There is no way that you are properly adjusting this carb if you don't know what the screws are for. Here are the instructions for a 3310.
The little screw in the center of the metering plate is the idle mixture screw. The large flat screw in the side of the bowl is the site plug. The screw in the top of the bowl is the lock down for float level adjustment, the actual adjustment is made by turning the nut.
Go to the Holly site to get the instructions for your carb.
FLOAT LEVEL CHECK AND ADJUSTMENT:
Primary and secondary float adjustments are set at the factory, but variations in fuel pressure could cause a change in these
settings. The following procedure shows how to make these adjustments:
1. Start the vehicle.
2. Observe the sight plug for the fuel level. If none is seen, the level is too low. Fuel should be even with the bottom of the
sight plug hole. If fuel comes pouring out of the sight hole, the float is set too high.
NOTE: A properly set float level will have the fuel level located at the bottom edge of the sight hole, as shown by the line in
Figure 12.
Figure 12
3. To adjust, shut down the engine.
4. Loosen the lock screw on top of the fuel bowl just enough to allow you to turn the adjusting nut. Hold the screw in position
with the screwdriver.
12
5. Using a 5/8” wrench, turn the adjusting nut in the appropriate direction: Clockwise to lower float and counterclockwise to
raise float.
6. Turn the adjusting nut in increments of 1/4 of a rotation.
7. Retighten the lock screw.
8. Restart the vehicle and observe the sight plug hole.
9. Repeat steps 1 through 8 as necessary.
IDLE MIXTURE NEEDLES
Idle mixture needles control the air/fuel mixture at idle. These have been preset at the factory and SHOULD NOT need any
adjustments. However, if you feel that adjustment is necessary, you can use the following procedure to do so. When tuning the
idle mixture, you’re actually tuning for the best manifold vacuum. Idle mixture needles are found on the primary metering blocks.
If you change one idle mixture needle, you must change the other idle mixture needle by the same amount. Here are the proper
steps for setting the idle mixture needles.
1. Attach the vacuum gauge to a manifold vacuum port on the throttle body (Figure 7).
2. Adjust each idle mixture screw (Figures 10 & 11) 1/8 turn at a time, alternating between each screw. Turn them equally,
until you achieve the highest possible vacuum reading without adjusting the curb idle speed screw. Turn screws in to lean
the mixture. Turn them out to richen the mixture.
Figure 9
3. Now that the idle mixture is set, it may be necessary to go back and reset the idle speed using the curb idle speed screw,
as shown in Figure 9.
4. If a vacuum gauge is not available, use a tachometer to obtain the highest RPM.