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Right fuel sightglass level loss when switching R to L tanks

F

Fowlplay

Registered User
When on left tank and switching to right tank the level in the right tank sightglass drops about 1.5 inchs. Then from switching from right to left tank the level in the right tank goes back up to where it was originally along with a few air bubbles. There is no movement on the left tank through all this. I have put all new hoses on the right tank to the sight glass and also does it with no gas tank cap on. Also used compresser to blow the lines out on right side. It is a stock 150 cub with stock fuel system. Any ideas of what this could be would be appreciated. Primer? Fuel selector switch?



Thanks
 
1.5 inch is like 5 gal dude. Is this some sort of riddle or is it April?
Did you just get this machine? Did this problem start after maintenance?
 
Not sure

Been in the family since 76. Didn't notice it till last summer. Gota just be a drawdown in the sightglass. No way it's messing with 5 gallons.
 
Are you standing on soft bushwheels when this happens? Or is it in flight and you are moving the rudder pedal when you lean over to move the fuel valve?
 
On 31's. Does it on the ground and in the air. Does it more frequent in the air. Strait and level attitude.
 
+1 on Brian's post. This was a problem on some early Legend Cubs. They stopped using the ball. I had one side where the ball got stuck sometimes so I removed it. Never bothered to so the other side because it seems to be ok. Just call me "one ball".

Rich
 
My balls work perfect! Not sticking at all. Even if they were it don't explain the air bubbles when switching left to right tank
 
Fowlplay said:
My balls work perfect! Not sticking at all.
Click to expand...
Whew, that's good! This is intriguing. I can't think of what could cause that fluctuation, s'pecially with no pressure in the tanks. How 'bout the header tank plumbing? Is it possible to get back pressure from the primer injectors? (thru the gascolator) Does the level change rapidly or is it a slow change?
 
Rapid change for the level up and down. I was thinking a pinhole in a line somewhere in the area of the primer. If I dink with the primer in flight I can make the level go up and down in the sightglass.
 
The only things that right sight-glass is attached to are the fuel tank and the rear header tank vent (via a tee at the top). A pressure change in the one shouldn't change the level in the glass. Are the fuel lines routed properly? (Front line to the windscreen line, rear line to the lower top header tank fitting, top header tank vent line to tee in the top sight glass line)
 
Last edited:
I have the same problem except I have never noticed any air bubbles. I switch form left to right tanks and it pulls the fuel level down in the gauge. Switch from right to left and the Right tank level returns. I have inspected the entire system as well, and found no problems. I have had this problem for over ten years with no ill effect. I have changed gas caps, gascolater, primer, and some of the fuel lines with no effect. I'm interested as well as to a cause. Tony
 
How many hoses big, on that right tank??? Does it maybe have the fromt header tank bent plumbed to it???

take pictures of that and fiel valve area with side panel removed
 
Gotta be a plumbing issue. Air trapped in a "high spot"?
 
maybe..... the top "Tee"(to rear header tank) on that sight glass has it's hole on tank end tank restricted down like the other 3? it should be larger/full size on originals.... ???
 
Last edited:
The only think I see out of the ordinary is the sightglass is flipped upside down. The vent T is on the bottom.

Rich
 
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