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Old 11-07-2011, 11:35   #1
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Join Date: Apr 2010
Location: Newport News VA
Boat: Egg Harbor sedan cruiser 1970
Posts: 829
Carb J tube issues drip fuel when engine running fast

Not enough to notice any engine issues when the boat is underway.
But I recall reving the engine and seeing some fuel pulled thru the J tube on a Rochestor quadrajet.
I was wondering if the air suction creates a negative pressure in the float bowl and the engine then wastes fuel without the operator being aware.
Most people would not even be aware if there was an issue as how many ever run even for testing with the arrestor screen off the top of the carb?

I went searching the web and found little discussion, some sites talked about how J tubes create a negative pressure in the float bowl air above the fuel and fuel gets pulled threw like it is an additional jet. Looking at the air horn on a rochestor, there is a large central vent connected straight to the float bowl, so I dont understand why a forward vent is even needed. Maybe it should be plugged.

for example this is the carb style here which shows the J tube on the forward vent.
Amazon.com: Rochester Carburetor for Volvo Penta: Automotive

this picture shows the carb top with the central slotted vent. If the forward vent was plugged, this vent is still going to work at the carb center. That vent is right next to the choke plate. So its J tube, choke plate, slot vent.
There is ALSO the secondary rod vent hole which pass the secondary rods up threw the air horn right next to the slotted vent and this also functions as a vent.
That slotted vent is sometimes also used to check float level with a flat gauge.

to see what I mean look here

Amazon.com: Rochester Carburetor for Volvo Penta
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