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fuel starvation or vapor lock

bushmaster

GONE WEST
GLENBURN,ME
help- pa18 stock fuel system. After 45-50 min of flight at cruise engine starts to sputter and run rough. loss of rpm and power. at full rich. I have checked fuel flow from both tanks no problem found. Checked fuel caps , carb screen, gascolator , fuel lineto carb, no problem found. IA thought maybe vapor lock. Installed insulation hose on carb fuel line checked all engine baffles. It does it when it is hot out 80-85. runs great till it starts this only way to improve running is pull on carb heat and leave on and reduce engine rpm to 2100-2200 w\full carb heat. A slow squrit of primer helps for few seconds. This is scarey in the wilds of maine . any ideas.
 
First thing I'd check, if you haven't already, is fuel cap vents. Do you have good vented caps, and which engine do you have? If the O-360, you might need different vented caps.

I've never heard of vapor lock on one of these machines, certainly not in in flight.

Finally, are you running auto fuel or avgas?

MTV
 
I have 0-320-150. I have checked vents in caps also landed and immediately removed caps as it was doing it and found no vacumn. Did a fuel flow test on each tank- 5 gals checked screen in carb where fuel goes in. It did it 1st time on combo fuel avgas and auto hightest. only running avgas now and still did it but only in cruise after 45 min plus wont do it local around lake flying. 1.5 hrs flying. This is acomplete rebuild with all new lines etc it run great till it start then cant stop it till it sets for awhile. I dont believe it can be carb ice as temp is 80-85 and i ran carb heat 10 min to get back on lake and land has done it 3 times now . I insulated fuel line at carb and gascolator flew for an hour and no problem but on return did it again . have to pull power back and carb heat on to get to run better
 
Level the plane as it would be at cuise attitude then with the fuel selector in the same position as the problem remove the fuel line at the carb and let it run into a series of clean 5 gallon containers and check for a slowing in volume. 15 minutes would be 2.25 gallons so you should see big change in the rate of flow. If so find the problem. Also Sputtering can also come from too rich. The primer working loose? Check valve in the Primer working like its supposed to? Also you could have a big hunk of crap sliding back and forth in the final fuel line to the carb blocking the finger filter. A tank full of water? When the plane is level do the same timed gravity flow test at the gasolator to rule it out. Do you have an egt? what's it doing at the time sputtering. Why is it spuddering? Does it do it if you climb for 15 minutes nonstop. Something simple and your just gonna have to find it. I think I'd take that carb apart and look at it. You have some crap floating around in there that finds its way to the fuel jet. before you pull it take a very clean white flannel cloth of some sort and drain the carb bowl catching everything that comes out. You might get lucky. Got carb you can swap with someone? Good luck.
 
good ideas i am on floats so I did drain 5 gal from each tank at carb end of hose in to a bucket no changes no water or dirt. I do think i will take gascolator and carb fuel bowls off and check. I do believe it some thing simple but not agood feeling when happens. I have not taken fuel selector apart - have to empty tanks but fuel flow was good so dont think thats it .idea of crap floating around is good one and what I suspected but can only get it at full cruise for 45-50 min. I have checked all baffles for cooling and oil temp runs 180-185 at 80deg . oil pressure 75 - 80 I have no egt
 
One of these pros might have some better ideas. Unlock your primer and see if theres any change. I would think if the engine runs better with the primer unlocked and slightly out its got to be the carb or whats feeding the carb. If you still can't find it swap a carb. If you do another flight test switch off the left mag then right just to check. Wish I lived by to help.
 
dont know what venturi I have . Did play with primer - gave it a little squirt and it ran alittle better for few seconds. also tried mags and made it worst as i recall. 1st time was only few min as i was almost at lake 2nd time was 30 min i was over river so did some experimenting ie mags mixture-which made it real bad engine almost quit carb heat on continuous and throttle back was best results and got home. 3rd time went about 15min but had better idea what to do -throttle back and pull carb heat on. played with primer little then landed. if after making checks and cooling down you take off again it runs fine
 
Ponder, ponder, ponder. Why does it run better with carb heat on and throttled back? The fuel system sounds like it might be the culprit, but what if there is a problem upstream of the air box that pulling the carb heat (AKA alternate air source) corrects? Pulling carb heat does allow air to bypass the filter.

