If your vehicle makes 6-inches of manifold vacuum at idle, then you need a #3 power valve. So if you get a 12-inches of vacuum reading, you need a #6 power valve. The rule of thumb is that the power valve rating (opening point) should be ½ the manifold vacuum at idle, tested in the manner described above. To check manifold vacuum, attach a vacuum gauge to the manifold vacuum port on the carburetor, place foot on brake, engage in gear and record the reading. If your QFT™ doesn’t have a choke it probably has a 4.5 power valve from the factory. A good rule of thumb is that if a QFT™ carb has a choke it comes with a 6.5 power valve. That is primarily determined by the amount of engine vacuum available at idle. Next, determine if you have the correct power valve for your vehicle’s application. After you have attained the highest rpm possible, you might have to turn down the idle adjustment on the throttle linkage to get the correct idle speed you want. However, if engine rpm stays consistent or increases when you equalize the remaining screws, make another ¼ turn adjustment on the first screw and adjust the others to match. If the engine decreases rpm when you are adjusting the others, take them all back to the original setting, because the settings were already correct. If engine speed increases with that one screw, adjust all of the others to match. What you are looking for is an increase or decrease in rpm. Start the engine and turn one of the idle screws in or out. The engine should be turned off while you are setting the adjustment on all of the screws. Regardless if you have a two- or four-corner idle adjustment on your QFT™ carb, start by setting all of the idle adjustment screws the same (1-1/4 turns from bottomed out in the metering block is a good place to start). Again, the vehicle must be up to operating temperature and check to see if your fuel pressure is holding at 6.5psi and the float level is still correct. Now you are ready to check and re-set the idle mixture screws. Along with these tuning tips, a check of your ignition system, as well as, the mechanical and vacuum advance curves in your distributor is suggested to eliminate any potential timing and ignition problems. Following these step-by-step tuning tips are necessary to make sure that your engine runs at peak performance. We went to Zach Baker, a technician at QFT that spends much of his time talking customers through the correct tuning procedures, to give us his proven step-by-step carb tuning tips to get the most out of your carburetor’s performance.
#YELLOW BULLET FORUMS HOLLEY CARB TUNING HOW TO#
There are many misconceptions about how to go about these adjustments, and of course there is no shortage of free advice. However, sometimes a little more fine-tuning is needed because the customer has a fairly radical street setup with a big cam with lower than normal intake vacuum, or the carb is not properly sized for the engine it is being used on. Engine testing all carburetors prior to shipment ensures that when a new QFT™ carb is bolted on an engine, it will start, idle and perform up to the customer’s expectations. Quick Fuel Technology® (QFT™) ships carburetors only after they have been thoroughly run-tested and adjusted on a typical engine.
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