lars

Around the beginning of the month, Forum member Brian (aka, “Bhebert449”) was having some trouble with his ’69 427 Tripower leaking fuel. He posted this:

https://www.corvetteforum.com/forums...from-carb.html

He had a bad fuel leak in the accel pump area:


He was given some good suggestions from Forum members, but he ended up pulling the carbs off the engine and sending them to me for evaluation and repair.

Although I do a lot of Q-Jet and Holley 4-barrel carb setups for people, I have been hesitant to take on the Tripowers simply because I don’t have any way to test them: I don’t believe it’s possible to rebuild a carb without actually testing it and setting it up on a running test engine. There are too many funny things that have been done to these antique carburetors over the years, and you can’t find or solve all the problems without actual testing.

My test engine is a 357 Chevy small block with Dart heads, a 268 cam, and headers – a typical street performance engine. I have adapters to quickly switch it over from testing Q-Jets to testing Holley square bores. When Brian asked me to help him with the Tripower, I decided to come up with a way to expand my test capabilities…

Part of the key is to realize that the end carbs on the Tripower do not flow any air at idle. Unlike the secondary throttles on a Holley 4-barrel, the Tripower secondary carbs are completely closed at idle. They also do not have a “real” idle fuel system, but they do flow a very slight amount of fuel at idle – just enough to keep the fuel in the bowls from stagnating and from rising due to slight needle/seat seepage. The end carbs are only fully functional at Wide Open Throttle (WOT) and under load. Since my test engine cannot test carbs under load or at WOT, and since the end carbs don’t flow air at idle or light cruise, it became obvious that I could test the carbs as a “system” without actually flowing air through the end carbs…

So I fabricated an adapter…. This was made from a Mr. Gasket 2-barrel to Spreadbore Q-Jet adapter, which I slightly modified with a little drilling & tapping, installed some hardware, and flipped upside-down:


I then fabricated an upper mounting plate that precisely replicated the Tripower carburetor bolt patterns and centerline spacing dimensions:


This was mounted to the Mr. Gasket adapter, allowing the center carb to be fully operational, and allowing the end carbs to be mounted in their correct positions to allow hookup and testing of all fuel lines and linkages.


The adapter was given a relief cutout to assure free movement of the accelerator pump, and I moved a few components around on the test engine to provide adequate clearance for the mounting plate and carbs.


When I received Brian’s carbs, I immediately test fit them to the test fixture. They fit perfectly, and the fuel lines were easily attached. Unfortunately, Brian twisted the fuel line to the center carb, but this was no problem with the test fixture: I simply fabricated a new line to fit the carbs as mounted on the fixture, thus assuring that the line will fit without the need to procure a new line set. Brian's carbs installed on my Tripower fixture for inspection prior to teardown:



Fuel line at the center carb is damaged, and will be rebuilt as part of the rework process.

With the preliminaries out of the way, I then tore the carbs down. The primary objective was to identify and correct the source of the fuel leak at the accelerator pump, but many more issues were quickly discovered. Here is what the teardown and inspections revealed:

• The fuel leak at the accel pump was caused by the pump housing screws being loose (finger-tight) - the diaphragm was not ruptured as some had suggested.
• Secondary floats were bottomed-out on their adjustment
• Stripped threads and loose hardware throughout - vacuum diaphragm housings were not sealed against the carb bodies, and this vacuum leak may have prevented the secondaries from operating at all
• Various incorrect and damaged hardware
• Only 1 carb (end carb) had a fuel filter, and it was missing the filter spring (all fuel bypassing the filter)
• End carbs had the stock "brown" (i.e. "very heavy") secondary springs installed, preventing full secondary opening (if the secondaries opened at all due to vacuum leaks at the carb body)
• Badly misbalanced idle mixtures
• Accelerator pump discharge nozzle seals never installed by previous builder - massive accel pump fuel leak bypassing nozzle
• Extremely lean (leanest possible) accel pump cam installed ("white" cam), producing poor accel pump shot for only partial throttle travel
• Secondary float springs installed backwards, forcing floats "down" instead of assisting "up." This may be why the secondary float adjustments were jammed into the extreme "low" position and bottomed-out.
• Choke intermediate rod seal plate in airhorn altered and damaged - not "floating" and causing choke to intermittently not close
• Damaged center carb fuel line
• Very low fast idle speed (fast idle tang not touching fast idle cam with choke closed)

