freeway drivability help
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 39
From: EXTREME southwest CT
The orifice can't be changed, there are three different vapor separators available.
.045, .060 & .090 orifice. Color coded with a white, blue or red dot.
I forget which is which without checking.
Yes there is a screen inside, and it can get sludged up.
There are threads here on the forum where people have cut these open and posted photos.
.045, .060 & .090 orifice. Color coded with a white, blue or red dot.
I forget which is which without checking.
Yes there is a screen inside, and it can get sludged up.
There are threads here on the forum where people have cut these open and posted photos.
Lead Driver
Joined: Jul 2009
Posts: 6,477
Likes: 19
From: Sheffield, AL
I highly suggest 2 items.....faulty/ leaking vacuum advance unit, easy to check AND the 3 way fuel filter next to the carb. I've mentioned this several times already, rust from the old fuel tanks will collect in it and restrict the flow resulting in the surging described, been there done that with both of these. Also vacuum leaks are suspected.
Moderator
Joined: Jan 2001
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From: Virginia
I don't know what the problem is, but I think we can figure what the problem isn't. If I am reading correctly, when you mash the pedal and need the power, it moves on with no problems correct? If that is correct, I do not think you have any sort of fuel delivery problem.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
I don't know what the problem is, but I think we can figure what the problem isn't. If I am reading correctly, when you mash the pedal and need the power, it moves on with no problems correct? If that is correct, I do not think you have any sort of fuel delivery problem.
Senior User
Joined: Nov 2000
Posts: 285
Likes: 1
From: sunny san diego
If im not mistaken there is no filter in the "three way" set up at the carb. Its a bypass to keep cool fuel moving to the carb when running. There is however a filter in the carb itself much like a Quadrajet which I just replaced, the old one was spotless clean with no signs of rust flakes or otherwise. From what read about the "hot fuel option" system there is a changeable orifice in the bypass that could become plugged which logically would up the pressure to the carb itself as well as potentially create boiling fuel as it stops flowing when heated up working hard.
Senior User
Joined: Nov 2000
Posts: 285
Likes: 1
From: sunny san diego
I think beetle is on to something with the faulty/leaking vacuum advance unit. If the diaphragm in the cannister on the distributor has a small hole, it would not likely show up at idle [unless you are using manifold vacuum] and would not show up under power, but would allow the timing to vary when at a steady cruise. Do as Franklin2 suggested and remove the vacuum line from the advance unit and temporarily plug it. If the surging/missing drivability issue goes away, then you will have narrowed it down significantly.
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Laughing Gas
Joined: Feb 2014
Posts: 815
Likes: 296
From: KY
I disconnected the cruise vac line to eliminate that one and half way to work did the vac advance disco. It seemed worst. I left it running when I pulled over to rehook the advance and when I plugged it in the idle ran up a bit so Im getting manifold vac at idle. I believe everything is still OE line wise with the thermos switches etc. I also later stuck my mighty vac on the advancer and it held vac no problem. As I mentioned on my way home I had to pick up our travel trailer. 20 miles home with a several mile long grade, down shifted into 2nd or nailed in 3rd and it never skipped a beat. Up on the flats and you feel nothing. Im pretty convinced I have a lean surge issue now. How it recently passed a Ca smog check is beyond me though.
Fleet Mechanic
Joined: Jul 2011
Posts: 1,916
Likes: 4
I'm just curious. Why are you getting manifold vacuum on your vacuum advance? Was it set like that on your emissions sticker under the hood or is that personal preference?
I much prefer ported vacuum.
If you're running manifold vacuum advance, I'd hope that whoever set your timing had unhooked the vacuum line before they set the initial timing. If the vacuum line is hooked up with manifold vacuum when checking initial timing, it can skew your results in a rather unpleasant way by advancing the timing to higher than what it really is.
I'd start by properly checking your timing with a light AND a vacuum gauge, then moving on to check your idle mixture with a vacuum gauge, then re-setting your idle speeds.
From there, it should be pretty obvious to go over all the vacuum lines that you've got and start plugging them off one by one to see which one causes an issue and which one doesn't. Use the process of elimination.
Just my suggestions.
I much prefer ported vacuum.
If you're running manifold vacuum advance, I'd hope that whoever set your timing had unhooked the vacuum line before they set the initial timing. If the vacuum line is hooked up with manifold vacuum when checking initial timing, it can skew your results in a rather unpleasant way by advancing the timing to higher than what it really is.
I'd start by properly checking your timing with a light AND a vacuum gauge, then moving on to check your idle mixture with a vacuum gauge, then re-setting your idle speeds.
From there, it should be pretty obvious to go over all the vacuum lines that you've got and start plugging them off one by one to see which one causes an issue and which one doesn't. Use the process of elimination.
Just my suggestions.
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From: Virginia
Some of the 460's had some weird complicated vacuum system giving the vacuum advance vacuum at different times. I would not be surprised if his problem is one of those temperature gizmos not working correctly, but it would take a steep learning curve and a lot of studying to figure if it was all working or not. It's a rat's nest of vacuum lines on those things.
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Laughing Gas
Joined: Feb 2014
Posts: 815
Likes: 296
From: KY
Roger that one 
dont forget the thermo switches and solenoids too. Never could understand what seems like 40 different Calibration schematics used covering the 302 thru 460. If you underhood cal code sticker becomes illegible you could be screwed as you'd never know what you have. There is a very in depth write up by a engineer for GM performance regarding the pros and cons of utilizing either source of vac. Pretty interesting stuff.
dont forget the thermo switches and solenoids too. Never could understand what seems like 40 different Calibration schematics used covering the 302 thru 460. If you underhood cal code sticker becomes illegible you could be screwed as you'd never know what you have. There is a very in depth write up by a engineer for GM performance regarding the pros and cons of utilizing either source of vac. Pretty interesting stuff.
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 39
From: EXTREME southwest CT
We have all read that article...
Seems Ford liked to use (restricted) manifold vacuum to the advance unit.
I think Gary Lewis figured out there are actually well over 100 different calibration codes for these trucks.
Seems Ford liked to use (restricted) manifold vacuum to the advance unit.
I think Gary Lewis figured out there are actually well over 100 different calibration codes for these trucks.
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Joined: Jan 2001
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From: Virginia
You said pulling off the vacuum line made it run worse? How about this; We could go with the theory that the advance unit is not getting the proper vacuum sometimes, possibly from one of those gizmos under the hood.
IF you could figure it out, what would happen if you hooked the dist advance unit directly to the ported vacuum source on the carb? You may have to get one of these other guys who know more about it to direct you to the correct port, and make sure you label everything so you can put it back, but I wonder how it would run like that?
This would be another "getting the engine to talk to me" experiments. If it actually would run better, then you could start examining what little thermo part or whatever could possibly not be closing, opening, or whatever.
IF you could figure it out, what would happen if you hooked the dist advance unit directly to the ported vacuum source on the carb? You may have to get one of these other guys who know more about it to direct you to the correct port, and make sure you label everything so you can put it back, but I wonder how it would run like that?
This would be another "getting the engine to talk to me" experiments. If it actually would run better, then you could start examining what little thermo part or whatever could possibly not be closing, opening, or whatever.
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