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Fuel filter always mostly without much in view 6 months 3 weeks ago #31389

  • jftman
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I ride daily for leisure generally at sun up or near sun down. Beating the heat and engine friendly.

i can ride fir 30 miles, at various speeds usually as fast as 75 mph at times. The engine is not starving for fuel.
i have a installed plastic clear fuel filter with approx 1.5 inch dia and 2.0 inches long. A common filter and sometimes if I leave it sitting over noght on prime it fills up almost completely. I will ride the bike and the engine uses up the extra amount to leave very little in view inside the filter.

It is quite obvious that there is some flow as the engine runs fine.
since the filter is like new, I’d say with 300 miles on it, could it be that it is free flowing through the paper element and as such the demand equals the fuel coming from the tank under some amount of internal vacuum due to the carb system.

i read some where that an ‘empty’ filter is a sign of a clean fuel filter. And this seems to be true here.
i know the fuel line routing is a matter of being done the right way and folks tell of their preferred routing.

The conditions in my situation allow for high speed sustained riding with out loss of power, so I can’t be worried but would like some confirmation.
perhaps when and if the filter canister starts gradually becoming a storage area for fuel, I can start considering buying a new one? 

 

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Fuel filter always mostly without much in view 6 months 3 weeks ago #31403

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Googled the topic.
Should motorcycle fuel filter be full?
If you are lucky enough to be able to see your motorcycle's fuel filter do not be shocked if it has a big bubble or even looks half full. A motorcycle fuel filter does not have to be full to do its job. As long as you can see fuel moving through the filter, you should be ok.

AnyOne also consider an empty fuel filter a good sign that the filter is still like new?

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Fuel filter always mostly without much in view 6 months 3 weeks ago #31404

  • Ted
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Mine's always been around half to slightly under.  Since it never acted like it was starved for fuel, I never thought anything of it.
1981 KZ1300, '98 Suzuki GSF1200S, '80 Honda CT110, '11 Honda CBR250, '75 Honda CL360, '00 Honda XR100R
Speak softly and mount a fast bike.

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Fuel filter always mostly without much in view 6 months 3 weeks ago #31407

  • Kawboy
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My "opinion" FWIW
I'm sure all of you at some point in time had an opportunity to put a length of garden hose in your old man's fuel tank and suck on the outboard end to syphon out some gas into the handy dandy fuel tank and if you were lucky, you managed to get the fuel to flow. if you were unlucky, and a air bubble got stuck at the upper part of the hose then no fuel was flowing. Something similar will happen here. When you have a fuel line that's running somewhat level in certain parts what you've created is something like a babbling brook. The fuel will run downhill and then will burble across that fuel filter until it gets to the other end of the fuel filter and then flow down towards the carbs. The more full the fuel tank is, the greater the "head pressure" of the fuel on the fuel line. A full tank will have just over 12 inches of head and 1 atmosphere of water pressure (14.7 lbs) 34 ft So 1 foot of water pressure would be 1/34th of 14.7 lbs = .43 lbs (less than 1/2 pound) and gasoline is 25% lighter than water so reduce that pressure down to .324 lbs/in sq.
So, on a full tank .324lb/sq in got the fuel down to the filter and then the air bubble (hiccup) Now the fuel will flow across the filter and then flow down to the carb and there's next to no head pressure here because the fuel filter is a couple of inches above the fitting going into the carbs.
If you wanted to do a little test for shits and giggles-
Pull the fuel line off of the carb fitting and allow the fuel to flow for 30 seconds with your existing set up with the filter on the horizonal and a bubble of air in the viewing of the filter.
Next, pull the fuel line off and block the end and lower the fuel line down and allow the air bubble in the fuel filter to rise up and enter the tank basically filling the fuel line with fuel. then raise the fuel line up to the point where it enters the carb fitting and then allow the fuel to flow out into a container and let it run again for 30 seconds. My money will be on a significantly greater flow of fuel when there's no air entrapped in the fuel line.
so the goal would be to route the fuel line in such a way that under no conditions can air get trapped in the line or if by chance air manages to get in, that the air will flow uphill and purge itself in the tank.

Somewhat long winded, but if you read my posts I tend to go deep into the engineering/ science of things. It's my fascination.

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