Carb jet recommendations
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Tuned
Joined: Mar 2005
Posts: 479
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From: Mesa AZ
Carb jet recommendations
Ok guys, the 390 is due back from the machine shop at the end of the week. I have found a factory 4barrel intake and a holley 600cfm 4 barrel vacuum secondary carb. Obviously it will require some tuning once its fully running. Im looking for a recommended primary jet size and power valve to START with. The carb currently has #60 jets and a #8 power valve. I have a set of #69 jets and a 6.5 power valve from my two barrel that was on it, at my disposal as well.
Trying to find a baseline to start with for cam break in, then I can mess around with tuning. It will be a pretty stock 390 with the exception of being .40 over.
Trying to find a baseline to start with for cam break in, then I can mess around with tuning. It will be a pretty stock 390 with the exception of being .40 over.
Posting Guru
Joined: Jan 2011
Posts: 1,670
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What's the # on the carb? Should be on the front of the air horn. Course this also depends on what your cam is and your vacuum pressure is. In 'general', your power value is 1/2 of your vacuum pressure. For instance, if your vacuum is 17 or 18, you might try a 8.5 power valve.
What all have you done to the motor?
What all have you done to the motor?
Posting Guru
Joined: Jan 2011
Posts: 1,670
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If your carb is still in stock form, I believe your primary jet is #64 and your powervalve should be 6.5. It should still have a metering plate for the secondary, if it hasn't been switched out.. Can I assume it still has the electric chock?
Of course I'm an old fart and most likely have no idea what I'm talking about.
Of course I'm an old fart and most likely have no idea what I'm talking about.
Mountain Pass
Joined: Apr 2018
Posts: 111
Likes: 4
From: I live in Oregon,
I tried running 68's on my 390 w/stock intake and they weren't enough had to go to 70 otherwise at cruising speed it constantly felt like it was starving for gas. soon as i gave it a little and the power valve opened no problem. I asked around the FE forum and C12H24 [Gerry] helped me out with that advice and he was right. One thing I should mention though is that I'm running alot higher cfm. than you 730 as to 600 you might not need so much jet but i have a feeling he would know. Good luck with your new engine
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Tuned
Joined: Mar 2005
Posts: 479
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From: Mesa AZ
I don't honestly know the measurement . Nothing has been machine off other than a small amount to clean the heads up.
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Thread Starter
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Tuned
Joined: Mar 2005
Posts: 479
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From: Mesa AZ
If your carb is still in stock form, I believe your primary jet is #64 and your powervalve should be 6.5. It should still have a metering plate for the secondary, if it hasn't been switched out.. Can I assume it still has the electric chock?
Of course I'm an old fart and most likely have no idea what I'm talking about.
Of course I'm an old fart and most likely have no idea what I'm talking about.
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Truck 390 pistons are sunken. Car pistons (or 360 pistons) are nominally .015. Significant compression ratio difference.
Hotshot
Joined: May 2003
Posts: 11,141
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From: south louisiana
Set the carb back to what it came with. Sounds like someone installed jets that were a wee bit too small. I'll have to look in my Holley book, but 64's and a 6.5 PV sound about right. Run it like that and then read the plugs before doing anything else to it. Set the initial timing, then the idle speed, then mixture screws, and check for slack in the accelerator pump linkage. The sec spring should be a black or brown.
Posting Guru
Joined: Jan 2011
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I agree with what baddad said but I would go one step more. Lock down the secondary so it doesn't open. Take it out of the equation. Tune the primary. Than bring the secondary into the equation. Unless you are very experienced in carb tuning, the secondary opening can very easily trick you. Secondaries opening can sometimes have a flat spot and can easily confuse someone with the idea of a boost of power when actually it's a lean bog.
when you add the secondary into the equation, use the 'paper clip' trick to see just how far the secondary is actually opening up - if at all. Search 'youtube' for procedure. There is all kinds of videos posted, I'm sure.
Also, something you might think about investing in, is a 'secondary quick change spring kit'. Makes things so much easier to fine tune the secondary system.
when you add the secondary into the equation, use the 'paper clip' trick to see just how far the secondary is actually opening up - if at all. Search 'youtube' for procedure. There is all kinds of videos posted, I'm sure.
Also, something you might think about investing in, is a 'secondary quick change spring kit'. Makes things so much easier to fine tune the secondary system.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
My book doesn't give the jet numbers for an 80457, but the 1850's came with #66 main jets. The smallest I can find for a 600 is #63. So #60 is way too small for a 600 cfm carb. Power Valve for an 1850 is a 6.5. The PV for an 80457 isn't listed but some did have 8.5's. That might be the correct one for a 600 on a 390 as it will generate more vacuum. Masked Rider's points about the secondaries sounds good to me. I recommended the stiffest springs there just to keep the sec's from opening too soon with the higher vacuum with this carb on a 390.
Posting Guru
Joined: Jan 2011
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Very interesting. Hmmm.
what are the 'date' #s under the model #s on the carb. Since there is a. '-1', this means it is a later carb. Wonder if the '--1' carb is getting into the snog series years and uses the smaller jet. I have never seen that large of jet in that model carb. But that doesn't mean much. I haven't seen everything. But I have seen misprints.
If I get a little time, I will try to find an old Holley carb book I have that lists just about every carb Holley has ever made. Even some of the experimental ones. There is a graph that shows throttle volume and min and max of jet possibilities.
Just need the time to dig it out. I think I know where it is. Well, maybe. Hmmmm. Haven't messed around with that stuff for some time.
69 jet, could very well be, what is the name of the book you got that from? Wonder if its the same as the old one I have. If the throttle plates are the same size, it must run rich with a 69. Perhaps for a racing application? I have NO idea. Now I'm interested.
what are the 'date' #s under the model #s on the carb. Since there is a. '-1', this means it is a later carb. Wonder if the '--1' carb is getting into the snog series years and uses the smaller jet. I have never seen that large of jet in that model carb. But that doesn't mean much. I haven't seen everything. But I have seen misprints.
If I get a little time, I will try to find an old Holley carb book I have that lists just about every carb Holley has ever made. Even some of the experimental ones. There is a graph that shows throttle volume and min and max of jet possibilities.
Just need the time to dig it out. I think I know where it is. Well, maybe. Hmmmm. Haven't messed around with that stuff for some time.
69 jet, could very well be, what is the name of the book you got that from? Wonder if its the same as the old one I have. If the throttle plates are the same size, it must run rich with a 69. Perhaps for a racing application? I have NO idea. Now I'm interested.