CARBURETOR TUNING

The stock carburetor is a Holley 575-580cfm vacuum secondary carburetor. This carburetor is not easily adjusted because it is preset for a stock motor and all adjustment holes are "Tamper Proof". If you are not strongly familiar with Holley carburetors and want more performance from your carburetor, I suggest buying a new Holley carburetor. Although the stock carburetor appears the same as every other Holley carburetor it is in fact very different in several ways. If you motor is basically stock, meaning with stock heads and cam a Holley 600CFM is a great start. Once you have the new carburetor remove the float bowls from the stock carburetor and install them on the new Holley. This will keep the vapor lines on the carburetor operational for emissions. Swapping this will not cause loss of performance. If you are not worried about emissions put the new carburetor on as is. If you have a modified motor you should choose a carburetor compatible with your modifications.  If you have not been to the Holley web site download their Holley carburetor tuning document and read it before touching the carburetor! This document covers some tuning and explanations of each part of the carburetor. The Holley site will also help you determine the proper carburetor for your application.

Holley's 4180 4-barrel Carburetor

If you are not highly familiar with Holley carburetors and the functions of the carburetor I suggest you download and read this document from Holley before touching the carburetor.
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Jets

The stock carburetor comes with size 62 jets in the metering block. This may vary so it is best to check by removing the float bowl from the front of the stock carburetor and check the jet size. There will be a number stamped on the jets. Make sure the new carburetor has the same size jets to start with. If not you can buy new jets or swap the old ones from the stock carburetor. Take a good careful eye and inspect the stock used jets before installing them into the new carburetor making sure there is no debris or other imperfections. Be careful not to strip out the jet when removing or installing. Final jet size will vary per motor, climate and elevation.

Vacuum Secondarys

An easy upgrade to this is a Holley Vacuum Secondary Adjusting kit. It allows you to manually change the point in which the secondarys open with the turn of a screw. There is also a Quick Change Secondary Spring kit which is cheaper and allows you to change the secondary springs with out disassembling the secondary vacuum housing. A long yellow Holley spring seems to work best on a slightly modified motor. If present the check ball should be removed from the secondary vacuum housing first before changing spring rates as this will also allow the secondarys to open sooner.

Accelerator Pumps

The stock carburetor comes with a 30cc accelerator pump. Stay with this size for starters. The after market carburetor will most likely have a 30cc pump as well. 

Accelerator Pump Cams

You can change the pump cam on the carburetor to change the amount/rate of fuel delivered to the discharge nozzles.  I would use the pump cam supplied with the new carburetor for starters. You may experiment later with pump cams to dial in your carburetor if needed.

Discharge Nozzles

The stock carburetor comes with size 28 nozzles. The discharge nozzle works with the accelerator pump. The gas pumped to the carburetor by the accelerator pump is discharged into the motor through the discharge nozzles. There is only one located on the stock carburetor. A double pumper carburetor has two discharge nozzles and two accelerator pumps. The nozzle sizes vary and allow more/less fuel to be pumped into the motor. Avoid putting oversized nozzles on your carburetor. Too much nozzle may cause poor performance, flooding, bogging or backfires. 

Idle Mixture Screws

The main idle AF mixture tuning of the carburetor is done with the idle mixture screws. The stock carburetor does not have these screws located on the metering plates like most Holley's, they have been relocated to the throttle body plate and blocked off behind "Tamper proof" plugs. A vacuum gauge is one of the most accurate ways of setting the idle mixture. Using the vacuum gauge, set the mixture screws so that you obtain the highest vacuum at idle at your desired idle RPM. Make sure all screws are set at the same amount of turns to keep the idle system balanced. Check and recheck your plugs as well for an indication of how you motor is running. On an after market carburetor without a vacuum gauge, adjust all the mixture screws all the way in (Clockwise) and lightly bottom out. Avoid over tightening! Turn all the mixture screws out 1 1/2 turns (Counter Clockwise). Start the motor and let warm up to operating temperature. Set your fast idle speed to the desired RPM. (Standard Trans around 700 to 900 Rpm's and Auto's from 800 to 1000 Rpm's) Next turn the mixture screws out 1/4 turn at a time until the engine speed stops increasing. If the engine RPM does not increase turn the screws inward 1/4 turn until engine speed begins to drop. Once you have reached this point turn the screw 1/8 turn the back the other direction in which you were turning the screw or until you have the most manifold vacuum at your given idle speed. (All mixture screws must be the same amount of turns out to keep it balanced). Do not adjust more than 1 turn either way. If no difference in RPM is noticed your carburetor is probably not the correct model for you application. 

Power Valves

The stock power valve is a two stage power valve, size is 11" / 6.5' hg. Use a single stage 6.5 in the new carburetor. Most out of the box Holley's have a 8.5 or 6.5 hg power valve so you shouldn't have to change these unless you use a super high lift cam or notice the power valve opening under high loads at cruising speed.