[Jack Gifford]

This new forum seems the ideal place to start a brainstorming session on linkage.

Like many others, I quit using vacuum linkage when I still had my first Tri-Power (in 1959). But mechanical linkage isn't the ideal solution for a street vehicle, since it limits center carb driving to about half-throttle. I would REALLY like to have almost full-throttle available on the single 2-bbl, as it was with the vacuum deal- especially in my GMC, which only needs the end carbs for a genuine race scenario. But vacuum isn't the answer, and I haven't come up with a scheme that looks promising. I thought about a solenoid (driven by a microswitch and relay), but the solenoid would need to be huge (I measured the force to start the end throttles moving off tightly-closed). Not only would it be physically large, but would draw a lot of current and would SLAM the end carbs against the full-throttle stops.

Let's kick some ideas around...

[tom s]

Jack,just remove the arm to the end carbs,put it on for the match races!Tom

[Tom Vaught]

Originally Posted by tom s

Jack,just remove the arm to the end carbs,put it on for the match races!Tom

x2 on removing the rear linkage rod. On the 64 Tube and Plunger linkage it would snap right off the ball studs.

I added a Moon fuel block and separate fuel lines to each carb which allowed me to add fuel shut off valves to the front and rear carbs.

The last thing I did was find some tire inter-tube material that would lightly slip over the "screens" of the tri-power and close off the air to the front and rear carbs. When the foams went over the screens you never knew the carbs were not getting air.

I learned this deal the hard way from a California hot rod transplant (who took a job as the sheriff in our town). It was either remove the carbs (after a EXCESSIVE speeding ticket) or block them off so that the engine was running like a 2 BBL engine.

But the reality was 10 minutes and the Tripower was ready to go again.

Tom V.

[tom s]

Tom,we both know the engineer in him wont allow that!LOL,Tom

[Tom Vaught]

Originally Posted by tom s

Tom,we both know the engineer in him wont allow that!LOL,Tom

Yes, Tom, I hear you. Maybe he will come up with a totally new way to get it done.

Jack is a smart guy.

If it were a Holley you could still keep the fuel in the end carbs because the Holley 2 BBLs have a small "bowl purge circuit" built into them to keeps the bowl fuel from going sour from lack of use.

Tom V.

[Jack Gifford]

Tom & Tom- I should have clarified that a "race scenario" might occur at any moment during my "normal" driving of the truck; I don't want to give up that "instant option"! I'd merely like to get a little more functionality in the 2-bbl mode, where I do 99% of my driving.

It seems that this particular Tri-Power (had it since 1961) may be somewhat unusual, in that I've never had a problem leaving the end carbs "dormant" but fully functional- no fuel leakage and no perceptible air leakage. The only fault with it (if you can even call it that) is the need to always let the end carbs "snap" shut by lifting quickly, as opposed to slowly getting off the throttle- otherwise some air leakage usually occurs at the throttle blades. A linkage system like I'm hoping to conceive would free me to use heavier return springs on the end carbs to alleviate the positive closing deal (can't use stiff springs with my mechanical linkage, as it would cause twisting of the center carb shaft- my linkage is on the passenger side).

I need to bring myself up-to-date on stepper motors- I suspect this may be a good application for them.

Or... try to design a varying-ratio linkage between a solenoid and the end carbs- which would provide a high force initially, then taper off to just enough to hold them fully open, along with a way to reduce solenoid current while they're open??

[Jim Doran]

@Jack, the solenoid idea should not be used to actually open the outside carbs, but rather it would close a pin that would enable the linkage to drive the end carbs. This way only shear force on the solenoid pin and no need for large current draw. I can imagine the linkage having some sort of slip fit design that the pin closes into a slot or hole and then the linkage is effectively solid and pushes in a linear fashion BAU.

[Tom Vaught]

If you just used a stock Vacuum Diaphragm set-up to open the end carbs and a vacuum reservoir along with a Modern air solenoid valve you could get the control you wanted and also calibrate when the rear carbs opened and closed. And you could use your heavier stock shaft springs.

Tom V.

[Jack Gifford]

Originally Posted by Jack Gifford

... I don't want to give up that "instant option"! I'd merely like to get a little more functionality in the 2-bbl mode, where I do 99% of my driving...

Update: I've come up with a scheme that may work. It's partially fabricated- awaiting a couple of parts. It's almost purely mechanical- a link from center carb to end carb which can effectively be two different lengths, to either operate the end carbs or not (not easy to describe, and no drawings were made). The only non-mechanical aspect will be a small solenoid to hold the mechanism in 2bbl-only mode normally, with a dash switch to select 3-carb mode. The only operating restriction is the requirement of throttle being at idle when switching to 3-carb mode. Wish me luck....

[Dick Boneske]

I've often thought how the availability of a little more center carb throttle would improve drivability of my Tripowers. Many of us know how lousy the vacuum linkage performed- by-allowing only wide open or completely closed end carbs, having no way to clutch off with all carbs open due to lack of vacuum, experiencing the thrill of the delay in the end carbs closing, etc.

A compromise to allowing the center carb to open fully without the end carbs is to change the geometry so the end carbs don't open 'til the center carb is at about 3/4 fully open. The can be done by raising the stud (for either the rod & tube or slotted rod) on the center carb bracket or lowering the stud on the rear carb throttle arm.

