For older mono-fuel propane carburetion systems, Technocarb made the Off-Road carburetor, which was a 4bbl venturi-style carburetor suitable for high performance applications with an integral throttle-body using a square-bore Holley flange. To suit a variety of engine sizes, the carburetor incorporated replaceable, pressed-in venturis. The following sizes (internal diameter at the vena contracta) and approximate flows with 4 venturis. Technocarb has not flow-tested their carburetors so the following ratings are estimates.
- 0.75" (400 CFM)
- 0.91" (575 CFM)
- 1.00" (675 CFM)
- 1.10" (810 CFM)
In contrast to the Impco 4bbl LPG carburetor, the Off-Road carburetor does not become leaner above its flow rating. Because of the nature of venturi carburetion, the fuel mixture actually starts off a bit lean at low flows and gradually becomes richer a flow increases. With the Open-Loop system (ie, no feedback fuel mixture control with an O2 sensor), the full load fuel mixture is set with a load-block valve in the hose between the carburetor and converter. The Feedback system (closed-loop operation with an O2 sensor) uses a stepper motor on the converter to control idle fuel mixtures and a stepper valve between the converter and carburetor for off-idle fuel mixtures. Both controls were coordinated by the EFS600 controller to ensure stoichiometric fuel mixtures at all times.
In January of 2012, Technocarb discontinued the EFS600 due to its high manufacturing cost and declining demand for their fumigation products. We are working on having the MegaSquirt EFI controller control the operation of the 2 stepper motors but this is in the preliminary planning stages. If you're interested in a feedback controller for the off-road carburetor, please let us know as it is difficult to estimate the size of this market and the viability of this product. Although it was a more expensive than an Impco 425-based system, it had more performance capability without the expense, complication, and hood clearance issues of adding a second mixer or carburetor.
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