Holley #4777/650

This carburetor was developed specifically for the DIRT Sportsman Modified series.  While this carburetor is not as restrictive as many of the other “straight leg” booster carburetors, it is power limiting.  When the rules were written, the intent of the restrictive carburetor was (1) to slow the cars down and (2) to equalize the performance of the engines.  They did accomplish slowing the cars down but VDL Fuel Systems has done a superior job of keeping the performance unequal!

Whenever a carburetor is improperly sized to the engine there are a variety of problems that arise.  The air/fuel circuitry was not designed for the air speed and booster signal of this application.  What results is a fuel curve that is too lean on the bottom and too rich on the top.  This is exactly the opposite from what the fuel curve should look like. VDL Fuel Systems has done extensive dyno and track testing to recalibrate the air/fuel circuitry, accelerator pump circuit, and idle circuit, producing optimum torque, horsepower, and drivability.
Airflow is of the essence with this carburetor.  VDL Fuel Systems takes each part of this carburetor and maximizes the potential airflow while still operating within the tech parameters.  VDL Fuel Systems has purchased tech tools so as to be able to take each dimension to the limit.  From booster dimensions, booster location, and venturi diameter to the throttle shaft width, this carburetor must be custom fit to the tech tools.  The performance gains from these modifications are significant and not something you can afford to leave on the table.

After the parts are machined to the minimum/maximum sizing, they are vibratory polished and recoated.  Assembly is done to tolerances unobtainable in high volume production.  Boosters are installed to within .001’’ on height, twist, and pitch.  The base plate is matched to the main body.  All gaskets are properly matched to the main body to prevent overhang.  And finally, the throttle plates are adjusted for optimum position angle on the flow bench.  VDL Fuel Systems produces the absolute best custom-built 650 CFM carburetor available.  Think “outside the box” and put yourself out front with a VDL Fuel Systems carburetor.
Application: DIRT Sportsman

  • Venturis blueprinted to maximum sizing
  • Air/fuel circuitry recalibrated
  • Idle circuit calibrated
  • Accelerator pump circuit tailored to application
  • Welded throttle shaft
  • Base plate matched to main body
  • Boosters calibrated, sized, and precision installed
  • All parts vibratory polished for enhanced airflow

Tuning Your Holley #4777/650This carburetor was developed specifically for the DIRT 358 Modified Series.  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 at the bottom of the sight hole so that you have to jostle the car 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 one round open. If you should lose your settings just back them both completely off, put 1 round in the primary and 1 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 Pressure6.5 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.