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junkyardjeff 08-29-2005 02:29 PM

Jet sizes for a 750 holley
I put a 750 holley on a stock 390 in my 65 galaxie,it seems to run rich and blows a bit of black carbon out the exhaust so I pulled the front bowl off thinking I blew out the power valve but when I removed the metering block no gas came out and it was dry so thats not the problem. It had 72 jets and I put in 67s and how far down can I go as I have a set of 64s that might go in. I have a feeling that a 750 is probably too big for a stock and high mile 390 but what kind of puzzles me is that it smells rich and will carbon up the motor but the plugs did not look like it was running too rich but I wil give the 67 jets a try but I was wondering who out there is running a holley carb on a 390 and what jets they have in it. Jeff 09-08-2005 08:14 AM

That is a big carb but you should be able to tune it for your 390. The way I do it is with an Edelbrock O2 senser. See here.

tresi 09-08-2005 03:27 PM

I usually end running 68 jets in a 750 so you're in the ball park, maybe getting on the lean side.

club327 09-09-2005 12:20 AM

Yep, #72 jets is what the 750 came with. I did exactly the same with my 750, in that I fitted #67 jets for a lean down as I only had one #68 jet in my kit. Found it cruised okay and that it had power, but it really stumbled on throttle openings. My engine's a 327 and the yes the carb was too big but was'nt as crazy as most would have you believe. It did however, have that benzine rich smell from the exhaust and we all here put it down to the venturis being too big to achieve an effective fuel pulling signal. Have since gone to a 600 vac sec unit. The smell's now gone and the throttle metering control is like EFi. The 750 should be great for your donk if it's been warmed up. Otherwise I reckon one of the Holley, Rochester or Edelbrock spreadbore units would be just the ticket for driveabilty and economy. Rob

junkyardjeff 09-14-2005 09:13 PM

The 67 jets were a little too small as the motor pinged a bit even with the timing back to specs and premium so I put in some 69s and it runs fine now,I am now going to convert to electronic ignition. I found another possible problem,I have a chrome aircleaner on it and it sits down lower on the carb and with the filter element I had on it there was not too much room for air flow so I found a taller filter and I think the motor will breath a bit better now. Jeff

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