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 over on the C3 Tech Section, but this info applies equally well to the C2 Tripowers, too:

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 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

Bluestripe67

Even though this work does not affect me, the write-up was very interesting.Your talent and expertise clearly showed the attention to detail that makes the difference. Dennis

DansYellow66

Thanks for the write up LARS - and the tip on lubing flared fittings. I usually go through 3 or 4 cycles of loosening and tightening new flare fittings to get them mated and seated but hadn't thought about applying lube to all the surfaces which probably would help a lot.

6T7L71CPE

WEIAND and Summit have a manifold that can use the Corvette Holley 2300 carbs. Summit even sells the whole setup with mechanical secondary carbs.

lars

That's an interesting manifold, as all 6 throttle bores are the same size (small). The Corvette/Holley Tripower had 466 cfm end carbs (350 center), which use larger throttle bores than the center carb. The Holley carbs also require unequal spacing between the carbs - the Weiand manifiold shown appears to have equal spacing, which would space the rear carb further away from the center carb than the front carb... compare it to an actual Vette Tripower intake - look at the throttle bores and the spacing between the center carb studs to the forward carb, and from the center carb to the rear studs:



This is done to "squeeze" the carbs as close together as possible, making them look "evenly spaced." The center carb has a metering block, which forces the carb in front of it to be further forward. The rear carb is smaller, since it only has a metering plate, allowing it to be scooted a little closer to the center carb, making the carbs "appear" to be evenly spaced (by making the "gaps" between the carbs the same):




Lars

Jebbysan

It appears the spacing on that Weiand would allow for three 2300 full idle setups......would have to be mechanical only. All three would have metering blocks. It also appears that this would be a good way to lose about 40 horsepower as well........

Jebby

ptjsk

That's is one heck of a comprehensive write up!

Thank you,

Pat