'61 348 3x2BC Vacuum Secondaries Tank
- Thread starter ThumperNZ
- Start date
Grape
Active Member
The carbs aren't modified except for a new check valve as opposed to the original one and aftermarket air cleaners. This is how the carbs came out of the factory boxes. This has been a drag car since the early 60s. The original owner purchased a ton of parts in the 60s that he included when he sold the car in 2013 including a brand new tri power setup. The secondaries work with the factory vacuum tank but not long enough to get down the quarter mile. Vacuum bleeds off through the slide valve too quickly. Yes it's dumb to try to make this work for drag racing but I can't help but try. I can easily hide another vacuum tank under the dash or in the trunk and tee the hose so I am also using the factory vac tank. There are plenty of tri power cars with mechanical linkage and for good reason. I just like to be different.
wristpin
Well Known Member
Perhaps there is a ruptured diaphram on carbs?
Grape
Active Member
Yes, I just used a new brass check valve. Image attached. It holds vacuum in the tank for a long time after the engine is shut off. With all the vacuum in the big air tank I rigged up, it is easy to hear and feel where all the vacuum is bleeding off when the throttle is held wide open with engine off. It is leaking right out the side of the slide valve on the side of the center carb where the 2 screws hold it on to the mounting plate. With the big air tank for a vacuum source, It takes about 40 seconds for the vacuum to bleed off and secondaries to close. I have no idea if it is normal for the slide valve to leak vacuum or not.
The slide valve should not leak.
But I would hope you could run quicker than a 40 second pass.
But I would hope you could run quicker than a 40 second pass.
Grape
Active Member
I will take the slide valve and see what is going on with that. I hope to only need 12.5-13 seconds once this is working properly.
Grape
Active Member
Sorry I couldn't get any closer to the back of the center carb with my phone to show the check valve. That part of the system is doing its job.
Grape
Active Member
I reluctantly disassembled the slide valve on the tri power and now understand how and why it leaks vacuum and there is no easy way around it. I block sanded the valve because there was a high spot on it but that made no difference.
The sliding "block" mechanism cannot be held tight enough to the face of the valve by the flat spring. I failed to take an image of this flat spring before reassembling the slide valve.
I did attach an image of the sliding "block" mechanism and how it has a nice gap where it leaks vacuum with the throttle wide open. A different type of flat spring could help this block stay tight to the slide valve. Outside of making a spring that functions properly, the only solution for drag racing with this original vacuum setup is a bigger vacuum tank than the original one but probably not as big as my air tank. I really don't want to risk breaking the tabs off the slide valve cover by taking it back apart so I will be hiding a bigger vacuum tank somewhere and hope to be competitive against James the next time I see him!
The sliding "block" mechanism cannot be held tight enough to the face of the valve by the flat spring. I failed to take an image of this flat spring before reassembling the slide valve.
I did attach an image of the sliding "block" mechanism and how it has a nice gap where it leaks vacuum with the throttle wide open. A different type of flat spring could help this block stay tight to the slide valve. Outside of making a spring that functions properly, the only solution for drag racing with this original vacuum setup is a bigger vacuum tank than the original one but probably not as big as my air tank. I really don't want to risk breaking the tabs off the slide valve cover by taking it back apart so I will be hiding a bigger vacuum tank somewhere and hope to be competitive against James the next time I see him!
Any chance you could seal the outside of that valve.
You better put high test in that tank if you want a better look at the tail lights
Grape
Active Member
There really is no way to seal the slide valve.
There is a pin at the bottom of the slide valve with strong spring pressure. This pin is pushing upward on the sliding block at full throttle so the secondaries can close when you back off the throttle. The little flat spring that holds the sliding block against the slide valve can't overcome the spring pressure from the bottom and the gap is created for vacuum to escape. It is a really poor design for performance. I just need to find or fabricate a big vacuum tank to fit under the dash and I'm all set for wide open throttle for as long as I need it.
Grape
Active Member
You may be right but I will get a better view than I did when I was racing with a single 2 barrel.You better put high test in that tank if you want a better look at the tail lights
How about some thin hard plastic.....I am thinking this stuff that I have seen in late model cars around electric connection in door panels....not sure what is but is thin and slick and most times white in color.
If you could make a gasket with this stuff to fit the housing that the slide rubs against if you get my drift.
It would slide easy and help it seal
If you could make a gasket with this stuff to fit the housing that the slide rubs against if you get my drift.
It would slide easy and help it seal
I just ran that one on two two barrels
Yeah Right,,,James!!!!!!!I just ran that one on two two barrels
