Running the Holley 390 cfm -
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post #1 of 2 Old 12-08-2018, 04:08 PM Thread Starter
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1995 YJ Wrangler 
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Running the Holley 390 cfm

This write-up is intended to aid in the dialing in of the Holley 390, List #R8007. This set-up is not ‘better’ than a properly installed and tuned factory or aftermarket fuel injection system off-angle, so keep that if you have it in good order. It is not the only carbureted choice available. However, it is considerably cheaper to obtain, is easily modified, has fewer expensive parts to break/replace, and is more than adequate for a daily driver, sand, woods, and mild trails (<10 - 12 degree angles). You won’t gain (IMHO) any distinct advantage from special ‘Offroad’ carburetors that can’t be dialed-in with the 390, and you will likely get better mileage and low-end performance form this little jewel than the larger offerings.

As supplied, the 390 is a 4160 configuration with vacuum secondary and a secondary metering plate, single pump shooter, 30cc. accelerator pump, and electric choke. The complete list of included part specs is located at the link below:

Having run this carburetor for over 20 years on various setups, there are some prudent changes to the box-stock product to immediately consider before installation as you will likely do many of these modifications eventually.

1. # 20-114 Metal accelerator pump arm.
The standard arm is now plastic (#20-78) and inferior to the long-wearing metal arm. Use of the plastic arm will necessitate frequent adjustment as it wears.
2. # 20-59 Quickchange Vacuum Secondary Kit
Tuning for the secondary is vastly easier with this kit. Definitely do this prior to install.
3. #20-13 Vacuum Secondary Spring Assortment
Get this for tuning to your vehicle. Order a combo kit (items 2 & 3) if you can find it.
4. #34-6 Secondary Metering Block Conversion
Removes the ‘plate’ and allows for using changeable jets on the secondary side.
5. REMOVE the sintered bronze fuel filter directly inside the fuel inlet. Buy a quality inline filter and place it between the pump and carburetor. See the carb pic below and note
the red arrow/box. This is where Holley hides the filter you should remove.
6. #6-513 Offroad Needle and Seat IF you plan on bouncing around, a lot, these are a go-to fix for flooding at off-angles and obstacles.
7. DO NOT REMOVE the Electric Choke and get tempted to run the manual conversion kit. Trust me on this.

You may find these units quite used (cheaply) as they have been around a while. A tune-up kit #37-933 will contain many of the other parts referenced here and salvage all but the most beat-up of units. Note the discussion of base-plate later as another option.

Your carb packaging will contain handy reference guides for pre-installation adjustments also available at this link:

Scroll through for all of the specific video instructions that address your combination.

Never assume that everything is fine out-of-the-box. Always check primary throttle plate position, secondary plate position, and choke setting and operation, and fuel bowl float setting or you could wind up taking it back off to make certain adjustments. See the following:

This carburetor can be run on 2 (adaptor required) or 4-barrel manifolds. You will appreciate the extra horsepower and mileage of bolting up to a 4-barrel manifold. Clifford and Offenhauser are typical choices. The Clifford is an open plenum design better suited to mid-range RPM and up usage. The Offenhauser is a ‘dual-plane’ design and separates the primary and secondary runs for increased low-end torque, thus a better daily driver choice. Regardless of which manifold you choose (or have) purchase a 4-hole carburetor spacer, preferably phenolic plastic like Spectre Performance #5764, or Summit Racing's SUM-G1412. Phenolic is heat-insulating and keeps the fuel bowls from percolating. The additional - 1” isolated travel helps raise fuel velocity and torque.

You MAY be restricted by your power brake booster or proportioning valve for air cleaner size. I run a 10” diameter topped with stacked filters (5.5” tall).

Now that you have installed the 390 and flushed the fuel line, plumbed in the inline filter, and run the pump just enough to fill the carb with fuel, stop and check for obvious leaks. Once satisfied, it’s time to make running adjustments. Adjusting the carb for proper operation will also involve redoing your timing settings, as your idle speed will be affected.

Once the motor has warmed, check and adjust the float (gas) level at idle on both the primary and secondary bowls BEFORE any other permanent adjustment. It’s ok to fiddle with the idle speed at this point, if it’s too high or low just to get in the ballpark. You will be changing it several times while tuning. Refer to the link above to find the instructions you’ll want.


The 390 will run on your 258 reasonably out-of-the-box even if you have built a 4.6 stroker. However, the cam, cylinder over-bore, head choice, gearing, tires, total weight, geography and usage will require tuning efforts to be well-satisfied with your ‘new’ system if you want the best out of it. Your upgraded 258 can produce from 210 to almost 300 hp depending on modifications made and have great street manners.


