I don't really know what the jet sizes are in Big trucks like yours, but my Unibodys both have .53's and would think theye'd be pretty much the same. I rebuilt another carb, that was dated 1960 and it only have .50's on it and am running that one currently on one of the trucks, so I would imagin you wouldn't want to be too big and still get a little economy on it. I rebuild my carb's using a kit from Auto Zone thats a GP Sorensen kit part number 96-567 and as $17.99. Do you know does you truck have either a govenor or a "de-popper valve" on it?? De-popper valve is for going down long grades.

                                             Michael

                                                Michael

I agree with Paul and Michael in that 12mpg in a F600 would be considered good and especially when taking into account the rear gearing that came in those trucks.  As far as fuel mileage goes in these big trucks, I came across a severly retarded camshaft in a previously never rebuilt and low mileage fire truck engine.  During the rebuild, I simply reinstalled the factory camshaft so it was in at 6° advance as measured on the intake lobe centerline and the customer claims that it more than doubled the fuel mileage.  The idle characteristics also improved dramatically over what it was prior to advancing the camshaft.

Besides jetting, other design factors that are influential on the fuel mileage includes camshaft timing, compression ratio, and ignition timing curve attributes.  That’s the (very) short version of what it takes to improve fuel mileage assuming the engine is in good condition to begin with.