Engine is a 400, with DuraSparkII
I was replacing the Distributor for a friend because the VacAdvance on his didn't work anymore. We put the dist in. Set base timing with VacAdvance capped off at 12 BTDC. Hooked the VacAdvance back up and went for a test drive. The thing now bogs when taking off from a stop.
He then wanted to try and hook a Vac gauge up and set the timing to the highest vacuum. The highest Vacuum came in at 36 degrees BTC base timing. We took the truck test driving and it drove great. No hesitation taking off and to my surprise no pinging.

Since then he has confirmed TDC and 0 on the balancer, rebuilt the carb and checked for leaks.

Does anyone have any ideas about what is going on?

Thanks in Advance

 

Okay, I'll be the first to bite...

Are you hooking the advance to manifold or ported vacuum? There is a debate aboutwhich is correct. One will be high vacuum at idle and low when the throttle is open. The other is just the opposite. I never can remember the terminology for the two, sorry. One, below the throttle butterfly, and the other is above. Switching the vacuum line between the two makes a big difference during acceleration. In one case the engine will fall on its face, while the other will make it launch. It is an easy test to move the hose and observe the results.

Hopefully someone more literate than I will expand on this.

There is a pretty good video on YouTube made by Uncle Tony of Uncle Tony's Garage that shows just what the distributor does in each case.

 

Vac advance has ZERO impact on acceleration Mech advance is what controls that.. Uncle Tony's video is out to lunch even he was hesitant thinking about it.
Also Ported and Manifold is the EXACT same thing, EXCEPT at idle, Ported is used to prevent surging/stumbling as at idle engine loads changes from things like AC compressors and the Alt.

The Bog from a stop in the OP's post may not be from the Dist but could be a carb problem that has manifested it;s self. Also not sure why the dist was replaced in the first. It is almost guaranteed the replacement is not curved the same as the original was. The ONLY reason to replace a Dist is if the bushings are worn so bad the armature is flopping about.

 

Optimal timing is based on rpm and and load. More specifically fuel burn rate which is affected by richness of the mixture, piston size, and piston velocity. Centrifugal advance only account for the piston aspects. Vacuum augments centrifugal by retarding when the mixture is rich and flame front is fast. Under light loads, with the butterfly relatively closed, mixture is lean, flame front is slow, and more advance is required. Wide open, mixture is rich, and spark should be retarded. In both cases, rpm by itself cannot make the full adjustment.

The op's issue may well be related to the carb, or something else entirely. The test I suggested is easy and costs nothing. That's the biggest reason I suggested it.

 

Oh brother you got some bad info from somewhere. The fuel-air ratio does not change with the throttle position short of WOT with a pump shot.
Under light loads, the vacuum is high cause the throttle is not open very much as less fuel AND less air is required to make the necessary power. The air-fuel ratio does NOT change from high load open throttle situations to light load closed throttle situations. Cylinder filling or mixture density DOES change from light load to full load but the fuel air mixture does not.

Vac advance is there strictly for maximizing fuel efficiency under light load and cruise conditions nothing else.

Under cruise and light load conditions when manifold vacuum is high cylinder filling decreases, even though the swept cylinder volume is constant. Under high manifold vacuum, there is less air and less fuel sucked into the cylinder, the mixture ratio stays the same, but there is just less of it in the same size space (the cylinder). As such the fuel-air mix is less dense (spread apart) this requires more time for the flame front to achieve a complete burn as the molecules are more spread out. Having a vac advance allows/gives the extra time needed for the less dense fuel-air mix to achieve a complete burn maximizing the work potential from combustion, thus increasing efficiency.

Also vacuum advance does NOT retard it ONLY advances in conventional single diaphragm dists. There are dual-diaphragm vac advance units that do retard but they generally are for emissions or speciality applications.

 

Thanks for the input and discussion. It took me a while to figure Vac Advance out when I got into it. It is a little counterintuitive, to me at least.
Speaking off.

The problem he has is that he has hesitation off idle. He can also put in a lot of timing without detonation.
Could this be caused by a mix that is just too lean? We didn't have time to tinker around with the carb anymore so I didn't touch it. Thinking back, I should have probably done something.

What I am thinking now is that maybe the Dist that was in it was set up to bandaid the problem that is now haunting the truck. Now that a new one is in there the bandaid is gone and the problem is apparent again.

From what I understand the thing should run just fine without the Vac advance even hooked up.

 

If he has a hesitation off idle chances are very high that is a fuel delivery related (carb). Either a lean hesitation or over rich bog. Over rich is usually pretty easy to spot as you get lots of black smoke out the exhaust once the engine picks up. The most common cause of an off idle hesitation is not enough of a pump shot from the accelerator pump to bridge the gap from the idle circuit to the main circuit in the carb when the throttle is opened.
Check to see if the hesitation is there with very very slow throttle application. If it disappears under a very slow throttle application then it is most probably an accelerator pump issue either the accelerator pump diaphragm is done for or the accelerator pump needs adjustment also potentially is an accelerator pump check valve problem.

And yes it engine should run just fine with the Vac advance disconnected. Actually, you would be hard-pressed to tell if it was disconnected, all you would notice is a little less power under very light acceleration at cruise. That would be the extent of the drivability differences with it disconnected, naturally you will also see a drop in the cruise fuel economy bit that is not really a drivability issue.

On a side note make sure the vac advance is connected to ported vacuum. Manifold can cause an off idle hesitation in some setups.

 

He is using ported. The thing is that the carb has been completely rebuilt. I am thinking that it might be related to a mistake made in the rebuilt, but I kinda want to rule everything else out before telling him to tear it apart again.
The hesitation only happens from idle to go. If he is rolling and then stepping on it it is completely gone.

Might have to look into the accelerator pump adjustment, just to double check.

 

My 2 cents: I fully agree with the above statement, but I'd hate to see somebody misinterpret what you've said. If the vacuum advance was disconnected (and supply line plugged) you wouldn't really notice any change in driveability.

But several years down the road, somebody vaguely remembers what you said. Only instead of thinking "if disconnected", they're thinking "if failed". And so your statement unfortunately gets twisted into something different and not always true. If the failure was the vacuum advance actuator jammed in the retracted position, you'd see no difference in driveability. But the most common failure is a vacuum leak preventing the system from working, and leaning out the fuel mixture at the same time. You'd definitely notice the problem then, but not because the vacuum advance wasn't responding.

Not trying to disagree or split hairs here. Just trying to ward off problems down the road when somebody doesn't remember exactly what was said.

My experience with a failed vacuum advance here:

https://www.ford-trucks.com/forums/1...e-failure.html