Early R&D was carried out for us by Norm Schenck at Competition Fuel Systems, in Michigan, who stuck an original 81 on his flow bench and got 109.6 cfm max airflow at 1.5in HG vacuum. 3D modelling then helped us develop a short venturi insert with a tiny step at the bottom so it is clamped into the 97 bowl casting by the new 81 base casting. In final manufacture, the real parts actually flowed 116cfm due to the super-smooth finish on the inserts and accurate turbulance-cheating fit. But most important, the booster signal curve is the same shape as the old 81, but a tiny bit stronger for even better response.
The 81 carburetor has a half-inch accelerator pump bore. We make 81 pumps so decided to make a press-in sleeve for the pump well, adding a small 0-ring at the bottom to stop the gasoline returning back up the gap between sleeve and pump wall.
After extensive testing, we decided on 0.036in main jets (original stock jetting was 0.035in) with a stock Number 71 power valve. We were also able to retain the stock 97-size air bleed sizes – 0.040″ high speed and 0.103″ low speed (idle)…and the stock 97 emulsion tubes.
While we did prototype tests with aluminum inserts in a 97 base casting, sleeving the 97 base casting down to 81 size was not a full production option. So we tooled up for a whole new cast iron base casting to original 81 spec.
This means 1.0625 inch throttle bores with new OE 81-size throttle plates, which are now also available to 81 carb rebuilders of course. The 81 always used the 97-style throttle shaft so we had that covered. Note how the transition ports exit just above the throttle plate.