In-line fuel filter placement (1969 CB350) | GTAMotorcycle.com

In-line fuel filter placement (1969 CB350)

jonnymedia

Well-known member
Hi all. I searched around and haven't not found a definitive answer on this one. Would really appreciate any tips.


THE SIMPLE QUESTION:
Does it matter where on the fuel line you place an in-line fuel filter. Should it, for example, be below the level of the float bowls, or lowest point (horizontal), or close to the petcock and vertical? If not, can it cause fuel starvation?

THE DETAILED QUESTION:
I was recently told by a shop that my tank is starting to show some rust, just a bit, so to not lose any riding time in the summer, I put on a pair in-line fuel filters with the plan the longer term solution fix over the winter.

As you can see by the photo, I've installed these two filters up close to the petcock. I didn't really put a whole lot of though into where I put them because at the time i didn't think it matter. (And yes, my yellow spark plug wire is usually on the outside of the fuel lines.. just quickly reconnected for photo taking sake).

At any rate, after I put the filters on, I've been experiencing some issues with bogging down at higher RPMs, or if the fuel tanks is not completely full, getting my left side firing at low RPM idle (and as you can see in the photo, that is also the one that is above the float level). I initially didn't put the two and two together, and as I've build this bike up from scratch myself, assumed it was, and doubled checked valve clearances, cleaned carbs, points, etc.

The lightbulb clicked last weekend. The bike runs like a dream on a full tank, but after 80km it starts to bogged down on acceleration, and did stall out. I was close to a gas station, so filled up, turned straight for home (and started to bog down close to home). I know this could also mean carbs or points, but i've re-checked those so many times that I need to find other suspects.

Does anyone have any thoughts on this? I could easily test this by taking out the filters and riding for another 80km, but I'm hoping this is one serious newbie mistake that a few wise words can clear up. Should inline filters be placed at the lowest point (meaning when they're horizontal), or do that just need to be below the float level, OR, does this not matter at all and I'm wandering down the wrong road.

Much appreciated.

a5upapem.jpg
 
My first thought would be that the filters are doing their job and getting clogged up with crap. A full tank has more height of fuel to push it though, where as it gets lower there is less difference between the height of the fuel in the tank and the float bowls themselves. No, it doesn't matter where the filter is located, but I would suggest perhaps that you get some larger (I mean more filter media, not coarser screens) filters if these are clogging that quickly. Have a look at these and see if they are clogged, let us know what you find!
 
Appreciate it Skapan.

Well, the fuel filters look pretty clean to me. Picture for your own look-see. However, noticed that, even if I shut of the petcock and run the bike for a little while, and then turn on, although fuel runs through, the filters never really fill up. The left side less so. Maybe I'm just assuming they should be full of fuel. Do I need bleed the air out / prime the gas lines? Never though of that actually.

Thanks again!

na3u2u9y.jpg
 
-Yes adding the filters will change how fast the fuel flows into the carbs.
-No it doesn't matter where in the line they are
-The longer the filter is there the more it gets filled with crap the slower the flow.
-The less fuel in the tank the lower the pressure head the slower the flow

What is the media size filter are you running?
 
Pressure drop through the filter is the issue. A filter with a larger filtering area should have less pressure drop.
 
Those filters look really small. I am surprised that they are restricting fuel flow that much, but it seems to be the case. I always use the plastic bodied filters about 1.5" in diameter with a pleated filter inside. Something like the one in the second row, second from the right.

http://www.germes-online.com/direct/dbimage/50039182/Plastic_Fuel_Filter.jpg

You look like you have the one on the bottom left, there is a huge difference in filter area. The bigger filter will likely help with your fuel flow issues, but won't look as good.
 
Old CB's dont like fuel filters. The fuel feed is so weak to start with, anything extra in the fuel system generally causes them to be starved for fuel at high RPMs.

The solution is to either get some high flow fuel filters (which will cost you like 60 bucks a filter, but generally they are servicable) or to fix your tank so you can remove the filters. If you search around on the vintage forums (Do the Ton, SOHC4, etc), you will find some good recommendations for filters for these old bikes. Id just take a weekend and clean/coat the tank though.
 
First off everyone, thank you again for the tips and advice. Has always been and was appreciated and helpful. However, this time around I might have wasted a bunch of your collective time.

Here's the problem. Obvious source of issue and one of many newbie mistakes, using plumbing clamps and over tightening.

*smacks head*

Live and learn.

u9ubujem.jpg
 
Thanks Ronnie. Yeah, they were installed correctly, cone up on these ones, but yeah. I had over tightened the clamps and crushed one of the ends.

Appreciated.
 

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