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| Performance tuning. | |
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ThermoQuad carbs generally do not require any extra tuning parts as long as the carb selected is the best choice for the application. For example, a converted 2 bbl engine in the 270 to 330 cubic inch range would only require an 800 cfm carb while a cammed up engine with headers, aftermarket intake and 9:1 to 10.5:1 compression would work best with an 850 cfm carb. T.Q. TUNING TIPS: Metering rods There are many metering rod combinations and while the perfict scenario would be to have as many different sizes as possible, one particular size(#stamped on the metering rod lengthwise)# is the best all-around. This # is the 1966 metering rod. Since metering rods will only show a marginal performance increase, the metering rod # is not as critical as the actual piston height adjustment: with the fast idle cam not engaged, find the metering rod piston's center slot on the top of the piston center and with a small slot head screwdriver, lightly bottom the piston and turn the slot counter-clockwise until the piston stops dropping, then turn the slot clockwise noting exactly when the piston starts back up, still lightly bottoming the piston, turn the slot 1 turn clockwise. This is the correct way to dial-in the metering rod tree/piston. For a richer setting, turn the slot 1/4 more at a time. For a leaner setting, back off the slot counter-clockwise 1/4 turn. Secondary air door adjustment To dial-in the 2ndary air door, several adjustments can be made. First, with the engine idling, push down on one of the 2ndary air door dimples and if the door is tight and the engine bogs under wot from a standstill, you can remove the choke pull-off linkage and bend the rear crook backward slightly which will lengthen the linkage. Re-install the linkage and push down on the air door dimple again and you should have slack before the air door tensions. This slack can be .100" to .185" measured from the bottom rear center of the air door to the air door stop(casting boss). This is a good baseline setting for a mildly cammed engine. For a more radical engine or engines with low vacuum at idle, keep the air door drop tight. You will notice a slit on the driver side front edge of the air door and a tang on the driver side metering rod cover. The tang on the metering rod cover acts like a vertical travel limit on the air door. Bending the air door corner backward will lean the 2ndaries since the dimples are closer to the 2ndary spray bars, weakening the venturi effect and will thus let more air into the 2ndaries. Conversely, bending the corner of the air door foreward will richen the 2ndaries by creating a stronger venturi effect and will let less air into the 2ndaries. Float level Allways use a float setting of 29/32" with the top gasket in place and the top plate upsude down. This is the best float setting and will often produce quicker starts hot and cold and will eliminate surging in hot tempratures. Pump nozzle id The accelerator pump cluster can be removed and the nozzle id's can be drilled with a pen vise. For stock to mildly built engines, drill the id's to .031" on a small block and to .033" on a big block. You can go up from there but larger id's should be carefully considered. Pump stem height Accelerator pump stem height should be set to 14/32" measured from the casting of the top plate adjacent to the stem to the top of the stem. To adjust the height, remove the pump linkage and bend the front crook tighter for more height or straighten the crook for less height. Adjusting idle mix screws Set the idle mix screws(measured from the casting boss surface to the end slotted of the screw)to 22/32" on mild engines and more radical engines to 24/32". This is a baseline adjustment. Air cleaners Drop base air cleaners will often put the underside of the top too close to the vent stacks and will cause an erratic/rich idle. Idle transfer slots Pay close attention to the idle transfer slots(rectangular)in the front of the primary bores of the base plate. When the carb is correct in cfm's and jetting, the slots will only show a square when the butterflies are in the normal idle postition. If the slot is a rather large exposed area, the carb will need the inital timing bumped up which will increase the idle speed and will allow the idle speed screw to be backed down, thus exposing less of the transfer slot. If the slot is less than a square, back off the inital timing and crank up the idle speed. For other topics not mentioned here, email me and I will be glad to help, DAVID.
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