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Tri, Tri Again Part IIPart II: Throttle Flange and Airborn Repair for Tri-Powers From the January, 2009 issue of High Performance Pontiac
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Last month, HPP documented Jim Taylor of Jim Taylor Engine Service in Phillipsburg, N. J. identifying, disassembling and cleaning a 1963 Tri-Power setup. The plan is to build a 1965 system using a 1965 intake with 1963 carbs and to make it concours correct for a 1965 GTO, a job that's increasingly difficult as original units become more and more abused. As work progressed, Jim found many incorrect internal components, and Taylor walked us through the hows and whys of a good induction system gone wrong. When we left off last month, the carbs and manifold had been disassembled, and Jim had begun repairing a center carb and its integral throttle shaft assembly, which suffered from plugged idle mixture screws, a warped body, and bent throttle plates. For this installment, Jim began to disassemble the throttle flange by grinding off the factory-distorted ends of the throttle blade screws with a carbide bit. He was then able to remove them with a screwdriver, slip the warped blades out, remove the throttle shaft return spring, and remove the throttle shaft. The photos and captions that follow tell the rest of the tale.  This is the throttle shaft...  This is the throttle shaft repair kit, which Taylor procured from Larry Isgro Carburetors (516-897-0401) (Larry also provides original screws, bushings, and throttle shaft kits for this application). From top to bottom: .314-inch broach, .316-inch broach, guide install tool, guide cutoff tool (which Taylor doesn't use for his repairs), a T-handle and a reamer. The 5/16-inch valve guides (the kind in your heads) in the foreground are cut to size to create the throttle shaft bushings, and the throttle shaft screws are replacement originals.  Taylor's T-handle reamer,...  Taylor's T-handle reamer, which measures .345 inches, is used to enlarge the throttle bore. This process is done by hand with the throttle flange in a vise, and is difficult because there is no pilot to the other side of the carb body and Jim made sure the reamer was centered before proceeding.  Pre-cut, .5-inch bronze bushings...  Pre-cut, .5-inch bronze bushings are pushed into the guide install tool, which collapses them. The bushing is hammered into the hole that was just reamed--this is an interference fit, and it takes a pretty good whack to get it in there.  Jim used a micrometer to determine...  Jim used a micrometer to determine that the diameter of the original throttle shaft was .310 inches, measured where the shaft rides on the bearings. The throttle flange is vertically soft-mounted in a vise with wood blocks on either side to protect it, and a .314-inch broach is selected to drive through the throttle bore with a hammer--this sets the inner diameter for the bronze bushings.  Previous welded shaft repairs...  Previous welded shaft repairs were quick and easy for sure, but nowhere near the correct screw-in or peened-over factory-type assembly. Jim will have to get this weld off to return the linkage to factory specs.  A trip to the grinding wheel...  A trip to the grinding wheel is made...  And Taylor grinds the weld...  And Taylor grinds the weld down until the linkage arm locator (the rectangular outline in this photo) is visible. Jim lets the metal cool a bit, and then he pulls the linkage arm off with a pair of pliers while keeping the shaft intact. Looking closer at the top of the shaft reveals some wear where the shaft rode on the original bushing. A small amount of wear is normal, but these scuffs are caused by a non-factory return spring that is too heavy.  In preparation for the correct...  In preparation for the correct screw, the throttle shaft is held in a vise and center-drilled with a .136-inch bit, then tapped with an 8/32-inch tap. An original screw with 8x32 threads (a number 8 screw with 32 threads per inch) is used to hold the linkage, and Taylor forgoes Loctite when he tightens the screw down with a screwdriver. The linkage is now factory correct, with no unsightly welds in sight.  Turning our attention to the...  Turning our attention to the bare throttle flange, we notice that this cast-iron piece has attracted rust in its bore. Jim attacks this problem with a smooth rat-tail file, which knocks down the surface rust.  With the heavy spots taken...  With the heavy spots taken care of, a medium-grit wheel cylinder hone is used to smooth out the internal surfaces.  