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Adding a Fuel Filter to the Savage (Read 3668 times)
Dave
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Re: Adding a Fuel Filter to the Savage
Reply #30 - 01/23/24 at 06:23:11
 
The attached photo shows the center finder diameters for the fuel inlet and a 5/16" filter.

There is no doubt that the biggest restriction to fuel flow is the small hole in the float needle seat.....that is the smallest opening in the fuel system. †The big problem is not that the smaller diameter hole on the filter restricts the flow of fuel - the problem that occurs is the small diameter nipple can often trap an air bubble that blocks the flow of fuel. †When air is in the nipple the fuel pressure from gravity flow from the fuel tank is not high enough to push the bubble down - and the air bubble does not have enough "lift" to come out of the nipple and flow upward. †The fuel system gets an "air embolism" and your carb doesn't get any fuel.....and this bubble can form days/weeks/months after you install the filter and leave you stranded and looking for ignition/carb problems that don't exist!




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Dave
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Re: Adding a Fuel Filter to the Savage
Reply #31 - 01/23/24 at 06:25:48
 
The fuel system operates under very low pressure - gravity cannot provide much pressure when the distance from the float bowl to the top of the fuel tank isn't much more than a foot......the bottom of the fuel tank less than half a foot. †With a full tank the pressure at the carb is less than 1/3 psi......with an nearly empty tank it is less than 0.15psi!

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ThumperPaul
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Re: Adding a Fuel Filter to the Savage
Reply #32 - 01/30/24 at 10:49:19
 
I like the math reasoning (area of a circle pi r sq stuff and psi)! †Along these lines, what is the diameter of the float valve feed? †Ultimately, gas has to squeeze through there to fill the bowl. †I donít know the size exactly, but I think itís 3.3mm on a Mikuni VM36.

Vacuum locks and air pockets. †Itís easy to see happen with a clear filter. †Question (and for a change I donít have a related problem!): †isnít it possible for the fuel line itself (no filter) to get a vacuum lock or air pocket? †Can the air pocket be removed by removing the gas cap and running the bike run a couple minutes. †Get it to burp? †Or am I worried about nothing and solving a problem that doesnít exist?
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Dave
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Re: Adding a Fuel Filter to the Savage
Reply #33 - 01/30/24 at 14:31:09
 
ThumperPaul wrote on 01/30/24 at 10:49:19:
I like the math reasoning (area of a circle pi r sq stuff and psi)
Vacuum locks and air pockets. †Itís easy to see happen with a clear filter. †Question (and for a change I donít have a related problem!): †isnít it possible for the fuel line itself (no filter) to get a vacuum lock or air pocket? †Can the air pocket be removed by removing the gas cap and running the bike run a couple minutes. †Get it to burp? †Or am I worried about nothing and solving a problem that doesnít exist?


The fuel line and factory petcock/carb nipples are large enough that that an air bubble can freely move upstream and exit the fuel system. †The small fittings on the fuel filters are small enough that the bubble can be trapped and it will not freely exit....the fuel wants to come down and I believe the surface tension of the fuel is stronger than the small air bubble - and the flow stops.

This situation evidently can't be resolved in normal operation, and some members have even had this occur after using a fuel filter for an extended period. †When they first install the new fuel filter the tubing and filter are empty - and the fuel flows down the tube and pushes the air out the bottom. †After that the fuel system is subject to periods where the bike is not being stored and air gets into the system from the carb end and the air travels up the tubing until it reaches the small fuel filter inlet.....and that is where it gets lodged and stays. †The rider is suddenly at a loss as to why his bike has stopped running, and he starts looking for electrical problems and carb problems that don't exist!

Your fuel petcock has a fuel filter screen in the tank......and the carb has a fuel filter screen on top of the seat for the carb needle.  If both of those are in place and working....I really don't see the need for a third filter - especially when they can be problematic.

Over and out.....if you choose to run a filter - you have been warned! Wink
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ThumperPaul
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Re: Adding a Fuel Filter to the Savage
Reply #34 - 01/30/24 at 15:01:17
 
Good answer, Dave.  I've already said I won't use an in-line fuel filter based on the problem noted.  I'm fine with the petcock screen in the gas tank and the little screen in the carb.

I was just thinking the fuel line itself could develop an air pocket.  And that you could probably get it to burp out of the fuel line and into the gas tank if you removed the gas cap and fired up the bike.  Glad you think this isn't likely or necessary and the line clears itself through the petcock and into the gas tank.  I like that answer.  It's consistent with my thoughts, but I needed something new to worry about on Woody.
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