Why does the DCOE need to be modified?
The 40 or 45 DCOE carburetor was never designed to feed the displacement of the Corvair engine, let alone the additional air flow demands when on turbocharged engine comes on boost. Weber DCOE carburators are designed to operate on an isolated runner, one venturi [choke] per cylinder, or, at the most, one venturi per two cylinders via a Siamese runner. These carburators are very sensitive to the pulsation of an engine and are metered as such. You can get your DCOE to run O.K. but it will never give you the all around drive-ability that the carburator was designed to give and you will be stuck with compromises regarding drivability and power output.
If jetted with small chokes, it will run fine at the bottom end, but will be rich on the top end. If you jet it properly for the big end, low speed, throttle response and cruise will suffer. Off idle and transition will be a pain to sort out, generally it will run lean. All this assumes that you understand carburetors in general and, for the most part, enjoy tuning Webers. They are great carburetors and can be easily tuned to fit your requirements, if they operate with in their design parameters.
The problems are due to the fact that the carburator, at low speeds, is too big for the engine and at high engine speeds the carburator will be too small. Look at the Carter YH that came with the car. The throttle plate is approx. 38 mm. and there is one of them. The venturi is approx. 35 mm. Your Weber has either two 40 or 45mm throttle plates that operate simultaneously. If jetted properly for high speed operation, when you floor it [at low speeds], both throttle plates open up. The air velocity across the venturi is insufficient for proper carburetor response at that low air flow. You will get bogging, hesitation, back firing, etc. If you jet the carburator for good low speed response, the flow goes sonic at high engine speeds and the carburator fails to meter properly. Your high speed mixture will be way off and your performance will suffer.
The modification to the DCOE Weber carburator to make it a load sensitive 2 bbl carburator consists of: splitting the throttle shaft, fabricating new throttle linkage pieces to be installed inside the carburator and grafting on a vacuum actuator that will open the secondary throttle plate in proportion to air flow across the primary throttle bore. The new linkage which is installed inside the carburator allows the secondary to open only as far as the primary is opened and is designed to close the secondary if the primary side is closed.
The opening rate of the secondary is determined by an airflow signal from the primary booster venturi. In this manner the secondary is controlled by the primary throttle opening and the primary air flow. For example, if the primary is opened 1/2 throttle then the secondary will only open 1/2 throttle IF there is sufficient air flow through the primary. If the primary throttle is suddenly snapped closed, the linkage forces the secondary closed independent of air flow.
Webers are very sophisticated carburetors and work very well. Everything can be changed on them to properly fit to the engine. Because they meter on a pulsing system they are more sensitive to each engine configuration. Each engine will have different vacuum and operating characteristics and in this manner Webers need to be jetted to each engine. The supplied jetting will be close and your engine should start up and run, but the final jetting and tuning will probably still need to be made. It is helpful for one to have a basic understanding of Weber carburetor theory and operation to properly tune a Weber. We would be happy to give you some solid direction and help as needed, but you will probably still need to make some adjustments of the carburator on the car to get the most out of the installation.
You have the option of using the same choke [venturi] size on both the primary and secondary side, or you may stagger jet the DCOE. In a stagger jet configuration, the primary is setup with a small choke and jetted accordingly. The secondary gets a larger choke and appropriate jetting. This makes the Weber a beautiful carburator for the turbo with a very responsive bottom end and good fuel economy. When the engine requires more airflow the secondary delivers it. You get a fantastic top end and a smooth transition from low speeds to high boost conditions all without the hesitation, backfires and poor throttle response you may have now.
New or used DCOE carburetors can be converted.
The modification includes a vacuum actuator for the secondary throttle shaft, secondary actuator linkage, primary and secondary control cams and other modifications to the DCOE to convert in to a progressive 2-barrel carburetor.
Your existing main and air corrector jets will be modified to help get your starting settings. Idle jets will be modified, if possible. If they can not be modified, idle jets will need to be purchased to establish your base line jetting.
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