Gas Caps and Venting issues

[bob turner]

We have a Cub locally that has had venting or line blockage issues. The left tank seems to feed fine until about six gallons are left, then it stops feeding.

Of course the first thing we looked at was the fuel caps, which are both now brand new machined aluminum vented. That was done by an FAA repair station with maybe a half century's experience. Apparently that didn't fix the issue.

I suppose it is possible that the left tank is hooked up improperly - I have not yet checked that. The left wing was pulled about five years ago for spar work.

My experience with wing tanks is that the vented caps are riding in a vacuum, and that feeding with vented caps is at best tenuous. The CC Cub hereabouts has those little ram air tubes on the caps, so presumably this has been a problem for some fuel systems on Cubs.

The question: Are these CC caps with the ram air thingies approved for all Super Cubs? Should we be looking elsewhere for a problem with such a feed issue?

 

[NimpoCub]

Interesting point... operating in a vacuum.
I got those spendy FAD vented caps, but/because they're an important part. (They cured my wicking moisture prob.)
Wonder if there's a diff in HOW they're vented. The ram air vents look gooney but I've had to blow into my sled tank to get it to start (2-man job ), and shop-vac pressure in my 300 gal tank to get the pump to prime. If it's just the last few gallons, it's not likely an obstruction, right??

 

[cubdriver2]

Bob have you tried doing a flow test on the ground, you could start out with say 10 gal and see how it drains till it's empty. you could do it at the gascolator sump or remove the hose from the carb.

Glenn

 

[mike mcs repair]

We have a Cub locally that has had venting or line blockage issues. The left tank seems to feed fine until about six gallons are left, then it stops feeding.

what type tank?

most likely the fuel line coming out of tank is not level or bellow outlet, would be my fist guess.....(atlee tanks needs outlet moved to BACK side of upright tube going to rear spar attach, down ontop of window frame)

or a kinked line

or old style fuel selector with detent washer upside down not allowing flow holes to line up in housing(hence restricting flow/line size)

or a collapsed tank that REALLY only holds 6 gallons less than it should....( because some one goofed and used a un-vented cap once....

 

[RAC cubs]

Those aluminum caps are junk I have a pair here new I will sell cheap. is the plane flying level ,Those caps can vent fuel out. if you fly next to some one with full tanks you will see fuel venting out,what do you mean it stops running,or just goes lean?

 

[Richgj3]

Early on Legend had the same problem with the caps that vent around the base. Switched to ram air caps now both tanks drain the same. Legend fuel system is either on or off, can't select left or right. With the old caps the fuel would come out of one tank or the other first. Sometimes it would transfer to the low pressure side and actually overflow. I guess those caps put the vent right in the low pressure area of the wing. I have seen many SC with ram air caps. In fact I never saw one without them.

Rich

 

[skywagon8a]

Do you have a both position on the selector valve or is it just left or right? A lot of planes which feed both tanks simultaneously have this situation. Has this been happening all along or just since the wing was repaired?

Has anyone used a vent interconnect between the tanks? Would there be any advantage to this?

 

[mike mcs repair]

since it's left, where did they hook the front header tank vent line? to front lower tank outlet or up to top of sight gauge?

if it has both position, then it must have a cross vent tube at highest point(if it traps gas in a low spot it will not cross vent right)

or would help if you tell us which version of fuel system it has... headerless, pennyan, original, both selector......

 

[bob turner]

Thanks guys. I have not been inside this wing - just following some very good mechanics. I got a pair of discarded machined aluminum caps from the last investigative excursion - use them on parked J-3s to preserve the corks.

I do not fly the aircraft often enough to have a real feel for what is actually happening - but I think it started with the wing repair. I will open it up and give it a good look. Meantime, I will be trying one of those CC caps. The airplane belongs to one of my students. You have seen it at Johnson Creek.

This is stock two tank Piper fuel, but installed after the aircraft left the factory. It is a very light 18-95 converted to flaps, fuel, and 160 HP. Gimme a couple days to look in there.

 

[n40ff]

Don't have that much experience but do know that the ram air cap is stock on the J5A-75 wing tank although it runs fine with a regular vented cap.......The J5 wing tank feeds into a 6gal header tank.

Aside, both my EAA bipe and Spezio use ram air caps. The EAA flow tested fine on the ground btw.