Carbs disassembled, cleaned up, and ready for (correct) assembly:


For assembly, the secondary diaphragms were given “purple” springs to allow slightly quicker (and full) secondary opening, and the very lean “white” accelerator pump cam was switched out for a “pink” cam, which continues to add accelerator pump fuel through a larger arc of throttle opening. This should provide smooth, strong power going into the secondaries at elevated rpm.


Incorrect hardware was identified and replaced, and the stripped threads were repaired. Here is an example of incorrect, short hardware installed in the secondary diaphragms: The thread engagement was so short that the threads in the carb bodes had simply stripped out:


Stripped threads in carbs:


The new center carb fuel line was fabricated, and the entire setup was test fit once again onto the test fixture and mounted up to the engine:




The center carb was primed with a few shots of fuel through the airhorn vent hole, the choke was closed, throttle given 3 strokes, and the engine fired up instantly to a smooth, stable 1250 rpm cold fast idle.

But, unfortunately, fuel was leaking at the center and rear carbs from an unknown source… this is why testing is important. The center and rear carbs were removed from the test engine and hooked up to an air leak test fixture on the work bench. Pressurizing the carbs to 10 psi with air and performing a bubble leak check revealed that the carbs had leaks at the inlet fitting gaskets. Removal of the fittings showed that both carbs had a slight defect in the gasket sealing surfaces. Once this was smoothed and burnished, and once the carbs passed the 10 psi air bubble leak test, the carbs were re-installed on the test engine, and the test was resumed…

The system running on the test engine. Float levels on all 3 carbs was set to the bottom of the sight holes:


Running cold with choke closed (using light upward finger pressure on the choke rod lever since I do not have a reverse-operating 427 choke coil on the engine) and cracked open by the pulloff with engine running:



Cold fast idle was set to about 1250. This was at initial startup at 35 degrees. It only took a slight “tweak” of the fast idle tang to get the fast idle speed right:


Here is the tang for fast idle speed adjustment. The tang is bent using a screwdriver in the slot to change the fast idle speed:


Cold fast idle air/fuel mixture should be in the mid-12’s. Brian’s Tripower is right on the numbers:


Running hot with choke wide open. Once engine temperature was stabilized, hot idle speed and hot idle air/fuel mixture could be set. Float levels were verified again, just to make sure:


Hot idle speed set to about 850:


Hot idle air/fuel mixture right on the numbers at 14.7:1 (1.00 lambda). This was achieved with the idle mixture screws at ¾ turn:


After 30 minutes of running, all lines and connections were leak checked for any evidence of fuel wetting. The problem area around the accelerator pump is bone dry:


All lines, fittings and connections are completely leak-free. Key to leak-free connections is two-fold:

1. Assure proper alignment of all tubes into the mating fitting - all connections should be able to be finger-tightened until they seat
2. All fitting threads and the back surfaces of the tube flares must be lubricated prior to assembly and tightening. Proper tightening cannot be achieved if the fittings are not lubricated. Assembly lube or bearing grease can be used:

Throttle response was excellent, and the idle is smooth and stable, both hot and cold. System is ready for pack & ship, and we’ll hope Brian has some fun with a correctly-running Tripower system on his car! (Although, this would look cool on a small block, even if it’s just a “dummy”…)

Lars

Jebbysan

Nice Work.........
Question for you......when I had my 400 horse, I often wondered if the gas would go stale in the bowls if you never got into it......it would never have a chance to replace any fuel in the bowls......
What do you think? Not like anyone would ever DO that, but for the sake of conversation.

Jebby

lars

Jebby -

As noted, the end carbs do have a small idle fuel bleed orifice - they "trickle" just a tad of fuel below the throttle plates at idle to keep the secondary fuel circulating. The metering plates even have an idle circuit hole drilled in them, even thought the secondary carbs technically don't have an idle circuit (that's adjustable).







