|Holley 4150 Carburetor Manual
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Fuel Bowl Pump Diaphragm (proper installation)
Before replacing the pump diaphragm cover, check for warpage by laying the cover on a sheet of emery paper which should be on a smooth surface. Remove the sealing bead, if any is present.
Place the diaphragm assembly on the cover, holding the pump lever arm against the diaphragm so that the diaphragm is flat. Don’t forget to install the spring into the bowl first. Install the cover and diaphragm on the fuel bowl and insert the screws finger tight. Release the lever, making certain the diaphragm is loose enough to flex without wrinkling at the edges, also the diaphragm must be able to be raised so that the flange of the steel diaphragm washer touches the inner surface of the cover. These two checks will assure maximum diaphragm travel. Tighten the four screws in two stages, 1/2 torque the first time and full torque on the final stage (5-8 lb. torque).
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Gas dripping from the venturi can happen on most carburetors, but we will talk about the Mercarb and the Rochester 2 Jet, 2 barrel.
The only time gas should come out of the venturi is when you pump the gas throttle. That is when an extra squirt of gas is required from the accelerator pump circuit. Make sure the engine is off when looking down the carburetor bore.
If it happens at idle, or cruising along, then you have a problem. This can be caused by a vacuum leak, or something plugged in the idle circuit.
Remove the idle mixture screws and blow out the circuit. Remove the venturi and blow out the small holes on the top. You may have to run thin wire down the holes to get them clear.
Still have the problem?
You could be getting too much gas. Check the fuel pressure to see if it is 4.5 to 5 lbs. Also check the float level. On the Mercarb turn the top upside down and set the float level.
Another thing to try. Start the engine and bring up to temp. Rev up the RPM to 2500 or so and cover the air intake with a gloved hand or rag while leaving the at same 2500 or so position. The engine will begin to die, but just before it dies let it recover. Repeat this several times. What happens is fuel goes through air/vacuum circuits and visa versa. If the idle circuits are dirty this often flushes them out!
One other possibility is “heat soak”. This happens when the engine gets hot and you turn it off, or come to an idle. The fuel boils over, causing it to flow out just about anywhere it can. If this is happening, try other brands of gas. Without ethanol if possible. Ethanol is not our friend.