Carbs only get enough fuel on ON

After cleaning and rebuilding the carbs this winter I have been trying to get them in sync and everything seems to be lined up, except for one thing. I can't get enough fuel to run when the petcock is ON. If the petcock is ON eventually I stall out. If the petcock is on PRIME however the bike runs wonderfully. Its almost as if the vacuum isn't strong enough to pull through the petcock. Any ideas?

 

Check the vacuum line to see if it is flexible enough to collapse. If it is, replace it and see if that works. It could be collapsing on itself.

That's the easiest check to do first.

 

Unhook the fuel line from the carb.
Unhook the vacume line from the carb.
In the ON position suck on the vacume line fuel should start to flow.
If you suck in fuel the diaphram in the petcock has a hole.

 

Vacuum line could be cracked, loose, or like Joe said, collapsing on itself.

Petcock diaphragm could have a hole or simply be less than flexible enough any more and need replacing.

 

Hound,
don't forget to say. If you get gas in your mouth, spit it out.

 

Out of curiosity... :idea: Umm. How much fuel is in the tank?

Does it run OK on reserve?

I know... I know... I'm just askin!

 

Just spit balling here, and don't take this as me thinking your not bright, but, are the fuel lines backwards? I ask because I did. I can be an idiot sometimes.

 

Everything is fine that was addressed so far. The line is definately vacuum line and is not collapsing the tank has fuel, the hoses are hooked up correctly. I am finding that I am beginning to be able to run longer and longer in the ON position whereas before I could only make it around 1 minute before I had to flip over to PRIME. Maybe the problem is fixing itself...that would be weird.

 

one thing that hasnt been mentioned yet is a clogged fuel petcock. speaking for myselft here i have had to open mine up twice and each time i found trash in it... of course my PO was an idiot and linned the tank wrong and so thats usually what i find pieces of tank liner.

the two screws on the face release a tensioner an then the switch slides out.... i shouldnt have to say it but empty the tank first in PRI.
check it for trash then re-assemble.

also not mentioned is that if the diaphram has a weak spot... usually in the middle ... it wont work correctly.