This throttle flange will...  This throttle flange will eventually be the base of a restored rear carb. As such, the front and rear carbs on a Tri-Power assembly have no idle circuits, so they absolutely can't have any vacuum leaks. Which means that at idle, the throttle blades must be tightly sealed in the bore. The out-of-place original throttle blades would have to be adjusted. This side shot of the partially assembled throttle flange illustrates the angle of the throttle blade's edge. Said angle allows for the entire face of the throttle blade to seat perfectly flush against the bore when closed (which Mark Erney measures with a smart tool as a 14.6 degree angle). If no angle were used with the throttle blades, they would stick in the bores.  Before the final assembly,...  Before the final assembly, Krytox GPL 105 oil is used to lubricate the shaft, linkage, and bushings. Taylor has found that if you either maintain the shaft's lubrication or halve the spring pressure, you can reduce wear that occurs from the return spring by 50 percent if lubrication is maintained--effectively doubling the shaft's life.  Once the throttle shaft has...  Once the throttle shaft has been seated, Jim twists in the throttle blade screws. It would seem that Loctite would be a logical solution to keeping these screws in, but this chemical can migrate to the bushings, causing the shaft to bind. Instead, Taylor peens the throttle blade screws to prevent them from working out and falling into the motor. He accomplishes this task without bending the delicate throttle shaft by mounting a homemade screwdriver head into the vise, setting the throttle flange on top of it so the blade is in the screw slot, and tapping with a small hammer. This method prevents pressure from being exerted on the shaft.  A thicker throttle return...  A thicker throttle return spring (left, from Ames Performance Engineering) replaces the used-up original (right). Taylor opines that at the rear of the assembly, the weight of the mechanism the spring has to operate calls for a heavier unit than stock. With a well-lubricated assembly, Taylor isn't worried about premature wear.  Our front carburetor actuator...  Our front carburetor actuator lever is installed with the aforementioned stiffer spring. The easiest way to install the spring on the front actuating lever is to hold the throttle body in a vise. With the end of the spring slipped into the hole in the throttle flange, the other end is slipped into the linkage's protruding ear.  Our good-as-new throttle flange,...  Our good-as-new throttle flange, replete with a reconditioned shaft, screws, and blades, new bushings, honed bores, and a new spring. Although a dead-on photo of the two blades could not be taken, this throttle blade-to-bore alignment was near perfect.  This Rochester 2-barrel bowl...  This Rochester 2-barrel bowl cover (left) is not from a Tri-Power, but Jim is going to be using it to demonstrate his method of fixing a broken air filter stud mount on an original Trip bowl cover (right). Taylor began by using a carbide burr to get the residual fragments of the original brace out.  He softly mounted the bowl...  He softly mounted the bowl cover in a vise, then drilled through one side of the air filter lip toward the top of the carb with a .159-inch bit, being sure to remove any burrs on the inside.  Jim's next step was to tap...  Jim's next step was to tap the new hole with a 10x32 tap. He repeats the drill-and-tap process on the other side, using care to drill the second hole so it will line up with the first.  A crossbar is positioned,...  A crossbar is positioned, and then two 10x32 set screws are test-fit into it.  Mark Erney cuts a section...  Mark Erney cuts a section of 25x20 thread union with a hacksaw. The height must be matched to the original height of the broken-off cast piece, which is .435 inch higher than the filter mount.  Erney uses a MIG welder to...  Erney uses a MIG welder to attach the union to the crossbar, allows the metal to cool, and then screws in the original air cleaner stud.  The new brace is now installed...  The new brace is now installed with Loctite on the set screws, and they are spun into the body until they sit flush with it. Taylor's set screw method works well because it allows for the original air filter gasket and a non-modified filter base to be used. Stay tuned for Part III next issue. We'll cover assembly of the Tri-Power.
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Jim Taylor Engine Service
120 S. 5th St.
Phillipsburg
NJ
08865
908-213-3456
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