Topic: Where's my gas?

[clgenelin]

BIKE: 1977 CB750K, Stock air box, K&N air filter, MAC 4-1 exhaust, #40 Idle, #120 Main, Needle clip 2nd from top, Screws out 1.5 turns, Intermittent accelerator pump issues, Iridium DR8EIX plugs, Light haze of rust in tank, Spin on oil filter.

1.  ACTION: Added a filter oriented horizontally just past the petcock using OEM fuel line.  The bike was fine on 30-min ride to work, but starved for gas on way home and died.  RESULT: Filter seemed to airlock.  Topped off with gas and stopped 3 times on way home to mess with line and get fuel back into fuel filter/line/carbs.

2. ACTION: Moved filter to vertical orientation near the inlet of carb #2 using vinyl 5/16 tubing from hardware store (temporary solution, I know vinyl is bad) and zip ties for hose clamps.  RESULT:  Fuel line seemed to be too long.  There was a bubble in line above filter and filter still never filled completely with gas.  Trimmed line and bubble went away.  Flicked filter until it was completely full.  Road bike and all appeared to be well.  2 days later, fuel line was empty. Bubble issue returned, frustration level escalated. 

3.  ACTION:  Removed filter all together and used 5/16 vinyl and zip ties to mimic original setup.  No bubbles in line.  Road bike and everything worked great.  2 days later, and fuel line is empty again.

QUESTION:  Where is the gas going?  Is there air leaking in because of the large fuel line and zip tie clamps?  I generally keep my petcock open all the time.  There is no evidence of leaking.  Fuel can only fall into the carb bowls, right?  The biggest problem with this setup is that it makes it a pain to start the bike and get fuel flowing back into the lines.  Any suggestions would be appreciated.

[brewsky]

Checked the tank and bowl vents?

Checked the petcock unhooked from the carbs?

[Vinhead1957]

The filter is the only part of the equation that creates the problem.   Just run with the one in the petcock!  Clean it once in a while.

The filter must have a crack or was not sealed well.

Do you have a lot of sediment or rust?

[Don R]

Sometimes air just appears. does it really affect the performance? My K0 finished the riding season with the same issue.  It has an old tank that I borrowed from my brother and i think the strainer is plugged.

[Retro Rocket]

QUESTION:  Where is the gas going?  Is there air leaking in because of the large fuel line and zip tie clamps?  I generally keep my petcock open all the time.  There is no evidence of leaking.  Fuel can only fall into the carb bowls, right?  The biggest problem with this setup is that it makes it a pain to start the bike and get fuel flowing back into the lines.  Any suggestions would be appreciated.

Open the oil tank and smell the oil for fuel, the fuel can leak through the carbs into the engine, If thats the case  the fuel will break down the oil and could lead to a catastrophic engine failure...

[phil71]

fuel filter orientation doesn't matter (unless you have the FLOW arrow in the wrong direction). What does matter is fluid-dynamics. you could coil a mile of fuel line in between the carbs and the tank , and as long as it's all fitted below the petcock and above the bowls, syphon will keep fuel flowing. Kinks are bad. Also check to see if it improves with the cap popped. It may have a vent problem and that's how you're getting airbound.

[clgenelin]

I'm also concerned about the fuel ending up in the oil.  When I originally got the bike a year ago, it was barely running.  One of the first things I did was drain the oil.  I remember it distincly smelling like gas.  Since then I've done a variety of things including removing, disassembling, cleaning, and rebuilding the carbs.  I'll check out the oil tank tonight.  If its leaking into the motor, I would assume its from a stuck needle?  The tank has a light coat of rusty haze that you can see, perhaps something made it to the carbs a jammed it up.

[Raul CB750K1]

If you think about it, a filter equals to a kink. If the surface of the holes equals the surface of the filter material, a filter is like reducing the section of the line by approximately half.

If you add a second filter, you are further reducing the section by another half. What is more, if the size of the holes is the same, the second filter serves no purpose. Actually, only the filter with the smaller holes does something.

My brother has a GPZ500 ninja and he was having issues with sputtering at high speeds. He was using an inline filter along with the petcock filter. As soon as he removed the inline filter, the issue disappeared. Gas is gravity fed. The velocity is constant because the pressure is constant, but the volume per unit of time depends on the line section. If you put two filters you are reducing the section by a factor of 4. It may happen that, simply, the velocity of fluid falling can not cope with the engine demands. The bowls get depleted, and the bike starts to sputter.

[flybox1]

Just run with the one in the petcock!  Clean it once in a while.

assuming he has one, and its working properly  
if you've gotta run one...this plus two 4" sections of fuel line and fuel is always flowing downhill