Tuning a Holley 390 4 barrel to run on an AMC 258

[Oregonite]

I have a Holley 390 4 barrel on my AMC 258 i6 / 4.2l with an Offenhauser intake and a header and HEI distributor. Currently I am running just on the primaries. I would like to try and get the secondaries working. I need to get a new diaphragm for secondaries. Is there anyone out there running the same setup and how is their engine performing with the secondaries hooked up. Do I need to change the secondary jets or just run what came on the carb from the factory.

 

[Peanut Butter]

I doubt that you will find hardly anybody that runs a 4 bbl carb with the AMC 258 i6 / 4.2l . The engine was never desined for that type of set up. The secondaries are for high rpm / speed levels. The AMC 258 i6 / 4.2l is a low rpm high torque application. Maybe somebody with some carb experience in other vehicles will chime in. The whole idea of headers, 4 bbl carb, etc. is high speeds......something the jeep is not designed for. Most people that want more power will go to a V8.

 

[Oregonite]

I doubt that you will find hardly anybody that runs a 4 bbl carb with the AMC 258 i6 / 4.2l . The engine was never desined for that type of set up. The secondaries are for high rpm / speed levels. The AMC 258 i6 / 4.2l is a low rpm high torque application. Maybe somebody with some carb experience in other vehicles will chime in. The whole idea of headers, 4 bbl carb, etc. is high speeds......something the jeep is not designed for. Most people that want more power will go to a V8.

Anything to help make an engine breath better is what it's all about. Just because it is a six cylinder doesn't mean you it can't be improved. There are lots of Jeep owners making these changes.

 

[hagar]

What you are doing was done heavily back in the mid 70s and 80s.
The 390 carb works well on small displacement engines.
Might be harder trying to find a Jeeper today using that set-up.
You just need to find the right Jeeper who can help you.

 

[Torxhead]

Years ago I did play with the performance Holley carbs. For setting primary jet size, I would just go by the fuel burn deposits on the spark plugs. For the secondary, I would just go with the same size or a bit higher. I would set the secondary rate opening by different tension springs for the Holley that was sold in a kit by whatever felt good as a good time for them to come on.

 

[Oregonite]

Years ago I did play with the performance Holley carbs. For setting primary jet size, I would just go by the fuel burn deposits on the spark plugs. For the secondary, I would just go with the same size or a bit higher. I would set the secondary rate opening by different tension springs for the Holley that was sold in a kit by whatever felt good as a good time for them to come on.

Thanks for setting things straight about running a small four barrel on the AMC 258 i6 / 4.2l . These engines were highly modified back when the Javelins were drag racing in the late 60's and 70's. I think it is fun to experiment with in-line sixes.:ty:

 

[Oregonite]

What you are doing was done heavily back in the mid 70s and 80s.
The 390 carb works well on small displacement engines.
Might be harder trying to find a Jeeper today using that set-up.
You just need to find the right Jeeper who can help you.

 

[mstar]

Just a FYI on your project.I ran a 390cfm holley on a clifford research manifold and headers. It did increase perf. but was a bitch to tune.We spent several days experimenting and got it to run fairly civil. This was mostly due to a very good carb man that knew how to modify circuits and make things work.this was way beyond just simple jetting and acc. pump,etc. adjustments.This was on a 1973 w/AMC 232 i6 six .Later went to a LT1 350 with Muncie 4 sp. Lot better.
You should be able to get it going with out to much trouble. mike

 

[tarry99]

I have a Holley 390 4 barrel on my AMC 258 i6 / 4.2l with an Offenhauser intake and a header and HEI distributor. Currently I am running just on the primaries. I would like to try and get the secondaries working. I need to get a new diaphragm for secondaries. Is there anyone out there running the same setup and how is their engine performing with the secondaries hooked up. Do I need to change the secondary jets or just run what came on the carb from the factory.

I think your confused............this is only a 390 CFM carb and the secondaries are hooked up ..............and are strictly vacuum operated.....meaning that until the engine RPM demand and signal from the engine intake track is great enough to support the additional air / fuel.........it will only run on the primary's......just the way the carb was designed to do..........Changing the diaphragm or related vacuum secondary springs will do little to nothing other than make it run worst until your motor RPM at WOT increases the demand....

And to my knowledge there is NO secondary jets in a Holley 390 nor is there a metering block back there unless someone modified it.

If your motor had a better intake track...........and the cylinder head had larger valves and a better camshaft to go with it and the port volume's were increased within so it would flow more air & fuel and the compression ratio was also increased........with all of that perhaps the motor would then have the ability to run at higher RPM's make more power while sending a greater air / fuel demand signal to the carb and thus the secondaries would open.

Short of that your just dumping un-burned fuel out the exhaust pipe.

 

[mstar]

actually the spring kit is quite useful on some applications. and the secondary metering plate does have removable jets on some models.they make the quick change spring kit for a reason. mike

 

[Crazy Horse]

Oregonite,
In the late 70's I had a AMC 258 i6 / 4.2l in my 77 CJ5 . Shaved the head to get the compression up ported and polished and added a Offenhouser dual port manifold and a 450 cfm Holley 4 barrel. Put a Clifford research header on it with a 2.5" exhaust sys. through a turbo muffler. Just doing this doubled the power output of the Jeep. The secondary's worked great and actually got better fuel economy than the Carter on and off the road as long as you weren't playing with the throttle. Steep inclines or the bouncies were never a problem. With the serial numbers you should be able to call Holley for replacement parts they are good about helping people in need and steering you in the right direction.

