Topic: Weird fuel flow problem

[American Locomotive]

Hey guys, I just got my CB750 back and running after a lash adjuster loosened up and made a bit of a mess. Anyways, the bike was sitting for about 5 days. I had run the bike just about out of gas previously, so it wouldn't start. It I turned the fuel on, but no fuel was flowing, and of course the bike wouldn't start. I took the fuel line off, and gas came out of the petcock just fine. I then blew into the fuel line, felt a little bit of resistance and then it started flowing. Hooked it back up, and the bowls filled just fine. I then took it for a quick ride, and after about 3 miles it started dying, and eventually completely stopped. I did the same thing - blew into the fuel line, and then it worked fine again and fuel started flowing.

What in the world could cause all 4 carbs to stop receiving fuel at the same time? Sticking floats? Some weird vapor lock? I do have an inline fuel filter, but it doesn't appear to be unusually dirty.

[BRG-BIRD]

I suspect the inline fuel filter. Check to make sure the inline filter is hooked up with fuel flow going the proper way. The other thing you can do is remove it and see if fuel flows properly.

[PeWe]

The tank fuel cap has a vent that might be clogged. If it work after you have opened the cap you'll know that the vent is clogged.
I had this problem on one tank. Got a stop after 5-10 minutes ride.

 I thought it was the petcock filter clogged by dirty fuel. I friend I called came and fetched me and the bike. I replaced the tank and covers with another set.  Months later when I had to empty the tank from all fuel I thought were dirty I understood what the problem was when the fuel started to flow when I opened the fuel cap.
I  have a new cap for that tank, old is too corroded inside.

[BobbyR]

Also check your fuel line routing. Make sure it has no bends in it. Everything should be going downhill.

[American Locomotive]

The tank fuel cap has a vent that might be clogged. If it work after you have opened the cap you'll know that the vent is clogged.
I had this problem on one tank. Got a stop after 5-10 minutes ride.

 I thought it was the petcock filter clogged by dirty fuel. I friend I called came and fetched me and the bike. I replaced the tank and covers with another set.  Months later when I had to empty the tank from all fuel I thought were dirty I understood what the problem was when the fuel started to flow when I opened the fuel cap.
I  have a new cap for that tank, old is too corroded inside.

I thought that originally too, but I can pull the fuel line off, turn the petcock on and and fuel comes pouring out without issue.

[pjlogue]

I have tried fuel filters in the past and they seem to cause more problems than they are worth.  When the filter was behind the cylinders it gets hot and on a hot day I think the fuel vaporizes and you get an "air lock"  and it stops flowing.  If the filter was up higher behind the valve cover it didn't seem to get the air lock.  You can have a "U" bend in the fuel line but don't have any "n" bends or it will trap air/vapors.

-P.

[flybox1]

Gravity fed carbs.  Fuel lines are always best running downhill, and there are ways to make this happen, even with an inline filter.

Put a catch pan under your carbs, open all bowl drain screws and then turn on your petcock.  You should see good fuel flow out of each bowl.
This will help clear any bubbles between the tank and the bowls.

You can rule out a tank vent issue by comparing flow out the petcock with and without the filler cap on.

[American Locomotive]

I switched to a low-restriction metal mesh type inline filter, and all of my issues seem to be resolved. It doesn't offer as good filtration as the paper filter of course, but I'm thinking in conjunction with the petcock's strainer screen, it should be good enough.

[BobbyR]

I sould have added that using Tygon fuel line helps the routing a great deal. It allows for gentle bends with a downward slant. No need for thick rubber with cord fuel line. There is no real pressure since it is gravity feed.