fuel problem

[Stripe]

I own a 1972 CB750.  A few weeks ago I had it fired up for the first time in a few months.  I saw that only one of the two fuel filters (One in each fuel line coming out of the petcock) was getting fuel.  I have disassembled cleaned and reassembled the petcock, it seems to work fine. When I open it fuel flows well out of both lines.  I have also inspected my fuel lines, they look good and are not blocked.  I got new fuel filters too.  I put this all back together and ran into the same problem.  Has anyone else ever had this problem?  Next I plan on cleaning the carbs or rebuilding them.  A few guys I talked to said they did not think the carbs were the problem but could not help me ether.  Can anyone help me out with what is going on? Thanks  

[LukeM]

My first guess would be you're getting fuel when it's set to RESERVE, but not to ON.  Reserve pulls fuel when ON doesn't pull anymore.  Most petcocks will give fuel when set to ON or RESERVE, but not both.  I'd double check against the factory manual for your bike, to see how it was set up at the factory.  It's possible the fuel lines have been modified somehow.

Good luck, and keep us informed.  
Luke M

[Re-run]

Does your bike have 1 or 2 fuel lines? I know some had 2.

Anyways, do you have any loops in your line or any place at all where the fuel has to go against gravity, basically up? Something that simple will cut fuel flow enough to cause issues. Also, you had filters in the tank and on the line? I know your bike should normally have an in-tank filter and that is all you would need.
Also, some filters are 1 way.
As a test, I would remove all filters outside the tank and give it a try.

[Stripe]

Thanks for the quick response guys.  I am having the same problem in both on and reserve, with a full tank.  As far as the filters I removed the filter in the tank and I am using the in-line filters only.  I was told to do this for better fuel flow.  I have had it set up like this for about two years and not had a problem until now.  The filters are new so I do not think they are the issue.  There is a place on both lines were the fuel must travel up, right before they connect to the carbs.  This has never been an issue before.  A friend suggested using a tee to vent the line.  I set it up last night, I installed the tee and ran the line from it up the side of the tank.  This did not help at all and I removed it.  A friend of mine is a bike mechanic and is coming over to check it out later.  I will let you all know how that goes.  Thanks again for the suggestions the help is much appreciated.

[Re-run]

The intank filter is fine. It actually works good enough so try getting rid of the in lines and make sure the fuel has a no incline to travel.