Jumper the start terminal (small) to the larger terminal (the one with the battery cable) and see what happens.
p.s. Make absolutely sure the bike is in neutral. We will all hear you if it isn't.
Exactly which terminals are you shorting? The two large or one small to one large. If it is the two large, the solenoid is bad. If it is the one small to one large, the problem is in the start circuit.
Someone may have put it there. The tube seat does not work with an o-ring. Discuss this with a store that sells hydraulic hoses and fittings or go to Weatherhead, Parker, etc. and check it out on the internet.
I do not know who illustrates parts in parts catalogues. I think someone throws all the parts in the floor and someone else has an "artist's conception" of how they go together. Guzzi is not exclusive in this area.
The fitting portion at the top is a tube seat. There is (or should be) a taper(37 degree i think) in the line to match the fitting. This will tighten to seal against any leaks. The bottom portion of the fitting might use the o-ring provided there is a taper in the threaded portion the fitting screws into. If not, you can use a copper or auminum washer to seal against a leak.
HTH
You will want this particular seal with the sealing portion toward the atmosphere. The low pressure of manifold vacuum will be pulling inward instead of pushing outward.
I do not understand the last portion about the snap ring? can you post pics about this so we can better evaluate this ?
Your answer is in your question. The idle speed screw adjusts the throttle angle and affects the r.p.m. The air bleed screws do almost the same thing. The screw(s) allow an amount of air to bypass the throttle plates and increase r.p.m. also.