I have bolted air filter backing plates to my twin 40 dellorto’s. They came with blanking grommets that fit over the small hole, the size of about one pence piece. Can anyone tell me if that hole should be blanked off or left open, thanks.

If it's like the Webers, the hole is the air feed to the jets. The main supply goes through the chokes, but the air for the jets comes in thriugh that hole, into the air space at the top of the float chambers and is then used as the air supply to the jets.

Many thanks