On multiple BIG97 applications, we recommend that the idle circuits are kept operational on all carburetors (ie. do not blank off the idle circuits on Secondary carburetors).
BIG97s are shipped with the top edge of the throttle plates set at the bottom edge of the transition ports, ensuring smooth low speed transition as the Secondaries begin to open. On a Tri-Power with progressive linkage, aim to keep the Secondary carburetor throttle plate positions set this way, linked so that they all close simultaneously. The idle speed is then set using the Primary carburetor throttle stop screw.
Once the idle speed is set, check that the Primary throttle-plate position is still below the transition ports. If it is too high (at or above the transition ports), this may indicate low idle manifold vacuum – typically seen with longer duration camshafts – and some additional initial (static) ignition timing may be required. In extreme cases, the addition of throttle-plate air holes may be required to get those plates below the transition ports. This is even more critical where the vacuum port is used as this port MUST not be exposed to manifold vacuum at idle.
Idle mixture is set in the usual way. Aim for matching idle mixture screw positions on both Secondary carburetors and not too much difference between Secondary and Primary.
NOTE: Traditional carburetor balancing can help with a multiple carburetor setup on a direct linkage. However, on a BIG97 Tri-Power with progressive linkage, secondary throttle plate position is more critical than absolute balance to ensure smooth running (see above).
The BIG97 ‘early plenum effect’ brings the strong torque/easy drivability characteristics of a dual plane intake with a power curve more usually associated with a single plane. The simple intake manifold modifications explained in our Tech Center article ‘How To: Maximise BIG97 intake flow’ enhance that effect, adding even more potential to your BIG97 Tri-Power.
Put simply, BIG97s have bigger throttle bores than a regular 97 so some intake manifolds may need opening up a little on the carburetor mounting to match the carburetor. You can use the supplied manifold gasket as a template. The deeper you can take that porting, the better your BIG97s will flow.