If fiddling with the primer helps though, that kind of shoots holes in my theory. Oh well. I'll be quiet again. :oops:
 
I had the same thing happen twice when I was getting some rust in my refueling system. rust would get through the gasculator and plug the screen in the carb. But I assume you have been there already. The screen is right where the fuel enters the carb. It is easy to get at. Even the spell checker doesn't know how to spell gasculator!

Ron
 
I would try a different carb too, one without the new one-piece venturi. See AD-93-18-03.
 
There have been incidents where the stratoflex fuel line fittings cut into the hose when they were installed and acted like a flapper valve periodically shutting down the rate of fuel. Something to check. What was done prior to this happening? A total rebuild of the airplane?
 
That is classic clogged fuel screen. Are you sure that finger screen and fuel bowl screens are clean? What happens is, when not running the junk clogging the screen falls loose back into the lines or bowl. When it runs, the fuel flow packs the junk tighter and tighter on the screen till it chocks off. Reduce the fuel flow, the junk relaxes from the screen, and it runs again. I've had this on several aircraft over the years. Inspect that carb finger screen CLOSELY, any dirt in it will be very fine because it has made it through the bowl screen, light should shine through the screen if it is clean. If these screens and lines are clean, I'd disect the carb and give it a good cleaning.
 
rock the wings....

When you drain your fuel out, ROCK the wings back and forth. Many times I have found trapped water in the fuel tanks. Rocking the wings back and fourth will generally allow the water to exit the tanks.

Mike
 
All good ideas lot i have checked I like your idea supercub md and will pull carb bowl this am and look. I have been running full rich when this happens lean even slightly make it worse immediately. I checked mixture control and it is going to full rich and works smoothly. I had finger screen out and did get tiny bit of debrie out of screen.
 
Also check continuity of mixture cable and lever arm, maybe the set screw is loose on the cable hold the lever on the carb firm and have some excercise the mixture control. You just never know.Also look at the shaft and arm relationship and make sure thats all tight. Can't wait to here what you ultimately find.
 
Someone out there in Maine has N2422H on floats. when that was new I was flying it on wheels and it quit on takeoff, so I landed on the remaining runway, checked everything (I thought) took off again, same thing. The next time it ran fine, went home and investigated deeper. Turned out the owner had been using car gas and the pea it the fuel gage swelled up and stuck giving an erroneously high reading. I was out of gas. Taking off from a rough field sloshed the fuel away from the port and starved the engine. Before, I always thought that those sight gages were infallible, and maybe they are if the pea is moving around but not when they are stationary. Sorry, I guess this wasn't on subject, but I was thinking it might be the same plane.

Ron
 
I flew a pa12 with a o320 that iced up on a very regular basis. Hot or cold weather but 80-90 degrees was the worst. We found it had a thick insulator type base gasket under the carb instead of the paper thin gasket that was called for. Seems the engine temp wasn't warming the carb body at all. After replacing gasket it never iced again.

S.
 
bushmaster said:
dont know what venturi I have . Did play with primer - gave it a little squirt and it ran alittle better for few seconds. also tried mags and made it worst as i recall. 1st time was only few min as i was almost at lake 2nd time was 30 min i was over river so did some experimenting ie mags mixture-which made it real bad engine almost quit carb heat on continuous and throttle back was best results and got home. 3rd time went about 15min but had better idea what to do -throttle back and pull carb heat on. played with primer little then landed. if after making checks and cooling down you take off again it runs fine

If one leans his aircraft right to the edge on both mags then switches to one mag it will run like wooster. Which is probably why yours runs like crap on one mag and better with carb heat. Like Supercub MD said that is probably the cause. Probably restricting the flow just enough that it takes 30 minutes to lower the level of the float to that point.
Had the same thing happen in one of my trucks recently. Ran one tank dry and sucked up some crap. Switched tanks and kept going but needed full choke to go uphills or anytime I mashed the pedal to the floor after that.
 