 

[Oregonite]

I think your confused............this is only a 390 CFM carb and the secondaries are hooked up ..............and are strictly vacuum operated.....meaning that until the engine RPM demand and signal from the engine intake track is great enough to support the additional air / fuel.........it will only run on the primary's......just the way the carb was designed to do..........Changing the diaphragm or related vacuum secondary springs will do little to nothing other than make it run worst until your motor RPM at WOT increases the demand....

And to my knowledge there is NO secondary jets in a Holley 390 nor is there a metering block back there unless someone modified it.

If your motor had a better intake track...........and the cylinder head had larger valves and a better camshaft to go with it and the port volume's were increased within so it would flow more air & fuel and the compression ratio was also increased........with all of that perhaps the motor would then have the ability to run at higher RPM's make more power while sending a greater air / fuel demand signal to the carb and thus the secondaries would open.

Short of that your just dumping un-burned fuel out the exhaust pipe.

I'm not as confused as you might think. I had to replace the secondary diaphragm because the original one wouldn't hold vacuum anymore. I am going to replace the metering plate with a metering block. For now I got the secondaries to work. I used the stiffest spring in the spring kit and it worked great.

 

[Crazy Horse]

:If your motor had a better intake track...........and the cylinder head had larger valves and a better camshaft to go with it and the port volume's were increased within so it would flow more air & fuel and the compression ratio was also increased........with all of that perhaps the motor would then have the ability to run at higher RPM's make more power while sending a greater air / fuel demand signal to the carb and thus the secondaries would open.

Short of that your just dumping un-burned fuel out the exhaust pipe.

I can tell you there was defiantly no unburned fuel running through that AMC 258 i6 / 4.2l . The secondary's came in when ever they were called upon by the go pedal, not necessarily at predetermined high rpms. No lags, sputtering, or delays in acceleration. You could hear and feel the secondary's come into power. And if I remember right the Holley 450 was a marine carb. That 6 would run nose to nose with a AMC 304 at the races.

 

[tarry99]

actually the spring kit is quite useful on some applications. and the secondary metering plate does have removable jets on some models.they make the quick change spring kit for a reason. mike

Yes Mike, Holley makes a ton of tune up parts for various performance applications......... But unless you have a motor below the Carburetor that has the ability to increase swept cylinder volume and signal demand to the secondaries it's not going to make a difference performance wise............. Open them up early or late if you can't burn the fuel there is no net gain!

Kinda like putting Lipstick on a Pig!

 

[Torxhead]

Yes Mike, Holley makes a ton of tune up parts for various performance applications......... But unless you have a motor below the Carburetor that has the ability to increase swept cylinder volume and signal demand to the secondaries it's not going to make a difference performance wise............. Open them up early or late if you can't burn the fuel there is no net gain!

Kinda like putting Lipstick on a Pig!

Right, it is a tractor engine.

 

[elwood blues]

I doubt that you will find hardly anybody that runs a 4 bbl carb with the AMC 258 i6 / 4.2l . The engine was never desined for that type of set up. The secondaries are for high rpm / speed levels. The AMC 258 i6 / 4.2l is a low rpm high torque application. Maybe somebody with some carb experience in other vehicles will chime in. The whole idea of headers, 4 bbl carb, etc. is high speeds......something the jeep is not designed for. Most people that want more power will go to a V8.

I think your confused............this is only a 390 CFM carb and the secondaries are hooked up ..............and are strictly vacuum operated.....meaning that until the engine RPM demand and signal from the engine intake track is great enough to support the additional air / fuel.........it will only run on the primary's......just the way the carb was designed to do..........Changing the diaphragm or related vacuum secondary springs will do little to nothing other than make it run worst until your motor RPM at WOT increases the demand....

And to my knowledge there is NO secondary jets in a Holley 390 nor is there a metering block back there unless someone modified it.

If your motor had a better intake track...........and the cylinder head had larger valves and a better camshaft to go with it and the port volume's were increased within so it would flow more air & fuel and the compression ratio was also increased........with all of that perhaps the motor would then have the ability to run at higher RPM's make more power while sending a greater air / fuel demand signal to the carb and thus the secondaries would open.

Short of that your just dumping un-burned fuel out the exhaust pipe.

Yes Mike, Holley makes a ton of tune up parts for various performance applications......... But unless you have a motor below the Carburetor that has the ability to increase swept cylinder volume and signal demand to the secondaries it's not going to make a difference performance wise............. Open them up early or late if you can't burn the fuel there is no net gain!

Kinda like putting Lipstick on a Pig!

Right, it is a tractor engine.

You guys want to know why there's no traffic on this site? This is why. You want to know why all of the old regulars and guys like me that were here from the beginning of this sight have all basically checked out. This kind of ****. None of you had anything productive to add to OP's post, you just wanted to criticize his question and tell him that you "know" why it won't work. Not that any of you have actually tried, you just know.

Up until a couple of months ago, I ran a Holley 390 on my AMC 258 i6 / 4.2l , for like 4 years. It ran great, and got better MPG than a 2100 2bbl did.

I've stayed around and helped CJ out with this site because he's a great guy who puts this site out there for us, at a huge cost to himself, free for us.

I really think this has about sealed it for me. I'm locking this thread, I hope the 3 of you have fun running everyone away from this site. Organite, I apologize for hijacking and closing your thread, open a new one. Hopefully this time, the members of this once great site can help you out and not drag your idea through the mud because they don't like it.