jetting setting

R

ricspicup

Junior Member
in the midst of a 37 ford pickup rebuild . motor is 21 stud flathead with Mallory ignition, 4 in crank, 20 over bore , cyclone heads and thickstun intake dual carb motor rest of pick up is stock except converted to 12 v neg grd.
question is what jetting would I use and how do I determine the correct size? the carbs on had 65 size jets. On adual carb would you have both carbs the same size? They appear to be synced to open same time with current setup
 
Run them as they are if that's the way they previously operated then determine what change has to be made on both (if any)
One of the best ways would be to use an Air Fuel Ratio meter.
 
What carbs are you running? If you're using Stromberg 97's, then you need to know the jet size as well as the power-valve size. You would run the same jet sizes on both - exactly what you need will be determined by how rich/lean the engine runs.

I really can't see running jet sizes of 65 in ANY situation that I've seen (with an engine like yours) - I imagine it should be very rich. If you're running Stromberg 97's - probably around 45 jets, if you're running Holley 94's probably around 48 jets or so. If you're running Holley 94's you also need to know what power valve to run (as they're controlled by your vacuum signal). If you have 6.5 power valves, you may need to go down to 5.0 or so (you'll need to use a vacuum gauge to see where your vacuum drops during acceleration).

As 34PKUP mentioned, it really nice to have an AFR meter and an O2 sensor in the exhaust system (about 30 or so inches back from the engine). If you know how to read plugs - that is another method, though I find it quite hard to read plugs with todays fuel compositions.
 
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A quick check of the AF can be done the old fashion way. Bring the engine up to a fast idle (1000 rpm). Then run your hand over the top of the carb and watch the tac. If the engine rpm comes up your lean . Now at this point you can asume it rich, check the exhaust pipes befor you start.
 
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