Here's another possibility. About three years ago, I converted my L-21B from a 135 horse O-290 to a 150 horse O-320. No other changes. After a couple of minutes at takeoff power, the thing would start sputtering and lose a lot of power. It was obviously starving for fuel. Checked carb, screens, mags, plugs, etc. Nothing. On a fluke, we pulled the little hoses on the fuel lines, between the tanks and the headers. The inside diameter had swollen down to about half what it should have been. Guess it was from previous owner's (my brother) use of tractor gas. Apparently, the flow was adequate for the 135, but the greater demand of the 150 was starving it after the fuel had run out of the headers. The time delay you are experiencing seems longer, and I can't make the symptoms correlate to my experience, but if all else fails, it's something you might look at.
 
looked at it a lot this weekend only found little dirt by draining carb at pipe plug at bottom of bowl and flushing bowl. with fuel from tanks. I didnt notice v on carb it is a ma 4 model have mech going to look at venturi am. Have removed and looked at inside of fuel lines no problem seen. Mech. suspect venturi problem or header tank vent problem but I get no hiss-vacumn after landing and removing fuel caps . Did find 2 plugs with carbon dep. but not bad also cleaned air cleaner . Runs great for short time all I had but dont feel I have found problem yet but I will.
 
fuel flow

Ron: You seem to have a fuel starvation problem. The one piece venturi would not cause your symptoms. The primer leaking would cause an over-rich situation. The fact that it takes 45min. to appear would suggest a fuel feed or a venting problem. If in fact you have eliminated those two sources as problems, then your carburetor may be the culprit. You may have something restricting fuel through the mixture port. You also could have a problem with your float hanging up. Either way you would have to tear down the carb to check those.
You have not stated whether you were on a particular tank when it happened. The fact that it only shows up after 45min. of flight @ 6gal. of fuel used makes it sound more like a venting problem. I have never encountered a vapor lock problem on a gravity-feed, carbureted system. I would go back and make sure that you have eliminated the fuel feed and vent system as a source of the problem.
 
Motor rebuilt?

It does sound like its fuel related, It will be fun to see who figures this out. The detective work is always interesting. Several times you said after about 45 minutes it starts acting up at cruse speeds. Once throttled back it seems to run better to get you back, it sounds like its starving for fuel, but Im thinking it could be something else. I know you said it just had a engine overhaul. Is there a chance that a cylinder is overheating and trying to seize up, cutting back on power might allow it to cool off some to get you back, and sitting for awhile to let it cool off, letting it cool down enough to run for awhile again. Something to think about, Valves, Springs,Cylinder and rings ? Best of luck, We need to get something set up with our group of Cub people, Free hat for who ever figures it out.
 
May have found problem. Actually 2 ,found lodged debris in top of gascolator behind screen on top in inlet fitting that had to be loosen to come out then fuel flow increased. Also have started hearing a exhaust leak and found bad gasket blowing onto intake tube. new gaskets installed on exhaust flange and clean threads and retighten. Still looking at vent lines to header tanks ex. but i feel better already but not totally convinced yet will need couple of long test flights to be certain also replaced fuel line carb to gascolator as it looked ok but felt soft and was old. Wiil post as i know more . Great ideas and help on here!!!!
 
If the debris is above the screen, then it is getting to the carb. Keep pulling and checking that finger screen. Pull the line between the carb and screen and flush it along with all the fittings. If the line and bowl are not new, consider replacing them. A couple hundred bucks in parts is going to look real cheap if you end up in the trees.

The exhaust find is nice, but not your problem. I'm just assuming that changing tanks when it happens doesn't help, that all but eliminates venting or any other problems upstream of the valve.

I'll bet someone a cold beer that the junk in that line is the problem, just make sure you get it all out, find it's source and eliminate it. What did the debris look like?

Was your Cub converted to a larger engine from a earlier model? Some of the early models had a small bowl inlet, and need to be drilled larger. I don't think this is your problem, but something to check.
 
someone ealier asked if i have a 1 or 2 piece venturi. It is a 2 piece I think I saw info and pros and cons on this on another thread I will look for that . Debris was black ? hose matter etc. Debris was above screen and have cleaned finger screen at carb and replaced fuel line between gascolator and carb going to test fly in 1 hr has been run by I.A. on ground for about an hour .
 
someone ealier asked if i have a 1 or 2 piece venturi. It is a 2 piece I think I saw info and pros and cons on this on another thread I will look for that . Debris was black ? hose matter etc. Debris was above screen and have cleaned finger screen at carb and replaced fuel line between gascolator and carb going to test fly in 1 hr has been run by I.A. on ground for about an hour .it was always a pa18-150
 
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