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From: Buffalo
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This all started after I timed and changed the dist. vacuum source from full manifold to timed vacuum. I had to do this though because it was driving like crap on full manifold vacuum.
(5.0 L by the way). It starts just fine in the morning. Couple pumps, basically fires right up. After this initial start though, if the truck sits for more than 5 minutes, it cranks excessively and sometimes slow before it finally fires. Is this timing, fuel pressure (or lack thereof), something I'm missing? It all seemed to start after timing the engine so I'm wondering if I need to advance it a bit?
(5.0 L by the way). It starts just fine in the morning. Couple pumps, basically fires right up. After this initial start though, if the truck sits for more than 5 minutes, it cranks excessively and sometimes slow before it finally fires. Is this timing, fuel pressure (or lack thereof), something I'm missing? It all seemed to start after timing the engine so I'm wondering if I need to advance it a bit?
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
Well let's back up a bit.
The vacuum advance is normally connected where, as from the factory where? As a general rule that's where it should be, at least from a troubleshooting standpoint.
The vacuum advance is normally connected where, as from the factory where? As a general rule that's where it should be, at least from a troubleshooting standpoint.
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From: Buffalo
From what I can tell, it goes to the carburetor, so I'm assuming it is a timed port. But I'm really not sure where it's supposed to be, I can't seem to find a definite answer. I suppose there's manifold source at the carb as well. Vacuum diagrams suck in this subject.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
What I'm getting at, is either method of vacuum advance will work, but it should not be necessary to switch to one or the other port simply because it starts running poorly, something else is going on or it would have run poorly prior to that.
You could check to see if the advance diaphragm is good. That might cause some problems in the form of a vacuum leak. It may also be that the initial timing is a bit retarded since no longer using manifold vacuum.
For the most part though, the problem is not dependent on what vacuum source is being used.
You could check to see if the advance diaphragm is good. That might cause some problems in the form of a vacuum leak. It may also be that the initial timing is a bit retarded since no longer using manifold vacuum.
For the most part though, the problem is not dependent on what vacuum source is being used.
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From: Buffalo
I have checked the advance diaphragm by sucking on the vacuum line. No leaks, moves as it should. I'm happy with the way the engine is running, just not the way it's starting. I may just try advancing it a little and see what happens.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
Absolutely. But you initially changed it because it "started running crappy".
Find out why it started running like crap, and then switch it back. See where I'm goin' with that? What was the cause of initial fault?
If you CAN get a motor to run well on manifold vacuum, then that is preferred. Mostly though, this will not play well with vehicles that have automatic transmission, AC, and other vacuum actuated gee gaws.
Find out why it started running like crap, and then switch it back. See where I'm goin' with that? What was the cause of initial fault?
If you CAN get a motor to run well on manifold vacuum, then that is preferred. Mostly though, this will not play well with vehicles that have automatic transmission, AC, and other vacuum actuated gee gaws.
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Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
The vacuum comes right back up as the RPMs or speed levels off. However, it is load dependent ONLY. RPM gots nothing to do with it.
If you're steady cruising down the highway at 65 mph, the vacuum will be the same whether hooked up to manifold or ported. Hook up a vacuum gauge and see.
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From: Buffalo
Yeah I get what you're saying. It was right around the time when I realized 4 spark plugs were loose and the cap and rotor were pretty much shot (previous owner, not me) So maybe having done the tuneup corrected the problem. I'll revert back to manifold vacuum. Looks like that's stock anyways.
Pardon my ignorance, but why is manifold vacuum better than timed vacuum?
Pardon my ignorance, but why is manifold vacuum better than timed vacuum?
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
As you noted it has more advance at idle than otherwise would be the case. This keeps engine temperatures down, and smoother idle. Some automatic engine controls will switch to manifold vacuum in the event the engine overheats for this reason. But, certain emissions are increased by engine advance so timing is retarded back and then also cleaned up by catalysts. Screwy in some ways but that's what we got.
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From: Virginia
There were many different ways they hooked up the dist vacuum, they had many different configurations.
If you have a completely original setup, hook it up like it's supposed to be. If half of it's missing, I would hook it up to timed vacuum port(no vacuum at idle) if it was a automatic. You can go either way on with a manual transmission, but i think I still would run it with timed vacuum. When I let out on the clutch I want the engine to stay steady, not drop in rpms dramatically. That's what you are going to notice between the two different setups, with manifold vacuum you will have a very high idle that you have to try and turn down, and a engine that will want to stall once it's put in drive with a auto tranny.
If you have a completely original setup, hook it up like it's supposed to be. If half of it's missing, I would hook it up to timed vacuum port(no vacuum at idle) if it was a automatic. You can go either way on with a manual transmission, but i think I still would run it with timed vacuum. When I let out on the clutch I want the engine to stay steady, not drop in rpms dramatically. That's what you are going to notice between the two different setups, with manifold vacuum you will have a very high idle that you have to try and turn down, and a engine that will want to stall once it's put in drive with a auto tranny.
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