Lars

Sky65

Lars. What main jet size did you use on the center carb?

Thanks
Tom

lars

Since this is going on a stock, unmodified car, I kept the stock jetting in all the carbs: 1969 center carb uses 63 jets. 1967 uses 64 jets. The end carbs use the metering plate stamped "-3", which has .076" diameter main metering holes, which are equivalent to a jet size of 72.

Lars

73racevette

Lars, first, nice writeup.

However, I must be missing something. Looks like you did a stellar job on the center carb but did not address the outer carbs other than checking for leaks, proper plumbing, etc. Seems like you need air to flow through the outer carbs to check for correct jet sizing and the plate you fabbed up does not allow for that.

What am I missing??

Thanks

lars

As discussed in my write-up above, the end carbs only operate under load at wide open throttle once the center carb is near its max cfm flow limitation. That cannot be simulated without a dyno or the actual car in a road test - it cannot be done on a test engine under a no-load condition. The end carbs were rebuilt and set up to stock factory specs. The float levels were set and verified, and they were leak checked. No other testing is possible without the engine being on a dyno or in the car. The "correct jet sizing" for the end carbs for a stock 427 has been exhaustively researched and determined by GM and Holley during the development of the 427 Tripower engine, so that spec is known. Re-jetting and fine-tuning is justified whenever engine mods are done, which must be done on the specific engine using a dyno, or with wideband sensors in the exhaust on the actual car during actual load testing. Keep in mind that the 427, in rough numbers, will push just short of 300hp up through 3000 rpm, which is when the end carbs will start to open. To attempt to fine-tune secondary jetting by running the Tripower on a no-load engine, whether the carbs are hooked up to the manifold or not, would be impossible. ("Correct" jetting of the center carb can also not be verified on a no-load test engine, since the center carb (primary side of any carb) will run on the idle and transition circuit under any no-load condition, regardless of rpm - the main metering circuit/main metering jets cannot be verified under no-load)

Lars

suprspooky

I should have known it was too early for an April Fools post. Nice job Lars, I'm a believer in "the rebuild isn't done til you (the re-builder) run it.
You are a glutton for punishment though, now we just need for you to build a water brake. I'll make you an adapter to run all three Carbs if you build the brake. I can't imagine opening all three Carbs no-load

lars

They would never open at no load - not possible to obtain enough mass air flow through the primary before redlining the engine... A while back, I did try hooking up a Powerglide transmission with a high-stall converter and a trans brake to the engine, but the tranny would overheat before any meaningful data could be obtained at a stabilized high-load/high-rpm setting, even with a pretty big transcooler installed.

Lars

derekderek

i guess too much effing around to mount iron sbc trips manifold to run these carbs?

69ttop502

Excellent Lars, very interesting thread!

Jebbysan

I guess if I had read every sentence, I would have known that!
Thanks though.

Jebby

lars

I don't think there is an iron SBC tripower manifold for the Holley carbs, and the aftermarket aluminum ones do not have the correct carb-to-carb distances or the large bores for the end carbs to replicate the BBC setup. So there is no such animal... But as noted, whether you have the end carbs hooked up to the manifold plenum or not is irrelevant, since they will never see any airflow under any no-load or light-load operating condition. The only thing you might be able to check is to verify that the throttles are closing and sealing completely, but you can do that with a visual inspection during the rebuild process.

Lars

jim-81

Sounds like you need a dyno now! haha. Nice write up.

lars

I could think of several reasons to justify an engine dyno.... If I had the floor space to put one in and I would seriously consider it. I can barely fit 5 cars in the workshop now with the engine run stand.

Lars

A10pilot

Thank you for all the great information. I have the same setup on my car and this write up will help me understand how it works.

suprspooky

Here's a factory Doc. about the Tri-power setup/operation, some may find it useful.

derekderek

oh. this is your spacer plate with these 3 carbs.

dmaxx3500

you really need to build a BBC eng

just to do it

lars

Yeah, I need another engine in my shop like a hole in the head...