Carb T-Junction (for air?) on 1981 CB750K

[brettp]

Hey, everybody. Bought a 1981 CB750K a couple days ago. Cleaned the carb bowls, jets, fuel passages, etc., it also has the stock air box (I'm really second guessing air pods given the nightmares I've read on this forum) and the fuel diaphragm removed from the carb (it is gravity fed from the tank).. I have NOT verified valve spec or ignition timing. With that said, she idles just fine but acts lean at 6k with popping. I know AMC has repeatedly said the valves being off will decrease vacuum, which then prevents the slides from raising all the way; I will look at the valves soon.
To my question...before I do the valves, I noticed a small black plastic nipple on a t-junction between the 2nd and 3rd carb. The plastic nipple can rotate/pivot in place. I cannot for the life of me figure out what that passageway is for, nor to what it needs to be connected. While the bike idles, I can cover it with my finger and the bike doesn't seem to notice. Any help answering what that is for is greatly appreciated. Thanks.

 

[mrtwowheel]

I would like to know the answer to your question also. I am working on two banks of carburetors right now from two different bikes. Both bikes had a hose connected to that barb and the hose was just lying over the air box, not connected to anything. I had put many good running miles on one of those bikes, it was just time to reclean the carburetors.

Scott

PS, there is no small barb on the airbox that the hose could go to. Maybe it hooks to that green mystery wire by the battery??????

 

[brettp]

I think I found the answer. Its a shared breather for the carbs that vents to the atmosphere. If you want to put a hose on it, you can, but put the end somewhere gas can spill without getting all over the bike or your rear tire. But I think it would be a problem to cap it.

 

[brettp]

And for anybody who finds this thread via search results for a XB that's won't rev higher than mid-throttle, I read a lot on this forum from username "amc" and discovered the vacuum sensitive of the carb's aluminum slides/piston valves is EXTREMELY sensitive. I didn't think it would matter at the time of the original post but I had the air filter out and air box cover off the bike. It results in more air flow, and a resulting decrease in vacuum, preventing the slides from lifting all the way. Once I reinstalled the the air filter and put the cover on it revved like a champ. This also means, as amc had stated many times, engine compression is key for these bikes. Without it, your (stock) carbs will NOT operate correctly. This also leads into pods -- you will have to manage/understand that vacuum change in order to correctly tune the carbs (pilot and main jets, needle adjustments, and maybe pod adjustments like restricting their air flow).

 

[mrtwowheel]

brettp, you must be right about the hose, three votes from my bikes. Forever more to be known as "The Hose To Nowhere".

 

[brettp]

brettp, you must be right about the hose, three votes from my bikes. Forever more to be known as "The Hose To Nowhere".

I like pointless nipple better