'61 348 3x2BC Vacuum Secondaries Tank

ThumperNZ

Well Known Member
Seems like we're missing the vacuum tank for the '61. Can someone tell me where it was positioned and a pic of what it looked like?
Also, how does the vacuum system work, we have the centre carb vacuum valve, correct 3x2BC carbs, etc. The vacuum hose that runs from the back of the center carb seems to be missing a check valve, and now I see mentioned on other threads, a divorced vacuum tank.
As far as I can tell, the front carb vacuum can pulls the secondaries open, but does this happen when the vacuum collapses at 60% throttle? Or is there a spring in the vacuum can that is held under tension by vacuum which is then released by vacuum decrease when 60% throttle is achieved.
 

Fathead Racing

Well Known Member
Supporting Member 7
On the 60 it is positioned behind the driver side inner fender wheel house. Tucked up between the inner and outer fender. I think there is a pocket stamped in the inner fender. I'm going to look at my assembly manual. I myself have seen only one of these tanks in my lifetime of working on three two systems, I regret not pulling it from the junk yard car a 60 Chevy when I saw it. I have never seen one in use and believe they were only used on the high lift solid cams.
 
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ThumperNZ

Well Known Member
I just got back from the chassis dyno where we tried a vacuum tank from a '57 TredleVac and a 3/8" check valve hooked up to manifold vacuum. The secondary system works with this setup, but not without it, so it looks like there is definitely a vacuum tank fitted, logically as you say between the firewall and inner fender on the drivers side.
 

Lt.FrankDrebin

Well Known Member
This is what happens, and it actually depends on having a healthy vacuum supply: You need to have a check valve at the rear of the center carburetor. With the vacuum tank (same as the power brake can), once vacuum gets in there, it stays in there via the check valve. When you floor it, the slider valve allows that reserved vacuum to pull and hold the secondary carbs open. Now, you need the fitting (#7 on the above diagram) (3/8" hose to 1/4" hose) just after the check valve to splice in the hose from the valve-to-tank to the slider valve. The original is VERY hard to find, but I have a repro in stainless steel if you need one. I just converted my 3x2 system back over to the vacuum system and it works beautifully. I had an issue at first where the secondary carbs held open for a second or two after completely letting off the throttle, but I have since remedied that.
 
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Grape

Active Member
Is the check valve and vacuum reservoir always needed with vacuum secondary tri power with no power brakes? I get conflicting answers from different people. The manual clearly shows the check valve and tank with tri power. Mine seems to work fine without those parts. I did plumb in a Dorman check valve and vacuum tank but it made no difference that I could tell. It just hissed loudly for a few seconds after shutting off the engine. Oh, there is an original check valve at ebay right now for $159. It has the extra hole for an auto trans vacuum modulator.
 

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La Hot Rods

Well Seasoned Member
Supporting Member 15
If you are hearing a hissing sound when you shut the engine off then it sounds like your check valve is not working.
 

Lt.FrankDrebin

Well Known Member
The first 348 3x2 system did not use a check valve and vacuum reservoir, instead, it was a vacuum pump-boosted system. This was found on early '58s. Late '58 thru '61 used the check valve with the vacuum reservoir. I don't know how well the 3x2 system will work without those two components. Either there won't be enough vacuum initially because vacuum drops off that far into the throttle valve opening and also, there won't be enough vacuum to sustain the opened secondaries. So either your secondary carbs won't open or will open slower than they should, or, they'll close before you want them to. Without the reservoir and valve, there just isn't a "deep" enough vacuum supply to properly operate the system.
 

ThumperNZ

Well Known Member
From my observations on the chassis dyno, there is no way the secondaries (front & rear carbs) would open with only a check valve when we tested, obviously the volume of the vacuum tank has a lot to do with it because once a vacuum tank was connected, no problem. Snapped shut as well when the throttle was closed. That's the function of the vacuum valve on the center carb.
 

