91 nighthawk vaccum to petcock issue

[91 cb750 knighthawk]

PLEASE Help, I have a 91 cb750 nighthawk not getting any fuel, I replaced the petcock to find out that that is not my issue. It’s the smaller vacuum line that look like goes into the engine. I am not getting any suction to the Petcock is not pushing fuel thru to the carb. Please if anyone can provide guidance. I know this sounds like a common knowledge issue but this is my first motorcycle! Thanks

 

[Matt]

PLEASE Help, I have a 91 cb750 nighthawk not getting any fuel, I replaced the petcock to find out that that is not my issue. It’s the smaller vacuum line that look like goes into the engine. I am not getting any suction to the Petcock is not pushing fuel thru to the carb. Please if anyone can provide guidance. I know this sounds like a common knowledge issue but this is my first motorcycle! Thanks

Just ran through this issue today, myself. this little diaphragm assembly is a real PITA! I originally thought I was flooding my engine with fuel, but it turns out the new fuel kit I just replaced with the large diaphragm and small diaphragm & spring were partially culprit. On the 3rd attempt to figure this out, creating suction on the other end of the line (vacuum line off the fuel petcock), I could tell the diaphragms were not sealing properly inside the petcock assembly. What this meant is that air was flowing past the diaphragms -they were not sealing properly to allow the vacuum effect of pulling fuel through. On the 4th attempt I could tell an edge of the new larger diaphragm was slightly rolled, so I opted to reuse the original large diaphragm that didn’t have any holes or tears to begin with. From replacing that large diaphragm for the second time, I believe the issue was that I didn’t have the large diaphragm nipple seated deeply enough into the metal center ring. After pulling suction with my mouth from a long hose, I was restricted in my effort and could tell things were now set up properly. I do like the safety effect of having the vacuum assembly for safety reasons but any little mistake while assembling this thing causes true nightmares.
Happy hunting! 92 CB750 nighthawk guy.