jaimslaw;

Can you take pictures of what you are talking about, and mail them to me (poolforum AT gmail DOT com)? I think I know what you are asking, and I think you'd be fine doing what you want, but your terms aren't 'technical' (and specific) enough for me to be sure. That may be making others hesitate, too. They don't want to tell you "OK", and then find out they misunderstood and let you damage something.

But, in general, you can do whatever you like to a passive air intake to a venturi jet, and not damage anything. Likewise, in general, if you have a powered blower, you can NOT block the air intake to it, without damage.

Ben