If you live between sea-level and 500 ft. elevation #51 main and secondary jets are a good initial choice for hot weather while you may go to #50 if your rig is very light or even #49 during the winter. The primary side already has #51 jets, so you only need to purchase two for the new metering block if you do that conversion. Higher elevations may need to go slightly smaller due to the (lack of) air density. You do not HAVE to keep secondary jets the same size as primary ones, although you should install jets in ‘pairs’. Jetting will be determined overall by gas mileage, consistent behavior, and less by raw performance, although after doing performance tuning you may find you have too much or too little jet and change accordingly. Overly rich jetting causes poor mileage and black smoke from the tailpipe. Overly lean jetting will cause hesitation and jerking under light part-throttle cruise. Reading your plugs along with how the vehicles behaves will give you the information needed to change in the proper direction. These are nearly indestructible parts, so grab a selection of used if you can. See below:

Initial Tune for Idle

Idle air control screws (2) should be initially dry-set at 1 1/4 to 1 1/2 full turns out (counterclockwise) from a lightly closed (all-in clockwise) position.
See this:

Shooter Selection

After adjusting the distributor for best static timing, and eliminating ping (as tested by rapid accelerations in 4th gear), and getting the best vacuum reading at idle, you’ll be testing the performance and tuning the accelerator pump cam and shooter sizes. You will be looking for ‘bogging’ by ‘flooring’ the carburetor and rapidly accelerating. The vehicle will either bog first and then ‘jump’ forward or it will start quickly then ‘die’ only to start accelerating again. Each is related to a different cause. See this:

Immediate bogs mean the shooter size is likely too small and not allowing enough gas into the motor. The 390 is stock with a #25 shooter. For many applications (larger than stock tires) the #28 is a better choice. If you have a heavily loaded Jeep (armor/winch/hardtop/cages/etc.), or are geared with a stock 2.73 or 3.07 you may need to be in the #30 to #35 range.

Accelerator Cam Selection

The 390 comes stock with the orange cam in the #2 position. AFTER getting your shooter size roughed in,you will want to try changing to the #1 hole before deciding to go with another cam. Cam Kit #20-12 has a variety of cams to tailor your vehicle’s idle-to-main circuit transition. Vehicle weight and gearing will determine when the gas needs to arrive. See this chart for gas flow by throttle position:


Everything starts to wear out. You can run the same carburetor for 10 – 20 years with only minor overhaul. One thing will happen though that deserves mention.

At some point, the sleeve bushings that seal the throttle shafts will wear enough that you will notice very slight weepage around the ends of the shaft where they meet the main body. This is normal and cosmetic until the wear advances and starts to cause a fluctuation in the vacuum signal that affects the idle quality. This will occur sometime around the 15,000 – 25,000 mile-mark. At this point it is time to re-bush the shafts. However, without highly specialized equipment and expert skill, this isn’t a DIY. You have a few options when you get here. I suggest calling Holley Tech Line and placing an order for Part #12R7800-3AM. This is a complete, bolt-on base-plate for the 390 carburetor.

This base-plate is hard to come by. Holley does not list it anywhere for sale as a spare part. It must be pulled from the factory production line at a corporate level (remember the COPO orders for Ford and Chevy in the 60’s and 70’s?) and will require about 3 weeks to be fulfilled. This is quicker and cheaper than getting your base-plate re-bushed and far more reliable! The assembly can be purchased by phone through Holley. Above you'll see the plate removed from the main body. Whether you're ordering/installing this or adjusting it as supplied on the new carb, note the 2 idle screws. The secondary MUST be adjusted from the bottom, so DO THIS BEFORE you mount the carb! Circles denote where the proper adjustment will be measured at the transition slots.


Using either a 7120 (best) or 0636 head casting is preferred. Because these are designed for fuel injection manifolds with the small ‘injector hump’ at the top, fitment of your 4 barrel manifold MUST be done carefully.

Clifford instructions state you should ‘remove the manifold locating pins from the head’ prior to installation. You DO NOT NEED TO DO THAT! Installation is harder and more prone to leaking doing this.

The late FI manifolds use these pins to exactly locate the position and have a pair of bosses with holes that slip over the pins. Both Clifford and Offenhauser were designed to locate in a similar way on the older heads, but about ” higher up (they were developing a different design, so IF you have that model, that may be OK). You have two very good options.

1. Grind a half-moon into the bottom of each boss directly below the existing pin hole on each end of the manifold. Do so gradually until you have the fit you want.
2. Cut the dowel pins down. They will resemble cabinet shelf pins when done (full where located in the block and cut at the exit).

Either way should be done gradually with trial fitting until the manifold fits up to the gasket with at least 1/16th “of compression around the port opening.

While this sounds more complicated than just ‘removing the pins’, either of these ‘hacks’ will give you a resting point for the manifold and eliminate any future slipping (not to mention not having to try and hold it while tightening bolt!).

Once mounted, your carburetor will be ‘sideways’ on the manifold. Unlike a motor-centered V-8, the mount is to the driver-side of the motor. Clifford and Offenhauser cause the primary end to be pointed directly at the motor (90 degree turn) instead of in line with the center axis. This is meant to be, not wrong, for even flow. However, this does present some tuning challenges as one sight-plug screw and one idle screw face the firewall. A placeable hand mirror is a good tool to have.

Throttle control can be challenging, but Mr. Gasket, Lokar, Spectre Performance, and Summit have a universal set-up that is ideal for this configuration and exactly replaces the factory cable. Return spring setup can vary. Dorman carries assortments for this. I recommend two separate springs for even throttle tension and control.

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post #2 of 2 Old 12-23-2018, 08:25 AM
2004 TJ Wrangler 
Join Date: May 2008
Location: Lake in the Hills
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Thanks for the work involved in figuring out a good use for the 390.
Thanks for writing it all down.

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