Since I finished my 300 rebuild over the past few months I have been trying to get my holley 390 cfm 4bbl carb to run right. I was starting to get discouraged because I had an off idle stumble that only last a few seconds. It would sometimes back fire through the carb. After alot of reading about how to tune a carb I went through a series of different accelerator pump cams, discharge nozzles, and secondary springs. I figured I had to be lean off idle when transitioning from the idle circuit to the main circuit. I kept going up in discharge nozzle size and now its running great. The stock size was .025 and I went to a .040 discharge nozzle with the hollow screw. The stumble is gone now. Only thing left now is to get the plugs a little closer to a tan color. I swapped the orginal primary jet size of 51 2 to a 55 jet to richen it a bit. The number 1 and 6 cylinders where running a bit lean. Any one know why the orginal jets are 51 2? Thats the number stamped on them. After the engine cools I will plug the plugs and check the color again. Thanks for any info.
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1982 Ford F100, rebuilt 300, offenhauser dual port intake, holley 390 carb, EFI exhaust manifolds,walker downpipe, comp cams 260 cam, .020 over, melling oil pump, pioneer harmonic balancer....
1995 Ford F150 300 I6, M5OD and 8.8
i just did my Holly/Offy conversion and am getting the same off idle bog. I figure I'll start with a straight .035 discharge nozzle first. i saw a post that mentioned something about going up 30% from the stock .025 discharge nozzles on our 300s, so I figure .035 is a good place to start. these things are high at $10 each. That's alot to put into a something that my not work or be the correct calibration.
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79 F-100 Custom
Big Block six
Hurst Mastershift 3speed floor shift conversion
Holley 390 4V, Offy intake, EFI manifolds, Chebbie rockers
Summit Racing 3.50 gearset
i just did my Holly/Offy conversion and am getting the same off idle bog. I figure I'll start with a straight .035 discharge nozzle first. i saw a post that mentioned something about going up 30% from the stock .025 discharge nozzles on our 300s, so I figure .035 is a good place to start. these things are high at $10 each. That's alot to put into a something that my not work or be the correct calibration.
Yea I had the same off idle bog as well im currently using a .037 discharge nozzle and the hollow screw that holds the discharge nozzle in place. The holley part number from summit racings website is HLY-121-7. All it has is a hole drilled through the center and two more holes in the side of the screw to allow more fuel to the discharge nozzle when using discharge nozzles larger than .037. If you use the standard solid screw it can restrict the amount of fuel. If you buy a discharge nozzle that is .040 or larger they come with the hollow screws. Check this part number at summit HLY-121-140 and look under notes how it says Includes hollow nozzle screws.
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1982 Ford F100, rebuilt 300, offenhauser dual port intake, holley 390 carb, EFI exhaust manifolds,walker downpipe, comp cams 260 cam, .020 over, melling oil pump, pioneer harmonic balancer....
1995 Ford F150 300 I6, M5OD and 8.8
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