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Carburetor Tuning and Adjustments
- QuestionWhy is my engine running rich and my mixture screws do not have effect?AnswerThe first thing you need to check is your float level. No fuel should run from the sight hole unless you shake the vehicle. Next check the engine vacuum at idle. (in gear if A/T) if it is 12" or more a 6.5" power valve will usually be fine. Anything below 12" divide in half. For example 9" vacuum will use a 4.5" power valve. Another possible cause associated with performance camshafts are exposed transfer slots. You should not see more than .025"-.030" of the transfer slot exposed past the throttle plate at idle.
- QuestionHow do I tell what size power valve I need?AnswerTo properly size a power valve, take a vacuum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vacuum reading in gear at idle and if the vacuum is below 12" divide that in half for proper size. Example 9" of vacuum in gear at idle will require a 4.5" power valve.
- QuestionHow do I know if a vacuum or mechanical secondary carburetor is best for me?AnswerFor street cars, the vacuum secondary carburetor works best on mid-weight or heavyweight cars with an automatic transmission. They are more forgiving than a double pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.
- QuestionHow do I adjust the mixture screws on my carburetor?AnswerTo set idle mixture screws: 1. Get the engine up to temp. 2. Shut the engine off. 3. Install a vacuum gauge to a full manifold vacuum port. 4. Turn the screws all the way in until lightly seated. 5. Next, back them out 1 full turn and restart the engine making sure the choke is completely off. 6. While at idle, back them out 1/8 turn at a time, alternating between the 2 or 4, until highest vacuum is achieved. Be sure you keep all 2 or 4 screws even. You can also do these adjustments without a vacuum gauge. The adjustments are the same except you will adjust to highest engine RPM at idle.
- QuestionAfter I shut my car off and come back out the next morning there is gas on the throttle shaft and puddled on the intake. What causes this?AnswerThis is usually caused by percolation. This is when the engine is shut off and the engine temperature rises it causes the fuel to boil in the bowl and leak out of the boosters. There are a couple of things you can do to resolve this situation. One is make sure the fuel level is not too high. You can also lower the fuel level about 1/8" below the sight plug hole and this will cure it sometimes. The heat from the engine will rise into the carburetor at times and will cause the fuel to boil. Installing a phenolic heat spacer between the carburetor and the intake or a heat shield can cure this. These parts will prevent heat from getting to the carburetor and boiling the fuel.
- QuestionHow do I adjust the fuel level on my carburetor?AnswerTo adjust the fuel level you will need to take out the sight plug on the side of the fuel bowl. With the vehicle running loosen the lock screw on top of the fuel bowl and turn the nut clockwise to lower the level or counter clockwise to bring up the level. You want it to be at the bottom of the sight hole. If you turned it too far down it might bend the tang on the float. If you bent the tang on the float, you'll need to get back to a good starting point. You will need to take out the lock screw and turn the nut so the needle & seat is 1/2 way through the thickness of the nut then put the screw back in. Take the fuel bowl off and hold it upside down & level, then bend the tang on the float so the float is level in the center of the fuel bowl. This will get you back to a starting place.
- QuestionWhere do I hook up my electric choke?AnswerThe positive (+) spade on the choke cap needs a wire from a 12v that is hot anytime the key is on (from fuse box to anything that is only hot with the key) The neg. (-) spade on the choke cap will go back to the carb for a ground.
- QuestionHow do I adjust the fast idle on my electric choke?AnswerTo adjust the fast idle you will need to hold the throttle wide open (without the engine running). This will drop a lever under the choke housing with a 1/4" screw in it to slow it down. Turn it clockwise to speed it up turn it counter clockwise to slow it down.
- QuestionHow do I adjust the pump shot on my carburetor?AnswerWITH THE VEHICLE SHUT OFF, look down in the carb and move the throttle and see if the two sprays of fuel shoot as soon as the throttle moves. If it is not instant back off the nut on the pump arm 1 full turn counter clockwise then check it again. Do this until it is instant.
- QuestionI just bought a new carburetor and it has a warning that it will not work with an overdrive transmission. What can I do about this?AnswerThis warning was placed with the carburetor because if not installed properly with the correct transmission kickdown bracket for the GM 700R4 it will result in premature transmission failure. Part 20-95 and 20-121 with the kickdown mounting stud part #20-40 will allow the proper adjustment of the cable.
- QuestionHow do I tell what size carb I need?AnswerYou take the engines cubic inches and multiply by the maximum RPM and divide by 3456. If you have an automatic transmission you will need a vacuum sec. carb, a standard transmission can use a Double Pumper carb. You can also use our Interactive Carb Selector Tool to help you find the correct carburetor for your particular application.
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