Most of our setups already are configured with the center carb stud in the top hole and the rear carb stud in the bottom hole. To further increase center carb opening, another hole needs to be drilled to move the stud to the locations described above. As you can see on the attached pictures of the '64, '65, & '66 mechanical linkage, there is no room for a "higher hole" in the center brackets, but the rear lever could be drilled closer to the throttle shaft to achieve the desired increase in center carb opening before the end carbs engage.

I'm going to try this on one of my GTO's and will report results. But, as has already been stated, simply removing the slotted rod or rod & tube will give you all the throttle you want with the center carb.

[Jack Gifford]

Dick- going past 1:2 ratio (center-to-end-carbs) would be asking too much of the small diameter center throttle shaft. With decent return spring rate on the end carbs, there's already some twist of that shaft with the normal 1:2 ratio.. Which is why I'm trying to come up with a scheme to select the "single carb" mode without opening the hood.

[Tom Vaught]

Jack, Holley Carb Engineers did a nice job with their 340 and 440 "6-Pack" vacuum carbs set-ups. The systems work by signaling the vacuum pods on the end carbs with "Venturi signal" (same as a 4 bbl Holley carb does).

So if you used a simple "normally open" BLEED CIRCUIT to the end carbs, the engine would only see 2 BBL operation anytime the engine was running. (The 6 pack carbs use external signal lines to the vacuum pods on the End Carbs so easy to add a vacuum control switch/solenoid to that vacuum circuit).

So when you wanted the end carbs to work like a "6 pack carb" set-up you would CLOSE
the bleed circuit (using a switchable power circuit) and now the carb set-up would act like a normal 340-440 "6-Pack" vacuum carb installation. The internal vacuum bleeds that remove vacuum from the pods (after "Light Speed Mode") would need no recalibration. So all of the parts could be stock Holley parts.

Tom V.

You could use a stock Vacuum Carb Tri-Power set-up with the same mods but they use no venturi signal to the end carb pod. So you would need the factory vacuum can for that and it would be more of a on-off switch deal vs the Holley venturi signal set-up.

[Jack Gifford]

Tom- ever since putting my first Tri-Power together (1959) I've been leery of using vacuum actuation. Pontiac hadn't figured a way to instantly "dump" the air from around the diaphragm, to allow the end carbs to "snap" closed (not surprising, as it's not easily done). First of all it's unnerving when trying to pull back in after passing a car, and the end carbs stay open for almost a full second. Secondly, "snapping" the end carbs closed is the best way to have them reliably seal vacuum at the throttle blades.

[Dick Boneske]

The vacuum setup doesn't take a second to close the end carbs, but it seems like A LOT more than a second.

I'll never forget when I put J2 on my '57 Olds and tried it out for the first time. I waited 'til there was about a block of space between me and the car in front of me. I punched it, felt & heard the end carbs open, backed off, and thought for sure I was going to hit the guy in front of me. All turned out OK, but that experience that will be in my mind forever!

[Tom Vaught]

You have a "Diaphragm Can" and a steel vacuum line with several inches of "space" that has to be filled with air again before the "Can" will completely close. Holley carbs with their "Built-in Vacuum Diaphragm" attached to the carb uses a "Bleed Circuit" into the secondary air horn area to allow air to replace the "vacuum signal" much quicker. Therefore the secondary 4-bbls barrel carbs close almost immediately.

The stock Vacuum Secondary Pontiac/Olds/etc system was poor at best in design.
Almost unsafe.

Tom V.

[Jack Gifford]

Originally Posted by Jim Doran

... I can imagine the linkage having some sort of slip fit design that the pin closes into a slot or hole and then the linkage is effectively solid and pushes in a linear fashion...

That's basically what I've done (re: my 5/28 post). But I changed my mind and decided to keep it 100% mechanical, with a choke cable into the cab to enable/disable 3-carb operation. Took it out for a spin- I like it! Although throttle must be at idle to switch modes, that's easy enough to do without stopping or even slowing down much. I've finally got the Tri-Power setup I've always wanted!

Side note: this arrangement is especially useful to me since the 2-bbl mode is so impressive in this truck. The whole package just seems to have "come together" great- the engine's torque curve, HydroStick's gearing, 3.08 posi rear, etc. Anybody want to challenge me to a single-2 bbl race?

[Jim Doran]

Thanks for the update. Who knows, maybe your stealth ( outside carbs on demand ) will be something the super stealthy FAST guys take up.

Curious, given you an only switch modes at idle, what situations would cause you to switch from single to tri or vica versa ?

[pfilean]

with a choke cable into the cab to enable/disable 3-carb operation

Pictures - pictures - pictures ?

Sounds like an interesting solution. How did you do it? When you engage the end carbs I presume they are still progressive.

[Jack Gifford]

Originally Posted by Jim Doran

... what situations would cause you to switch from single to tri...

Well... since racing on public highways is not legal I guess that only leaves an occasional run at the nostalgia drags.

If I can get a photo that would help explain it, I'll post it.

[Jack Gifford]

Not great photos- I'll try explaining.
(Center carb held at just shy of half-throttle for the photos)
The slotted link to the rear carb has an arm pivoted just ahead of the slot (arm is on the far side and hard to see- it's black). The choke cable's spring hooks into the top of the arm. In the first photo the arm is held up to not engage the rear carb arm stud (2-bbl mode). In the second photo the arm is lowered to engage the stud (Tri-Power mode). The rear of the arm has a curved face to match the round shoulder of the stud, to assure it doesn''t slip off.