He folks,

I have a 67 big block, original drive train, original six pack carbs. At about 4,500 rpms in 2nd or 3rd gear, I can feel the additional fuel surge, but I can't tell if both the front and the rear carbs are opening. Seems like years ago, the surge was more abrupt. I am getting slight wetness in the gasket under the front carb so I know the front carb is actuating. All the vacuum hose connections appear to be good, although the hoses are probably 20 years old they are supple and not showing signs of cracks. I also know there has to be real operating load on the engine to generate sufficient vacuum to open the front and rear carbs, so I am assuming there is not way to rev the engine with the car sitting sitting still to test the vacuum actuators and carbs. Is there any way to test to determine if the rear carb is actually opening and pushing fuel into the block? I'd rather not get into opening up the vacuum actuators and fussing with diaphragms and spring adjustments, if possible. This one is above my pay grade.

Thanks

FCR

 

On the primary carburetor (middle) there should be a small hole in the passenger side venturi about midway down that you can bearly see, that's the vacuum pickup for the secondary carburetors. It allows low pressure (partial vacuum) that is created by the air rushing through the venturi to lift the secondary diaphragms.

With the engine off and choke open, if you open the throttle blades on that primary carburetor wide and take a air chuck and blow exactly past that small hole you can replicate that low pressure and you can watch the secondary blades open. You need to get that air right above the hole so it blows over it.

 

The first thing I would try is disconnecting the vacuum advance to see if the surge is still there. If the surge goes away the vacuum advance has too much advance for today's unleaded reformulated gasoline. both Lars and I offer a limiter for the vacuum advance that limits the vacuum based advance to 10degrees where the factory used 16 to 24 degrees of advance when lead was in gasoline

I hope this helps Henry @ olescarb

 

I guess I'm a bit embarrassed to ask, but could you describe how to disconnect the vacuum advance? Thanks much.
FCR

 

Disconnect the vacuum hose at the distributor. Plug it with a golf tee or something similar.

Patrick

 

If the surge problem is when you are operating at high load conditions (wide open throttle) you may also want to try loosing the gas cap to help vent the gas tank, we have seen a lot of surge problems caused by gas tank venting issues. there is an article on the issue at The Shop magazine in the March 2020 issue starting on page 60 https://nbm.uberflip.com/i/1206768-march-20/65?

Henry @ Oles carb

 

Hi FCR,

You could also test the front and rear carbs one at a time in order to isolate which one is opening or if both are opening. First disconnect the black vacuum line which attaches below and next to the vacuum pod on the front carb and then plug it with a golf tee. Then take the car out and drive it and see if you still feel the surge when you nail the throttle at the same place you normally feel it. If you do then the rear carb is working. Then re-attach the vacuum line back on the front carb vacuum pod and detach the vacuum line to the rear carb vacuum pod and plug it with a golf tee. Then go out and drive the car and see if you can again activate the surge at the expected time. If either the front carb or the rear carb does not initiate the same surge when you are running on one and then the other, then at least you have narrowed it down to which secondary carburetor to focus on if indeed it is either the front or the rear carburetor.

Once you narrow down the issue to either the front or rear carb then the problem should be easier to isolate. I know that you don't want to get down into the weeds but the faulty (non functional) carburetor may have a slight tear in the rubber vacuum diaphragm inside of the vacuum pod and so is not opening up the carb at the correct time and in conjunction with its sister secondary carb.

And you are correct in your assumption that you cannot stand next to your car with the engine running and the air cleaner off (to watch the throttle plates) and reach over to the accelerator rod and push it to WOT and hope to see either the front or the rear secondary carbs open up. It won't happen. You have to be driving it and have it under load. Good Luck in your hunt and let us know how it goes.

C.J.

 

You can unplug one carb but it's still going to be connected to the other carb via the linkage.

 

The front and rear carbs are connected to each other with hard linkage between the two carbs. If one opens, they both open. One cannot open any more or less than the other if the linkage is correctly adjusted - they both open exactly the same, and they open at the same rate. It's physically impossible for one to open and one to not open.

...and just for clarification: GM never made a "six pack." Mopar made a Six Pack, and so does Budweiser and Coors. GM made a TriPower, which was referred to as a "Triple Power Pack" in GM technical literature.

Lars

 

All very helpful guys, really helpful, and apologies for the "six pack reference". Ha! good that you pointed that out. I'm just wondering how many people caught me using that term over the years and didn't want to say anything. I don't even buy beer six packs any more. At my age it would take me a year to drink it all.

There seems to be a little disagreement, healthy, of course, over the best way to test. And it seems, if the linkage point is correct, one actuator can't be tested separately from the other. Maybe, given these are "born-on" parts, now more than 50 years old, I should just bite the bullet and have a good carb guy restore both vacuum actuators with new internal parts and re-tune the carbs. Its so hard to find a mechanic that has a good feel for these tri power set ups. Guess I was hoping for an easy way to determine If I need to do that.

Thanks again, and best wishes.

 

Lars. Good information to know.

I had always been under the mistaken impression that the metal linkage on the driver's side was there to purely ensure that both secondary carburetors closed completely and simultaneously when they were supposed to. I used to think that it was only the vacuum pods on each secondary carburetor which controlled the opening of the throttle plates inside of the front and rear carbs with the different colored springs inside of the vacuum diaphragms determining the when.

