1,000 miles before it failed. I replaced it with the 30 year old stock one which still works. They don’t make them like they used to.

Might be be interesting to disconnect the vac advance and cap the line just to see how much of your trouble was lack of vac advance and how much as vac leak.

Agreed. The actuator that sprung the leak was five years old. It was part of a NAPA reman distributor. I had saved the original distributor and my first thought was to rob that actuator, but lo and behold it had a leak too, just not as severe.

For the life of me, I can't remember why I changed the original distributor five years ago. I think one of the centrifugal springs had broken and replacement was easier than sending the old one out for repair and calibration.

To replace the actuator yesterday, I had to get a complete distributor from Carquest and rob it. Nobody had just the actuator, and Carquest was the only place within 30 miles.that had a distributor in stock. This is new manufacture, probably all the way from China, so I will look into finding a spare actuator when I get back.

I wondered the same thing. Not going to mess with it at the moment, though, as the truck is running great again. Not going to poke a sleeping bear. But I favor the vacuum leak itself as the primary fault.

FWIW, my 1948 Willys CJ-2A never had vacuum advance, only centrifugal. Runs great like that, although I realize the centrifugal profile was probably different to compensate.