Carb setting gurus?
Senior User
Joined: Jan 2000
Posts: 161
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Carb setting gurus?
If any of you have played with holleys a bit and know a thing or two I'd appreciate some help. I'm due to get my 390 back from the shop any day and need to get the carb dialed in before I fire it up. Here's the background: 390 w/Edelbrock aluminum heads, performer RPM intake, .525in lift, 224 degree cam, 9.5:1 compression. I run a '76 F250 (roughly 5500lbs) with 4.10's and 38in. Swampers, and a 4 speed. I just took the engine in for rebuild, it was in the truck before. Problem is, the 750 double pumper on it never allowed it to run just right. I tried different jets and power valves but it always had a big bog under 2300rpm. It also ran a little rich but that's OK, I just didn't jet it down that much, I settled on 73's and 80's. If you all could give me a good baseline jet and power valve setup I'd appreciate it.
Post Fiend
Joined: Feb 2000
Posts: 5,130
Likes: 8
From: Brea CA
Carb setting gurus?
Chilly,
I think you will always have a bog a low rpm because the 750 is too big to flow properly at WOT in that rpm range even on a 390 with 90% volumetric efficiency. The 428CJ ran a 735 vacuum secondary and you can’t beat it. A 600 vacuum secondary would be a lot more tractable but if you must run a double pumper on the street, the 650 would be a better choice all the way up to 5000 rpm. Double pumpers run rich by design no matter what size jets are in them, try starting out with 72’s and 78’s. Get the high flow power valve with the rectangular slots that opens at 6.5”. Your cam is not too radical but will lope at idle so check manifold vacuum to make sure it is above 6.5” while idling or else the power valve will be opening up and make it impossible to tune. I would guess yours would idle between 10” – 12”. Check your throttle plates to make sure they fully seat around the bore when closed and don’t use too heavy a return spring or else you will wear out the throttle shaft. Good luck and happy tuning.
Barry
I think you will always have a bog a low rpm because the 750 is too big to flow properly at WOT in that rpm range even on a 390 with 90% volumetric efficiency. The 428CJ ran a 735 vacuum secondary and you can’t beat it. A 600 vacuum secondary would be a lot more tractable but if you must run a double pumper on the street, the 650 would be a better choice all the way up to 5000 rpm. Double pumpers run rich by design no matter what size jets are in them, try starting out with 72’s and 78’s. Get the high flow power valve with the rectangular slots that opens at 6.5”. Your cam is not too radical but will lope at idle so check manifold vacuum to make sure it is above 6.5” while idling or else the power valve will be opening up and make it impossible to tune. I would guess yours would idle between 10” – 12”. Check your throttle plates to make sure they fully seat around the bore when closed and don’t use too heavy a return spring or else you will wear out the throttle shaft. Good luck and happy tuning.
Barry
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