Join Date: Dec 2015
Thanked 51 Times in 50 Posts
Rep Power: 5
I could envision a crack or pin hole in the fuel line that could be sucking air while the engine is running, but would be too small to leak fuel. Fuel is thicker than air.
There could be debris in the fuel line, like rust or dead bug parts, that restrict flow enough that it can't feed enough fuel to supply a more fuel hungry cold engine, but is sucked past the restricted area with the added suction of a warm running engine.
My vintage motorcyle has rusty sludge in the fuel petcock that will require me to remove the tank to get to. In the meantime, I've placed an in-line fuel filter in the fuel line to keep it from getting to my carbs. It is a gravity fed system, with the tank sitting above the carbs and engine. The obstruction is just enough to cause fuel starvation when my tank gets below half full. My engine will start running rough and loses power. I can pull into a station and top off the tank and the bike runs great. It also runs rough when it starts and needs a lot of choke. After it reaches operating temperature it runs like a champ, as long as the gas tank is filled above half. It makes just enough extra gravity pressure to push past the restricted fuel petcock.
Rebuilding the mechanical fuel pump could make a big difference, as well.
I owned several VW Beetles in the old days. Even when their mechanical fuel pump was working well, a rebuild every 30,000 to 40,000 made a noticeable difference.
The acceptable pressure range was listed as 8-15 psi. The car would run ok with a pump that tested at 10 psi, but if the engine was older and had other issues, rebuilding the pump to get closer to 15psi made a noticeable difference in cold starting and acceleration.
Last edited by midwesterner; 04-21-2016 at 03:28 PM.