Ported vacuum source for distributer

[Eric VanBuren]

Even within the 2 basic models of carb Bill posted there are variations. The most common 2300, the last one Bill posted, and what should be stock on a 71 has it's ported vacuum source on the side of the metering body. It is that threaded 90 degree adapter on the left side.

Regarding the TVS vehicles that were equipped with factory AC did have the high temp switch. No single port will "hold" vacuum. When the engine is running hot it switches the connection to the vac advance canister from receiving ported vacuum to receiving manifold vacuum. That increases the timing which causes the idle speed and thus fan speed to increase and assist in cooling down the engine. Rigs that had air injection also have a TVS and it's purpose is to prevent the diverter valve from injecting air into the exhaust until the engine is warmed up. There is also another type of TVS on pre EGR rigs it is to control the road speed controlled vacuum advance. It connects to a "T" in the line leading from the ported vac source to the vac advance. Until the engine warms up it stays closed, once it is up to operating temp it opens and connects that circuit to either the solenoid on the firewall which is connected to an electrical speed sensor in the speedo cable or a mechanical speed sensor in the speedo cable. Both open the vac line to atmosphere via a filter so that there is no vacuum advance above aprox 43MPH. That in effect retards the timing to reduce emissions. When cold the vacuum advance functions normally to preserve driveablity during warm up.

[Infinite Monkeys]

I had considered the possibility of the throttle plates not closing and had checked it. It felt and sounded like they were closing properly, however I could be wrong. The return spring does have enough tension.
I have the bottom most carb (2300, correct?) and upon closer inspection I found a capped off port......This must be the ported vacuum fitting that has been closed off.
I will look for a replacement and connect through there. I did not know what it was for when I was rebuilding the carb, so I closed it off as I had found it. I think this will do it!!!
This leads me to the question: Why would someone close it off and why do some vehicles run OK with manifold vacuum to dist. It seems to me that this method is bypassing the original intent of a vacuum advance dist.

Wrong picture......Disregard

[Infinite Monkeys]

Right picture.

Also,
When I removed the fitting, a few drop of fuel came out. Is this OK.
Thanks again

[Infinite Monkeys]

"There is also another type of TVS on pre EGR rigs it is to control the road speed controlled vacuum advance"

What color is this and where does it go. I have vacuum lines coming from a canasiter on the drivers side firewall. It also has the speedo cable going through it. The vacuum lines are going to full manifold vacuum on the back of the manifold where the thranny modulator gets it's vacuum. this probably is not right?

Sorry about all the posts. Questions always come to me after.

[Jay Tabor]

Originally Posted by Infinite Monkeys

I cannot adjust the idle down or mixture with the dist vacuum connected. With it connected, the engine will run with the idle and mixture screws all the way closed. Still at about 950 RPM. I have checked for vacuum leaks, but I must be doing something wrong because everywhere I sprayed the carb cleaner, the engine increased RPM. There can't be that many leaks, or it wouldn't run. Or it really does have a ton of leaks.

I only have one fitting on the carb. It pulls vacuum all the time. Is it possible I used the a wrong gasket in the rebuild and this made it a manifold vacuum. Currently I do not have any emissions stuff, but that does not mean it did not come with it. I think a '71 did not have any.

you really need to start all over,,
perhaps have a reliable carb rebuilder work on your carb.
you state: '''''I have checked for vacuum leaks, but I must be doing something wrong because everywhere I sprayed the carb cleaner, the engine increased RPM. There can't be that many leaks, or it wouldn't run. Or it really does have a ton of leaks. '''

you also state you ''can turn the idle mix screws both closed and the engine still runs''
that should be your first clue your rebuild/install isnt any good .
so once you have the carb leaks fixed, then we will continue with dist vacuum advance issue.

[Eric VanBuren]

Yes that port that is plugged on the side of the carb is your ported vacuum advance source. It is hard to say how fuel got in there it may or may not be cause for concern. Why would someone remove and plug it. Likely because they listened to some self proclaimed expert telling the lie that ported vacuum advance was done for "emissions" and if it is for emissions it must be bad.

From the factory the rigs with the mechanical road speed controlled vac advance were plumbed as follows. Port on carb to "T", one leg of "T" to vac advance can, other leg of "T" to 2 port TVS, 2nd port on TVS to valve in speedo cable, other port on valve in cable to nipple on the underside of air cleaner at the back of the carb that is inside the filter. This did show up with emissions and was done to "retard" the timing above that 43mph mark. Actually it prevented the vac advance from functioning it didn't really retard the timing. This was/is an emissions control strategy that did cause performance issues namely reduced fuel economy, a common complaint about the early emissions controlled vehicles.

Other mfgs often used a "high gear retard" where there was a switch either mechanical on MT or operated off the hydraulics that engaged high gear on ATs. That signaled a solenoid to open and vent the vacuum advance. On later rigs IH used a solenoid but it was controlled by an electric device in the speedo cable and the vent was just a port on the solenoid capped by a piece of foam to act as a filter, instead of a nipple leading to the filtered portion of the air cleaner.

[Infinite Monkeys]

I will hook through the ported fitting and start over on the tune up process. It is entirely possible I used the wrong gasket in the rebuild. If things are not correct after the new port, I will put in the other gasket I have left over and see what happens. I did notice some small cracks in the carb base after I had cleaned it, they did not go all the way through, so I figured I’ll risk it and see what happens. Looks like this is what happens. At least the correct port issue is resolved.
I will follow up with what I find.
Thanks again.

[Infinite Monkeys]

I think I should title this... How to fix your truck for $1.60.
'cause that's how much the fitting cost to run ported vacuum to the dizzy.
Right now I have the idle at 800 RPM and the timing at about 10 BTDC. It's running a 1000 times better and my kids no longer need to hold their noses from the gas fumes when they come outside as the truck is running.

I think I will still get a Holley rebuild kit and make sure the gaskets are right. It's the metering body gasket that I think I may have gotten wrong. I had two options and I took a chance on one or the other. The directions did not specify on or the other. The one I got right now in is an Autozone....something. I am not expecting miracles from a 40 year old carb. I do know there are cracks in the base plate. But for now it's good enough.


Thanks again.

[Eric VanBuren]

Don't waste your money on an entire Holley rebuild kit that won't have the correct gaskets for that carb anyway. Pickup the pack with 2ea metering body and bowl gaskets, and maybe fuel bowl screw gaskets.

[Infinite Monkeys]

Does Holley have this gasket set? Also, I've seen an option for a thick base gasket. Is that much better to use?
Thanks