78 CB750k carbs

A couple of questions. I am trying to get my 750 running again, but am having some trouble with the carbs. If you are familiar with this model then you know it has an accelerator pump when you crack the throttle that sprays a stream of fuel into each carb. I can crack the throttle and it will start off of that initial spray of fuel but dies right afterwards. The only way I can get the bike to idle is to take the airbox off and hold my hand over the back of the carbs. I know the bowls are filling and I know the jets are clean. I have tried everything that I know to do. Any ideas?

I noticed that there is a tube directly in front of the fuel intake on the carbs that is not attached to anything. Does anyone know if it is supposed to be attached to something? I have a repair manual, but it only has the early model carburetors in it. I thought it might be a drain tube, but why would there be a drain tube pointing up?

Last thing. When the bike was running a few years ago, if I put my feet up on the highway bars it would kind of choke the engine. As soon as I put my feet up it was like I was running out of gas, then I put them back down and it ran fine. Any clues? Thanks.

Last thing. When the bike was running a few years ago, if I put my feet up on the highway bars it would kind of choke the engine. As soon as I put my feet up it was like I was running out of gas, then I put them back down and it ran fine. Any clues? Thanks.

That's real odd, no idea.

Try plugging that tube off and see it that makes it run any better. I'm not sure where it goes, mine doesn't have it. The only other things I can think of is clogged up pilot jets or real old gas.