Tuning Your Holley # 80507 390HP


A 390 cfm carburetor used in a racing application presents some unique tuning challenges because of the very small venturi size vs. the airflow pulled through them because of the high horsepower engine.  The following are some tips on tuning and maintaining your carburetor that we have learned over the years through dealing with hundreds of different applications and drivers.

Float Level – The fuel level in the bowls should be set about 1/8" below the sight hole so that you have to jostle the car heavily to get the fuel to come out the sight hole when running.  We set the floats in the shop to a particular setting; but fuel pressure dictates fuel level.  Therefore you must set this when you first install the carburetor.  If the floats are set too high the carburetor will be too rich at low rpm and/or part throttle conditions.  You will particularly notice this when coming off the corners.  It should be noted that at a no-load situation, such as in the pits setting the timing, the carburetor will always have a slight rich condition at probably 3000 rpm.  This is due to the fact that the strong booster signal and the fuel metering that enables us to make good torque numbers is pulling a lot of fuel in at this rpm/vacuum.  At the same time there is no way to burn the fuel because there is no load, and the engine will flutter.  Finally, after setting the floats, always give the carburetor time to burn off enough fuel to reopen the needle/seat so that your new setting is really what you are seeing.  Many times people lower the float too much because they haven’t waited a sufficient amount of time for the fuel to burn off and the new setting to take.

Throttle Blade Adjustment – The throttle blades should be set at an rpm that is as low as possible to keep from pulling through the high speed circuit/the boosters at idle.  Special attention should be taken to ensure that the primary and the secondary throttle blades are always adjusted as low as possible, with the primary never being more than 1/2 round open. If you should lose your settings just back them both completely off, put 1/2 round in the primary and do not adjust this again. Put 1/2 round in the secondary, and start there.

Idle Mixture Screws – Always adjust the 4 idle mixture screws evenly.  The settings on all 4 should be the same. Our original setting is 1 1/2 turns out. If for some reason your engine absolutely needs a different setting on one side or one corner you can just about bet something is wrong with the engine causing it to have a different signal/vacuum on that side/corner.

Fuel Pressure – 6.5 to 7.5 pounds.  You should absolutely know what your fuel pressure is!  Many racers have no clue what their fuel pressure is, and they chase engine problems for weeks when a simple fuel pressure gauge would have indicated the problem straight away.

Vent Tube Clearance – You must have at least ¾” clearance above the vent tubes.  It is ok to lower the vent tubes if absolutely necessary, but you will begin to run the risk of fuel spilling over in the turns if you are not careful.

Pump Circuit Tuning - Because of the differences in track conditions and driving styles you may need to adjust the pump circuit, i.e. pump cam/pump arm, to correct "off corner" stumbling issues.  If an engine stumbles two or three times after the driver steps into the throttle this usually indicates too much fuel and can be corrected by adjusting slack in the pump arm or installing a smaller pump cam. We recommend adjusting the secondary first to the point of even removing the secondary pump cam thereby disabling the secondary pump squirter.  If an engine has a "dead hesitation" and then picks right up and goes, this usually indicates not enough fuel on the pump circuit.  First check that both pump arms have no slack in the adjustment then proceed to increase pump shot with either a larger pump cam or a larger pump nozzle.  This adjustment should be done on the primary side first.