Driving a 348 3x2 - vacuum secondary carbs, manual shift

Lt.FrankDrebin

Well Known Member
As I'm in the process of going from mechanical progressive linkage back to vacuum-operated secondary carbs, I'm wondering how this car is going to drive. From what I understand, there is a delay in the closing of the secondary carbs after letting off the throttle. My car is a stick shift, so is that delay going to be unfavorable when shifting gears?
 

Don Jacks

Well Seasoned Member
Supporting Member 3
This was only a issue when at full throttle,so the answer would be not if you shift fast enough.I belive that there were some adjustments that could be made to eliminate,or at least reduce this tendancy.
 

Fathead Racing

Well Known Member
Supporting Member 7
I've experienced the vacuum open secondaries 348 three two four speed combo. I can't remember any issues when operating as the General intended.
 

real61ss

Well Known Member
Supporting Member 8
I've experienced the vacuum open secondaries 348 three two four speed combo. I can't remember any issues when operating as the General intended.
I agree, a few years ago I had a 58 Impala that had a 280 horse 348 with the vacuum linkage and a 4 speed (it was originaly a 3 speed) and I don't remember any issues with the carbs closing. The problem that I had was getting them to open, I had to reduce the spring tension on the throttle shafts to get them to open because the vacuum pull wasn't stronger enough to pull them open.
 

Fathead Racing

Well Known Member
Supporting Member 7
I agree, a few years ago I had a 58 Impala that had a 280 horse 348 with the vacuum linkage and a 4 speed (it was originaly a 3 speed) and I don't remember any issues with the carbs closing. The problem that I had was getting them to open, I had to reduce the spring tension on the throttle shafts to get them to open because the vacuum pull wasn't stronger enough to pull them open.
That's the reason almost everyone I knew switched over to progressive linkage. When street racing light to light the vacuum pot would just be opening by the time they would be getting on the brakes hard for the next red light.
 

rstreet

Well Known Member
Supporting Member 17
Back in the day a guy around here had a late year1961 FH 348/350 (I have verified the block) that had vacuum secondary system and he would only run from a standstill to full quarter. Never was really beaten but his 4:11 rear sort of really helped!!
Robert
 

La Hot Rods

Well Seasoned Member
Supporting Member 15
Sounds like it should work like a vacuum 4 bbl if set up right.
The flywheel and pressure plate is going to carry the RPM anyway when you let off.
 

dakota tom

Well Known Member
Supporting Member 2
I have a 1963 389 4sp Pontiac Catalina with the factory vacuum tri-power. End carbs are open or shut, not progressive in operation. They shut down right away but take about a second to open up. I just power shifted it. Found the dash to be bloody knuckle hard when the pin in the Borg Warner shifter would break on the 2-3 shift.
I too had problems with the carbs opening after I rebuilt the carbs. I had set the linkage between the end carbs to open together. I found the one end carb has a slot in the arm where the linkage hooks. Readjusted it so that the one carb opened slightly before the other. The vacuum pot could then open the carbs up.
 

poison ivy

Well Known Member
as i recall, 1958, 348, 280, stick, the tri-power stock, vacuum, worked best for me, when properly lubed, and adjusted, without binding.. bogged when vacuum dropped. and a gear really helped.. i used a 4:56 and it REALLY helped..
 
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Lt.FrankDrebin

Well Known Member
Sorry to bring this post back up, but just today, I got the car on the road. I'm happy with the activation of the secondary carbs, but when I let off the throttle, it still feels like it's floored for another 2 seconds. Not acceptable for me, especially when you can't coordinate the pedals for shifting. Two things I've noticed: the little bit of play between the carb throttle lever and the accelerator pump lever seems to matter a lot in the operation of the vacuum slider valve and also, I have the late '58-'61 check valve/vacuum tank setup; not the earlier '58 vacuum pump setup. I was operating the secondary carbs via the slider switch with the engine off and only 7" of vacuum and the carbs opened and closed within 1 second. Now that I have the vacuum leak fixed, I have 20" of vacuum. Could this be too much vacuum, requiring more time for the vacuum to bleed down? Maybe I need some kind of pressure regulator after the hose T and before the slider switch?
 

Lt.FrankDrebin

Well Known Member
Well, I varied the vacuum between 7" and 20" and end carb shutdown was the same. Maybe the end carbs need to be deactivated sooner. It seems like a bad design to have those carbs deactivated when the center carb returns to idle. So now I'm thinking of rigging up a spring to keep the slider valve in the closed position only unless the center carb lever pushes it open, at ~70% full throttle, and then it returns closed at below ~70% throttle. I'm thinking that it takes longer for the secondary carbs to close while driving is because there is vacuum between the carburetor and the air cleaner, which is where the secondary carb vacuum system is pulling air from.
 

Lt.FrankDrebin

Well Known Member
I looked into this issue a little further by taking the vacuum slider switch apart to see if I can make any adjustment in there. Turns out that it had been oiled or greased at some point, but now is sticky enough to cause drag to prevent the spring from returning the valve to the "off" position, without requiring the help of the accelerator pump lever. I cleaned it up thoroughly with lacquer thinner, blew everthing out with air, and then "colored" all contacting surfaces with graphite. Operates real nice and smooth now and the switch returns to the "off" position on its own; not when center carb goes to idle position.
 
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