C150-O200 Engine Quit

[weirdjim]

About 5 months ago, a fellow's engine went to idle about ten minutes into climbout and he stuffed it (undinged) onto the freeway. Engine stayed at idle (about 800 rpm) until he shut it off after landing.

Fired it up the next day on the ground and it ran at normal power for about two minutes and then went to idle. No combo of mixture, carb heat, or throttle had any bearing on the problem. Did it a few hours later, same thing. Did it the next morning, same thing.

Pulled the fuel line off at the carburetor and let it pour (steady stream) into a jug and convinced ourselves that it wasn't going to take over a quart a minute for an hour to keep the airplane in the air. Engine controls were tight and secure.

Pulled the carburetor (metal floats changed and overhauled about five years ago) and the only anomaly we could find was about a half-thimblefull of dirty water at the bottom of the bowl. Floats were free. Passageways appeared unclogged.

Put it all back together and ran it up and it went for a good half hour with no apparent problems.

Now we all know that unfixed problems will most probably recur. I'd rather not have them recur with this fellow on departure again, especially as it will be my signature in the logbook for the annual.

One thought I had was to take the carb down to the local ... um ... "aviation" carburetor fixit shop and have them flow test it for an hour or so. If there IS a clog in a passageway that isn't optically visible, this might find it, or it might have been a clog that we loosened and blew out the exhaust when we disassembled the carb and then ran the engine after reinstallation.

Thoughts?

Jim

 

[AdamZ]

Jim nothing close to an A&P here but could the reduced RPM have been caused by something else blocking the air flow to the engine? Nests, loose rag anything. I recall a few years ago Ron had an incident in his Tiger where tape or padding or somthing from the airfilter peeled or melted loose and got sucked in reducing the RPM.

 

[Witmo]

Sounds like the carb is the most suspect component. Another cause of uncommanded decrease in fuel flow is a defect in the fuel vent system. Primary fuel vent gets plugged and the vented fuel caps don't kick in until a vacuum builds in the tank decreasing the fuel flow. What usually happens if the vented fuel cap functions correctly is the engine cycles from the power setting selected down to idle and back as the fuel cap vent pops open and closed.

 

[Capt. Geoffrey Thorpe]

Likely a long shot - I ain't seen under the cowl of a C-150 for a long time, so I don't remember the layout - but it sounds like it could be a flex hose that connects the carburetor to the outside world (assuming there is one, and assuming that you still go through it with carb heat on...) - I've seen them collapse and choke the flow to the carb (on automobiles) at higher throttle settings, but be OK at idle. In that particular case, a wire reinforced flex hose (a.k.a. dryer vent) was used, but it was too long and the hose would kinda collapse sideways. A corroded wire in a legit hose might do the same.

Again, a long shot, but not that hard to look into. YMMV, FWIW, I ain't no A/P, etc.

 

[Tom-D]

These aircraft are equipped with a carb airbox with a flapper style airmixer valve, they have been known to crack and allow half of the flaper to close without opening the other side. this shuts off the air flow to the carb and gives you these symptoms.

remove the airfilter and inspect the flaper for cracks at the shaft area.

 

[Let'sgoflying!]

dont forget the exhaust outflow, could be blocked intermittently by loose baffles

 

[Dan Thomas]

dont forget the exhaust outflow, could be blocked intermittently by loose baffles

That's the one I'd go after, except that there are two mufflers on a 150's O-200 and both would have to do this at the same time. The other suggestions that something is blocking the intake are ignoring the fact that these would act like a choke and increase the mixture to the point that the engine would not run. Suggestions that have fuel flow diminishing similarly affect the mixture, leaning it until it would not run. Just try to get the engine to idle with the throttle wide open and leaning it with the mixture; it ain't gonna happen.
This one I've never heard of and I wonder if Jim is pulling our collective leg?

Dan

 

[Bob Noel]

>That's the one I'd go after, except that there are two mufflers on a 150's O-200 and both would have to do this at the same time.

well, assuming that loose baffles in one muffler would be hard to detect, the mufflers could have failed months apart.

 

[Let'sgoflying!]

That's the one I'd go after, except that

true enough, I admit to skimming the post

 

[Tom-D]

>That's the one I'd go after, except that there are two mufflers on a 150's O-200 and both would have to do this at the same time.

well, assuming that loose baffles in one muffler would be hard to detect, the mufflers could have failed months apart.