Grape

Active Member
From my observations on the chassis dyno, there is no way the secondaries (front & rear carbs) would open with only a check valve when we tested, obviously the volume of the vacuum tank has a lot to do with it because once a vacuum tank was connected, no problem. Snapped shut as well when the throttle was closed. That's the function of the vacuum valve on the center carb.
Good to know. I don't think it is possible to get the secondaries to open with the hood open and no load on the engine. I found the correct vacuum tank. I will keep hunting for the correct check valve.
 

Grape

Active Member
The first 348 3x2 system did not use a check valve and vacuum reservoir, instead, it was a vacuum pump-boosted system. This was found on early '58s. Late '58 thru '61 used the check valve with the vacuum reservoir. I don't know how well the 3x2 system will work without those two components. Either there won't be enough vacuum initially because vacuum drops off that far into the throttle valve opening and also, there won't be enough vacuum to sustain the opened secondaries. So either your secondary carbs won't open or will open slower than they should, or, they'll close before you want them to. Without the reservoir and valve, there just isn't a "deep" enough vacuum supply to properly operate the system.
Excellent. Thanks for that response. I'm interested in purchasing the repro stainless steel vacuum tee. I'll send you a message.
 

Lt.FrankDrebin

Well Known Member
With a good functioning check valve and a vacuum reservoir, you can operate the secondary carbs with the engine off. As a matter of fact, I was able to operate my secondaries about 5 times with the engine off before the vacuum supply finally gave out. But let's not confuse the valves. The check valve at the rear of the center keeps a vacuum supply in the revervoir after vacuum gets in there. The slider valve chooses between the vacuum supply and venting to the atmosphere for the vacuum actuator pod.
 

ThumperNZ

Well Known Member
pvs409 (Paul) was good enough to supply a vacuum tank which I attempted to fit today. To get it behind the LH hood hinge it appears that the hinge will need to be removed temporarily. The Assembly Manual in the Brake Section has a page that shows the bolting dimension, which references from an existing dimple on the fender skirt. I've checked and can't find the dimple, so now it seems that its a place the tank and mark the holes location unless Pauls friends '61 Convertible 348 3x2BC can be measured.
 

neilswheels

Well Known Member
This doesn't sound good. I'm planning on running 3x2's on my 348 in my 40 ford, but thought that a progressive linkage would be all I need, do I need to sort out a vacuum can, pipes, valve as well? From reading the above, it appears that the vacuum opens the secondaries, is there no progressive linkage on standard cars, or have I misunderstood?
 

Don Jacks

Well Seasoned Member
Supporting Member 3
The cars that have been mentioned were set up that way from the factory,Neil. On your project,go with the original plan of using progressive mechanical linkage and leave this troublesome vacuum actuated stuff to those who are willing to spend the insane amounts of money necessary to find overpriced,difficult to find parts just to make their cars "Factory Correct".Such behavior is necessary for a "show car",but both unnecessary.and unwanted on a street car with an engine that has been modified with aftermarket parts,such as cams,improved heads,stroker kits,ect.Most of the time,back in the day,when this vacuum junk failed,they were coverted to mechanical linkage anyway.The last years that Pontiac had the tri-power option[65 and 66] came from the factory come thru with mechanical linkage.
 

Lt.FrankDrebin

Well Known Member
My car is a driver and I was so happy to get rid of the progressive linkage and go back to vacuum. It just took too much pedal pressure to get the secondary carbs to come in with progressive linkage. If you go with progressive linkage, you won't need any of the vacuum stuff. The 348 3x2 system came from the factory with vacuum linkage, not progressive linkage.
 

Don Jacks

Well Seasoned Member
Supporting Member 3
If you mean the ports that supplied the vacuum switches,diaphrams,yes.I just didn't want you to think that you had to dig up this stuff.It's difficult and expensive to find here,and you live in England for cry'in out loud.Add in shipping,taxes,duties,ect.,and it becomes expensive to the point of being stupid,imo.
 
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