Considering you have a certified aircraft, there should be a "correct" set-up.

FWIW

Jack

 

[bob turner]

If venting is a problem, flow testing while parked is useless. My J-3 wing tank started with a standard vented cap, and getting it to flow was problematic. Converting to the ram air cap made it flow perfectly and consistently. With a skinny little tube that goes down the window post, it flows at about twice the consumption rate at cruise. More later.

 

[mvivion]

Super cubs have been running just fine for decades without those snorkel vented caps. My understanding is that those caps were required on the CC 180 hp cubs in lieu of increasing the size of the fuel lines to provide positive pressure on the system. I don't know that to be a fact, but it makes sense.

I've run a half dozen 150 and 160 SC with plain caps such as the Atlee caps. I'm not familiar with the caps you've described.

I'd say whether the snorkel caps help or not, I'd track down the source of the problem and get it fixed. There may be other issues as well.

MTV

 

[King Brown]

Nothing worked for me regardless of price until I went with the snorkels for the 180hp.

 

[swoeric]

has anybody seen the gas caps that ride in a header tank and have a gauge that can be read rather than just the floating hanger. i think i have seen these on heating fuel oil tanks but also on one or two aircraft

 

[Mark Lund]

I used to have one, might still be here. It was in some J-3 stuff I had, it looked like it would work in a J-3 tank instead of the cork and welding rod gauge.

Mark

 

[mvivion]

Nothing worked for me regardless of price until I went with the snorkels for the 180hp.

The 180 hp engine flows more fuel. The snorkel caps provide some additional pressure to permit staying with a gravity flow system, I believe. Shouldn't be necessary on an airplane with a 150 or 160 or less hp.

MTV

 

[supercub24a]

MTV is right ! you need to check your system ! Mikes repair is also on a good track also you did mention 95 HP now 150HP I do know the older 125's and 135's you had to drill an extra hole in the gasolator when converted to the 150 hp but you more likely have something else besides a venting problem
Steve C

 

[bob turner]

If I had been sure it was a venting problem I would not have asked. I agree, something is going on. Why would it feed 12 gallons, then stop? I like the theory of a collapsed tank, but they were both exposed at the last annual.

 

[supercub24a]

Did not mean to sound harsh so sorry if i came across that way.
so how does it quit when its down to six gal. level flight ? hard climb? sitting on the ground? ect.

If your main line is plugged on your left tank at the screen or at the selector valve tee you will still feed from the forward line with full tanks ,but when you get lower on fuel in a climb on takeoff, our on the ground it will not flow enough fuel , its only feed by a 1/4 inch line after removing fuel screens changing carbs ect. it was fuel lube at the brass tee plugging the main L/H fuel line.
Steve C

 

[Fortysix12]

What Mike the mechanic said, Sounds like if the fuel does not flow while sitting on the ground than someone has looped the feed line higher than the fuel tank exit port. We had a 12 that was rebuilt and both lines looped almost to the top of the wing root. We discovered this on a trip north and with still 10 gallons in the tank a hard bank to the right then hard bank to the left caused the fuel to break the syphon and the result was a 3 mile deadstick to an airport.Anybody in Sylvania GA remember that day.

 

[mike mcs repair]

.....
If your main line is plugged on your left tank at the screen or at the selector valve tee you will still feed from the forward line with full tanks ,but when you get lower on fuel in a climb on takeoff, our on the ground it will not flow enough fuel , its only feed by a 1/4 inch line after removing fuel screens changing carbs ect. it was fuel lube at the brass tee plugging the main L/H fuel line.
Steve C

good thought..

also probably remove hoses connecting tank and line splices, make sure no one tried to pinch off hose with vise grip like method and collapsed aluminum line end some....

 

[bob turner]

I will be checking all the lines. It only quit on me once, and that was on final approach at sea level with engine slightly above idle, and nose slightly down. I had flown for about 50 minutes on that tank.

 

[nanook]

Are you sure the float in the carb isn't hanging up? (dragging on the side of the bowl)......

 

[39-J3]

I have a friend that had the same problem with his piper colt. The problem was that Mudd Dobbers had plugged up his vent line. The fuel would drain out fine until he was around half a tank then the engine would starve for fuel.

Larry

 

[n40ff]

That's usually my first try, disconnect the vent line('s) and see if I can blow through them.........