Modern Vespa

My MP3 400 has started to bog down under acceleration. I use it mostly for freeway commuting 70 miles round trip. Being on wide open throttle for most of its use made it easy to ride around. However, the problem has been getting steadily worse. Today, I set aside some time to look for an obvious cause. I cleaned the air filter and replaced the spark plug. I have been running injector cleaner thru the system for a few fill-ups. The symptoms still persist. While I was inspecting for anything obvious, I noticed the injector "T" only had a hose connected on one side. Luckily , I have access to a MP3 250. The 250cc has hoses connected to both sides of the "T" connector.
Thinking this may be the problem I scoured the engine compartment for where it may have dropped. I can't find any signs of an unconnected hose.
Can anyone with experience with MP3 400 help me with the solution to my conundrum. If the hose is missing where could it be, where does the other end connect? Thank You

There isn't a second hose to find the MP3 250 has 2 hoses because the pressure regulator sits on top of the pump. Yours has only one because it has a different system. Others might chime in and be able to tell you where your pressure regulator is.