Carb vacuum help
Thread Starter
|
New User
Joined: Mar 2015
Posts: 23
Likes: 0
You have two lines to the vacuum advance? Interesting.
I've heard of that setup, but never seen it. From what I've heard, one of them only comes on when you fire up the truck and actually retards the timing (the lower the timing, the easier it is to start). This allows you to run more advance.
I've heard of that setup, but never seen it. From what I've heard, one of them only comes on when you fire up the truck and actually retards the timing (the lower the timing, the easier it is to start). This allows you to run more advance.
Moderator
Joined: Dec 2007
Posts: 7,986
Likes: 104
From: Boise, Idaho
Speaking of which, I just read this on a post over on FSB when someone asked about the ported vacuum switch (the 4 port thing on the thermostat).
Explains what the PVS does, and why you have two lines running to your distributor.
I still have my ported vacuum switch just sitting there on mine, I wonder if I could rig that back up.
The PVS valve is a temperature sensing valve found on the distributor vacuum advance line, and is installed in the coolant outlet elbow. During prolonged periods of idle, or any other situation which causes engine operating temperatures to be higher than normal, the valve, which under normal conditions simply connects the vacuum advance diaphragm to its vacuum source within the carburetor, closes the normal source vacuum port and engages an alternate source vacuum port. This alternate source is from the intake manifold which, under idle conditions, maintains a high vacuum. This increase in vacuum supply to the distributor diaphragm advances the timing, increasing the idle speed. The increase in idle speed causes a directly proportional increase in the operation of the cooling system. When the engine has cooled sufficiently, the vacuum supply is returned to its normal source, the carburetor.
I still have my ported vacuum switch just sitting there on mine, I wonder if I could rig that back up.
Thread Starter
|
New User
Joined: Mar 2015
Posts: 23
Likes: 0
Speaking of which, I just read this on a post over on FSB when someone asked about the ported vacuum switch (the 4 port thing on the thermostat).
Explains what the PVS does, and why you have two lines running to your distributor.
I still have my ported vacuum switch just sitting there on mine, I wonder if I could rig that back up.
Explains what the PVS does, and why you have two lines running to your distributor.
I still have my ported vacuum switch just sitting there on mine, I wonder if I could rig that back up.
Moderator
Joined: Dec 2007
Posts: 7,986
Likes: 104
From: Boise, Idaho
Ah, true. I hadn't looked back at your vacuum diagram before posting that.
I think that the V8s, from what I remember from working on my brother's 351, has more than one of those ported vacuum switches. They're usually 3 prong too.
I think that the V8s, from what I remember from working on my brother's 351, has more than one of those ported vacuum switches. They're usually 3 prong too.
Thread Starter
|
New User
Joined: Mar 2015
Posts: 23
Likes: 0
Ended up throwing a newly rebuilt carb from National Carbs on it. Made a huge difference, the stalling issue is gone. Unfortunately, now that I have a fully functioning carb, my other weaknesses are starting to show. Getting dead spots on partial throttle, likely due to the disconnected/faulty EGR valve. Have a new one en route, should be here Monday. Also replacing the cap, rotor, wires and plugs while I'm at it. Going to convert to a GM HEI ign module and TFI coil for good measure. All vacuum lines have been replaced and tested for leaks with the carb cleaner trick. I was having some off idle stumbling and dieseling, likely due to the EGR (in part) but this morning I removed the ported vacuum line to the distro and instead running straight manifold vacuum without check valve. Holy cow, this thing likes the extra timing, had to play with the base to find a happy spot without surging or pinging. When the EGR goes back in I'll need to start all over...but that's ok. This truck is easy as pie to work on if you understand the basics.
Moderator
Joined: Dec 2007
Posts: 7,986
Likes: 104
From: Boise, Idaho
Awesome on the carburetor. That has to feel good.
Sounds like you're really getting that thing running tip top.
And yes, it's all one big congruent system, so as you fix some things, the weaknesses of other parts will become more apparent.
Keep us posted on the HEI, I'll be curious how that goes.
Sounds like you're really getting that thing running tip top.
And yes, it's all one big congruent system, so as you fix some things, the weaknesses of other parts will become more apparent.
Keep us posted on the HEI, I'll be curious how that goes.
Thread
Thread Starter
Forum
Replies
Last Post
annaleigh
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
11
May 15, 2015 05:24 AM
mmmmoore24
1973 - 1979 F-100 & Larger F-Series Trucks
6
Jun 25, 2014 02:57 PM
grey77
1973 - 1979 F-100 & Larger F-Series Trucks
29
Apr 7, 2006 02:15 PM
