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Carburetor Essential Tuning And Setup Tips - Four-Barrel Fundamentals
Twelve essential setup and tuning tips for the classic Holley 4150-series carburetor
4. Power Valve Power Play
The power valve (known as a metering rod on a Carter-style carburetor) is a vacuum-activated feature that’s used to enrich the fuel mixture to prevent detonation or stumbling. They’re available in different sizes to match the flow requirements of the engine. On manual-transmission applications, a standard 6.5-inch power valve is adequate if the vacuum reading at idle is above 12 inches. For automatic applications, if the idle vacuum reading is below 12 inches, divide the number in half to determine the correct power valve size. For example: a 9-inch vacuum reading requires a 4.5-inch power valve.
5. Installing The Electric Choke
A key-on hot lead wire is connected to the positive (+) spade on the choke cap; and the negative (-) spade on the choke cap is wired back to the carb as a ground. It sounds simple enough, but many installers get it wrong. Don’t be one of them.
6. Electric Choke Fast Idle Adjustment
With the engine off, hold the throttle wide open; this drops a lever with a ¼-inch screw in it beneath the choke housing. To slow the idle, turn the screw counter-clockwise. Turn it clockwise to increase the idle speed.
7. Basic Fuel Level Adjustment
In the fuel bowl, a fuel level that is too low can cause stumbling when the throttle is opened quickly. Follow these simple steps to adjust it:
- Remove the sight plug on the side of the fuel bowl
- With the engine running, loosen the lock screw on top of the fuel bowl
- Turn the nut on the fuel bowl clockwise to lower the fuel level and counter clockwise to raise the fuel level
8. Mixture Screw Setup
Getting the mixture screw adjusted properly is a must for preventing lean or rich conditions. Follow these steps:
- Bring the engine to normal operating temperature and turn it off
- Turn the mixture screws all the way in, and then back them out three full turns
- Restart the engine
- With engine idling at temperature, turn one screw in a quarter-turn and the next screw in a quarter-turn, repeating the process in quarter-turn increments until the engine rpm drops
- When the rpm drops, turn the screws back out 1⁄8-turn
9. Selecting The Right-Size Jets
Whether you call them shooters or jets, installing the correct-size components is essential for smooth operation and stumble-free acceleration. Because every application is different, settling on the correct shooter size (orifice diameter) often comes down to trial and error tuning. Stumbling at take-off without black smoke from the tailpipe means the shooters/jets are too small. Stumbling with black smoke from the tailpipe means the shooters/jets are too large. Crisp, stumble-free acceleration and optimal vacuum mean the shooters/jets are just right.
10. Troubleshooting Bogging And Hesitation
Bogging or hesitation is annoying and gives the impression of low horsepower. It occurs when the secondaries open or “come in” too soon; and it can be corrected with a heavier secondary spring. Conversely, sluggish performance, especially at wide-open throttle, may be due to a secondary spring that is too heavy. Swap in a lighter one.
11. Overcoming Stumbling
Stumbling is a common malady experienced with a new or rebuilt carb that makes one wish for electronic fuel injection. But it’s pretty easy to diagnose and cure.
A stumbling condition at take-off is usually due to an inadequate accelerator pump fuel shot—assuming the shooter/jet size is correct (see Step 9). Inspect the pump shot with the engine off. Look into the carb and move the throttle; fuel should spray the instant the throttle is moved. If it doesn’t, turn the nut of the pump arm counter-clockwise one full turn and check the spray again. Continue until the spray is immediate with the throttle. If the pump spray is immediate upon inspection, check the following:
- Inspect the pump diaphragm for a hole
- Check the pump passage for debris or other impediments