Carb jeting


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By slumlord444 - 5 Years Ago

The 312 in my T Bird is prety much a copy of an engine that Ted built several years back and did an article in Y Block magazine. I've always ran the E carb setup on mine. I'm in the process of switching to the Blue Thunder intake that Ted ran and a similar 750 Holly 4 barrel. Teds jetting was 77 primary and 89 secondary. My carb came with 76 primary and 80 secondary. I think this is close enough to start with. My good friend mechanic buddy says I should go with 89's in the secondarys. Ted was getting around 427 horses out of his. Opinions?
By Ted - 5 Years Ago
Unless the Holley carburetor is the exact same list number, primary and/or secondary jetting can vary simply due to different air bleed or emulsion sizing within the carbs.  The Holley 600 and 750 carbs are good examples of this as there are a myriad of each of these out there with different list numbers and have different jetting requirements as a result.  On the HP carbs with the removable air bleeds, it's sometimes necessary to change the air bleeds rather than the jets which can further complicate transferring the air/fuel requirements from one engine to another.

My recommendation is to always start out with the factory jetting as a baseline and go from there.  Second guessing an engine’s fuel needs before actually running a specific carburetor can be misleading.  For determining an engine’s fuel requirements, the wideband sensors located in the exhausts are considerably more accurate than reading the spark plugs.  To compound problems with determining the correct jetting, weather conditions and altitude also play into this.