Thanks for jumping in on this thread. We are all lucky to have your expertise here on the Forum. Cheers. C.J.

 

That's true and correct: The mechanical linkage, the way it attaches to the slotted hole in the center carb throttle lever, does ensure that the secondaries are "forced" closed when the primary (center) carb closes. But the linkage also hard-ties the two end carbs together, which forces both of them to open at exactly the same rate and the same amount. Yes, the vacuum diaphragms do the actual opening, and the opening rate is determined by the stiffness of the spring installed in the diaphragms. As long as there is no binding or linkage issues, there is very little that can go wrong with the operation of the secondaries: If the diaphragms are not leaking, if the vacuum hoses are hooked up, if the throttle linkage allows the center carb to fully open, and if correct springs are installed, the secondaries will open when adequate venturi vacuum is generated through the primary carb (high airflow at WOT). It's a very simple system, and nothing can prevent it from working if it is in basic operating condition. But the two end carbs will always open equally and uniformly due to the hard-linkage between them.

There is nothing to "restore." There are only 2 parts in the actuators: The diaphragm and the spring. If the diaphragm is not leaking, there is no point in replacing it. If the spring is the correct stiffness, it will allow the actuator to work. You can remove the entire actuator and put a vacuum hose hard up against the vacuum hole in the lower actuator casting. Just suck on it. If the actuator retracts, it's working. Use springs that are soft enough to allow the secondaries to open below redline rpm without a bog. I have found the "purple" springs to be a good choice in most applications. Verify that the carbs have the correct jets and metering plates in them, adjust the secondary linkage per the specific instructions in the Service Manual, set the float levels and idle mixture, and you're good to go!

A '69 Tripower system being assembled and set up on my work bench right now. I use a bench assembly jig that has the exact correct carburetor spacing so I can correctly adjust the secondary linkage and verify that all fuel lines are correctly aligned to assure leak-free installation once on the actual engine:


Lars

 

I went through this a couple of years ago with a friend's L-71...practicing for a NCRS PV the engine didn't pull as I expected.

The end carbs are opened based on VENTURI vacuum in the center carb. They don't open suddenly but gradually due to increasing venturi vacuum as air flow increases through the primary carb with increasing revs. The mechanical linkage is there to ensure that they positively close when the driver lifts off the throttle.

So back in the garage, I try to use the closing linkage to open the end carbs... no go, they are jammed. With a little persuasion I got them unjammed. Then we started the engine and I blipped the throttle REAL HARD - briefly to WOT and could see the end carbs start to open. (The air cleaner was off.)

Back out on the road with the air cleaner back on the owner applied WOT. It took off and pulled like gangbusters to 6000 with lots of induction noise. It passed the PV a month later, not without problems, but nothing to do with the engine.

With the way the carbs are set up and the relatively mild cam in the L-68 there should be no noticeable surge. Floor the throttle at 1000 revs in any gear and it should pull smoothly and linearly to the upper end of the rev range. Like Lars says, it's actually a fairly simple system, but needs occasional attention.

The proper way to set up the carbs - various checks and adjustments - is in the '67 CSM. Don't leave home without it.

Duke

 

Lars

 

Nobody mentioned the paperclip on the vacuum diaphragm rod to determine if it is opening, why, only works for 4bbl?

 

In addition to the venturi pick-up in the primary carb, there are also venturi pick-ups in the outboard carbs that aid in the opening too. Crappy pic below of a secondary carb with a wire sticking through to the pick-up.

Side note: FCR, we used to call them "Tripps" also.

 

The proper operation is such that the low pressure (vacuum) created by air rushing in the primary carburetor at the pick up hole is bled off by the pickup hole in the secondary carburetors.

When the low pressure signal (vacuum) from the primary carburetor overcomes the secondary bleed holes it lifts the diaphragm and air starts rushing past the secondary blades and bleed hole at which point that hole changes from bleed to vacuum and aids in further lifting the diaphragm to WOT.

 

Yes, the paperclip test method works, but the diaphragm rods on the Tripower are very short, so the clips will often get pushed off the rod when the secondaries open. But at least you'll know that they're opening...

As Duke implied in his post above, the secondary opening is very smooth and gradual on these systems, so with the air cleaner in place (muffling the engine sound), it can actually be difficult to tell that the secondaries are coming in. Additionally, the stock diaphragm springs are pretty stiff, which delays the secondary opening until the rpm is pretty high. I usually install the "purple" springs and then tighten it up one spring color from there if the car bogs.

Tripower system I just finished building and installing this afternoon. Secondaries have the purple springs, and they definitely open. The previous builder had stripped the finishes off the carbs and painted them with bronze metallic epoxy... real pain in the *** to fix a mess like that and make it look right again:



Lars

 

I thought I had seen a print ad from back in the day that had a GM car with the hood open and a six pack of beer on the fender, The tag line that said something like, "This is a six-pack, referring to the beer, and this is TriPower", referring to the carbs???

I tried searching for it on the interweb and each time I did a search for six pack ad, Google thought I was searching for six pack abs....... Oh well..... I ordered some magic powder that says I will be ripped just in time for Summer!!!!!