Thorp Air Command - T18.net

Right on John - I took your advice and actually just took off and rebuilt the fuel line firewall aft last week, as I had a bit of minor weepage around the tank fitting, anyway - it was all original and needed changing - had teflon tape at all the fittings and I should have done it a long time ago - no weepage now - the screen had a few little naughty bits within its confines - while the tank was empty, I vacuumed out a few other naughty bits in the bottom of the tank - little bits of rubber and other foreign matter. When I installed the carb last year, I installed a new fuel line FWF and a new gascolator - removed and cleaned it while doing the fuel lines last week as well. Fuel wise, the system is as clean as a catheter - it's just a matter of getting some positive pressure in that outdated vent.

For those who don't know, I overfilled the tank with avgas when departure fueling from Gary Green's fly-in - we got vacuum in the tank and flamed out just at rotation.

Good to hear you got the fuel supply running ! I try to fill the petrol tank to about an inch below the top , but I had miss judged the nozzle on occasion and had the sucker over flow . With a facet fuel pump and the engine driven pump I was not to worried on take off . Had a positive fuel flow to the carb . As a side note when I was having carb problems and had the fuel line disconnected from the carb and turned on the facet pump , I had more than adequate fuel flow thru the mechanical pump...bypass . With the facet/engine pump I show about 3-6 psi pressure on the gauge . As a side note with the fuel line disconnected to the carb I had gravity flow thru the gascolator, thru the facet pump (off) , the mechanical pump (engine off) to the disconnected line to the carb . Not sure about the flow per minute ? I like having positive head pressure to the carb ! PUMP Me NOW , PUMP Me tonight !

RB

It sounds like you really went through it with a fine-tooth comb, Fraser. Good job!

Forgive me for mentioning the "Arkansas Incident", but I think you shouldn't blame it on overfilling the tank necessarily. That led to the problem, but I believe it was the mostly the result of a poorly designed/installed vent system on that airplane, where the vent line came down, went up, then down again, forming a real "p-trap" on the right side of the cockpit where the vent line ran. Your immediate reaction and skill as a pilot saved the day. We were a little lucky that we weren't over the trees at the end of the runway, but you did everything right. I'll fly again with you anytime.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

No worries about mentioning it John - thanks for the undeserving kudos.

Sharing this kind of info (bad vent line routing) and how you saved your bacon (how did you do it ?) would benefit someone else . Also where did you find a clean pair of panties to replace the soiled one from that day ?

RB

Well Rich......there I was, surrounded by bogies......John kinda made it sound more than what it was - we were just about to be airborne when the engine quit....all I did was turn off at the next taxiway, jump out and open the fuel cap - Dick Wolff has the airplane now....Would imagine this got his attention - having said that, I really overfilled it that day, and that was the only time it ever happened.

As far as the underwear......I was sporting the kilt.

Good to hear you got a box of Depends Kilts ! Not sure how the engine runs on the ground with an over full tank and a plugged vent line ? Is there a reason why we can't use a vented cap ?

RB

It's critically important to create some pressurization of the system with the designed vent, if you have a gravity flow only system like very many of us do. Even with fuel pumps, it's a good idea. Obviously, a vented cap would negate any pressure produced by the vent.

_________________
John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

JR : Guess I was not thinking that day ?

RB

I do not really believe the vent pressurizes anything. The gravity system will work just fine as long as the drained fuel can be displaced by air. Hence the vent. When I had all those troubles I wrote about years ago, it was because the vent gets filled with fuel. The fuel goes in cold, expands due to the ambient heat, and fills the vent line. My line if perfectly vertical. Yet when it is full of fuel, the carb will starve due to the vacuum. Make sense? My vent bung at the top of the tank extends about 3/4 inch down into the tank. When you fill it all the way up, the fuel expands into the vent line. I got tired of flaming out at ten feet on take off. I installed a facet ticker pump that runs off the master. Never flamed out since.
Cubes

Are we talking soda straw theory here ? I always thought it was strange that a small vent 1/4" tube going up and over to the top of the tank could "pressurize" the fuel . There may be a small amount of ram air going into the "tube" , but an open air way to allow the fuel to flow (gravity) would seem to be the key. Hince the "Soda Straw Theory ." I too have the Facet pump and have overfilled my tank to overflowing...not by choice . The Facet pump gave positive pressure to the carb in this un planned situation . BOOM !


RB

I have over filled my tank more than once that fer sure, the fuel will run out the vent and then stop,and believe you me it will run a steady stream untill the siphon is broken it happened when I was up in Susanville a couple years ago. They were filling our tanks for free when we left and I got greedy and filled er up. It probably ran a half gallon back out on the ground, but I have never had it quit on take off. My RC airplanes always had the vent pointed at the prop, blast vent we called em,these were .062 vents and it really made a difference in the way the little bugers ran. I thought about running a 1/2" vent line from the 3/8" hose barb on the top of the tank down to a 1/2" opening facing the prop blast, the 3/8" outlet would never be able to shut off the 1/2" hose in a gravity senario and thus stopping a siphon from occuring, if the 3/8" tube in the top of the tank were to fill with fuel it would then just dump into the vent hose and dribble out or it would run back into the tank. In any case if the supply to the vent hose could not seal off the hose then the siphon would not occure. The 1/2" inlet on the vent would however catch a little more blast and probably have to be screened to keep critters that are impervious to gas fumes from intering in, just a thought...Dan

Tony Bingellis always said you should have the vent pointing forward to help add a little "head pressure" to the fuel system. Of course it needs a screen of some sort to keep out bugs and to prevent anything from building a nest or whatever. When I was a student pilot I took the "hangar queen" Cherokee up one day since all the other planes were already rented and found I had no airspeed as a critter had built a nest deep inside the pitot that you could not see on preflight. Anyway, I don't claim to understand the science, but if you do not have positive pressure you could have a negetive pressure situation that would actually prevent fuel flow. So you should have a pressurized vent even if the static fuel flow test seems to be adequate. That's what Tony always said!

The reason we do not have a vented cap is that the cap is in the cockpit area. Years ago, when is was having flameout troubles, I drilled a small hole in the cap under the key. That was at the Rapid City fly in. I flew home choking on gas fumes! I will NEVER do that again! I'm not saying the vent can't pressurize the tank. I'm sure it can. But the big deal is to VENT, not pressurize. Remember, these are simple float carbs. Pressurization is not going to flow more fuel to the engine. We are just concerned about starving the float bowl, mainly on takeoff.
Cubes

I believe you will find that a Marvel carb requires about 1.2 pounds of fuel pressure to operate correctly....do I hear fuelpump