With 1 muffler blocked you would loose 2 cylinders of a 4 cylinder engine, I suspect you would notice.

But it would not return to the same RPM upon failure each time. This one is hitting the 800 RPM each time it fails, = intake blockage, producing the same MAP each time.

 

[Capt. Geoffrey Thorpe]

If memory serves me correctly (not likely, eh?), if the housing for the throttle cable isn't properly crimped into the end fitting (at either end - depending on where it bends and how it is secured) it could slip out and let the throttle close. Bring the throttle knob back to the idle position would force the housing back into place - for a while.

Less likely would be if the housing split somewhere along a bend.

 

[Dan Thomas]

If memory serves me correctly (not likely, eh?), if the housing for the throttle cable isn't properly crimped into the end fitting (at either end - depending on where it bends and how it is secured) it could slip out and let the throttle close. Bring the throttle knob back to the idle position would force the housing back into place - for a while.

Less likely would be if the housing split somewhere along a bend.

I'd though of that, but Jim had said that no amount of fooling with the throttle or mixture or carb heat changed anything after it did this. I'd think that the problem would persist, not disappear the next time they started it.
But it's a good idea to have a look at the entire throttle cable assembly to see if the cable sheath is parting at some point (maybe immediately behind the panel) or coming out from the clamp at the engine end.
We've had a throttle cable break in flight (the music wire inside) as well as a couple of carb heat cable failures, so we now replace all engine controls at engine changes. They DO wear out and get tired. If the internal wire broke, Jim's throttle would come out in his hand.

Dan

 

[weirdjim]

This one I've never heard of and I wonder if Jim is pulling our collective leg?

Dan

Jim is not pulling your collective leg; the fellow was enroute to a flyin at Columbia airport in June? and the story and resultant blog went on for about a week in the local newspaper (California, Grass Valley, "The Union"); you can go back and look in the archives from that era if you like. The landing was on Highway CA49 near the Dorsey Drive overcrossing and you can check CHP logs for the incident. Since nobody was hurt, nothing was broken, it was not a citable offense, so you won't find a court case in the matter.

I'll be happy to post an N-number if you like; I don't know if the FAA even entered it into the database because nobody got hurt and nothing got dinged.

As to all the rest of it, you can bet that we squeezed the hose going between the gasculator and the carb, we very carefully inspected (magnifying glass and mirror) the throttle cable, including pulling off the center conductor and giving it the old "pull hard" test to see if it had a hidden break, pulled the fuel caps OFF during one test to see if a vent had gone goofy, and in general tried everything we could think of to locate a singular failure point.

Again, thanks for all the wags, ewags, and sewags, but nothing y'all have said so far wasn't something we investigated thoroughly. I'm sort of on the sidelines as the fellow uses an A&P for his fixit jobs and I'm only there as the annual doobie. (I stopped doing fixit a long time ago when I found out that teaching paid a hell of a lot more and didn't put my license on the line every time I fixed something).

Jim

 

[Jim in NC]

I had an old VW bug that had a problem that sounds somewhat similar to your problem. A crappy fuel tank (rust and scale on the walls flaking off) and the fuel filter had some small holes that would occasionaly let some stuff through, and get in the carb.

My thinking is that the small amount of dirty water and crap in the float bowl could be getting vibrated up and into circulation, and into a passage, then getting cleared when there was no suction.

My bug would run like crap, with barely enough power to keep running when it had its problems, then when you shut it off, it would run normally for about 20 minutes.

The problem finally went away when I cleaned out the carb, and put a new fuel filter (pleated paper) on it.

I hope you fixed it. I'll bet you did, and hope you can satify yourself as to the permenancy of the repair.

 

[Capt. Geoffrey Thorpe]

Again, thanks for all the wags, ewags, and sewags, but nothing y'all have said so far wasn't something we investigated thoroughly.

What? No thank you for just plain dumb ideas??? ;-)

Question. You said it comes back to idle - does it actually run like it's at idle (smooth)? Run rough and slow? I suspect that the pilot wouldn't be able to explain if it was a lean misfire or blubbering rich problem?

Finding the water in the carb is odd - but I'm not sure where to go with that.