C-150 Misfiring after takeoff

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btp44

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btp44
I have a C-150 Continental O-200 engine that is misfiring shortly after every takeoff. This typically happens about 10 minutes after takeoff while in a climb attitude at full power. The engine will drop to 2000 RPM for 5-10 seconds and then regain full power.

My first thought was water. However, I have never found any water contamination in the fuel. I have done the shake rock and hold the tail down method and still no water.

Is this just water that is not coming out when I sump the tanks or is there something else that could be causing this issue?
 
Bad mag or sparkplug or plug lead. Ignition is 90% of all engine performance problems.
 
If it is just a sag in rpm, without misfire then it's a fuel vent being partially blocked. What you describe is just enough time for the tank to need air to replace the fuel that the engine has used. If the vent isn't keeping up with that, it will sag and (then if its only a partial block) when the air is replaced it will run back up to cruise. A more complete block will stay 200 rpm drop that might be fixed by a lower throttle setting for a while to let the air catch up. If it is a complete blockage, the engine will quit. The mag can go bad with a coil heating up after a half hour, but that is an actual misfire on one mag or another and you usually don't get an rpm drop since the other mag is firing just fine. A plug or mag will reveal itself by being worse on L or R. Climb over the airport until this happens and then switch the mags to R to L to both. If you can't isolate the issue with that, check fuel venting.
 
It would be interesting to know if this aircraft will do this with carb air on .
 
skyking3286 said:
If it is just a sag in rpm, without misfire then it's a fuel vent being partially blocked. What you describe is just enough time for the tank to need air to replace the fuel that the engine has used. If the vent isn't keeping up with that, it will sag and (then if its only a partial block) when the air is replaced it will run back up to cruise. A more complete block will stay 200 rpm drop that might be fixed by a lower throttle setting for a while to let the air catch up. If it is a complete blockage, the engine will quit. The mag can go bad with a coil heating up after a half hour, but that is an actual misfire on one mag or another and you usually don't get an rpm drop since the other mag is firing just fine. A plug or mag will reveal itself by being worse on L or R. Climb over the airport until this happens and then switch the mags to R to L to both. If you can't isolate the issue with that, check fuel venting.
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A plugged vent will stop all gravity flow and the engine will quit. The 150 and many other Cessna models have, by AD, at least one vented cap to prevent that.

It's way too common to blame the fuel ststem, and enormous amounts of money are wasted looking for fuel system defects until the problem is finally traced to ignition. Ignition is easy enough to rule out if the airplane is properly maintained.
 
OK, so if it is a fuel vent partial blockage why (a) do the mandated AD vented fuel caps not take care of that and (b) a partial blockage is just that, why would the engine magically resume power and then continue on normally?

Or ignition. What in the system fails ten minutes after takeoff, continues for a couple of minutes, and then fixes itself and resumes flying?

Right now I'm not buying either one until somebody can explain what theoretically is happening ... SPECIFICALLY.

Jim
 
Fuel system modified by hack A&P. Climb builds heat and angle of climb reduces cooling air flow. Hear vaporizes some of the fuel in the line. Carb bowl goes low or bubbly fuel messes with proper atomization, engine stumbles. Not bad enough to kill engine.

Just an internet guess.
 
455 Bravo Uniform said:
Fuel system modified by hack A&P. Climb builds heat and angle of climb reduces cooling air flow. Hear vaporizes some of the fuel in the line. Carb bowl goes low or bubbly fuel messes with proper atomization, engine stumbles. Not bad enough to kill engine.

Just an internet guess.
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Would have to be messed up pretty bad to do that.
 
I know about vented caps not venting. My Chief had one that didn't vent and it had the same symptoms. Perfect climb. 10 mintues or so, it sagged from 2200 rpm to 1900 rpm. Stay. Engine ran smooth the whole time. If the fuel cap goes SWOOSH when you remove it. Bingo. I actually had video cam working when I had one of these events...this one was on takeoff after an extended taxi and run up. It was the cap in the event and it was a bit subtle to detect when you are busy with the feet.
 
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BTW, is it a misfire or an rpm sag.... misfire would be ignition. Sag probably air/fuel. Smooth sag or a noisy misfire?
 
skyking3286 said:
BTW, is it a misfire or an rpm sag.... misfire would be ignition. Sag probably air/fuel. Smooth sag or a noisy misfire?
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Let'sgoflying! said:
Yeah, does it bang and shake around, or does it just wind down or surge smoothly?
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It's more of an rpm sag. It drops from 2500 rpm to 2000 rpm but the engine is still running smoothly. It does not bang, shake, or make any unusual noises.

Also, I would like to add that it will only do this one time per flight. I have done multiple takeoffs and climbs during flights after this happening and it will not do it again.
 
Dan Thomas said:
A plugged vent will stop all gravity flow and the engine will quit. The 150 and many other Cessna models have, by AD, at least one vented cap to prevent that.

It's way too common to blame the fuel ststem, and enormous amounts of money are wasted looking for fuel system defects until the problem is finally traced to ignition. Ignition is easy enough to rule out if the airplane is properly maintained.
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Take a piece of tubing and put on your fuel vent, remove the fuel cap on that side and blow into the tube. If you hear air in the tank then your good. Don't worry about the fact that you feel a little weird doing it, it will save you from running dry due to plugged vent.
 
The once per flight thing makes this a real puzzle. If it's fuel vent, it would sag then recover, once it pulled enough it would sag again, etc. Rinse & repeat. The OP has said (post #17) it only does it once per flight. Even with repeated take offs and climbs it runs fine after this one drop.

If you shut it down after it does this once, does it do it again or only on the next start up from cold?

John
 
If you shut it down after it does this once, does it do it again or only on the next start up from cold?

So far, it has only done it on the start up from cold.
 
weirdjim said:
OK, so if it is a fuel vent partial blockage why (a) do the mandated AD vented fuel caps not take care of that and (b) a partial blockage is just that, why would the engine magically resume power and then continue on normally?

Or ignition. What in the system fails ten minutes after takeoff, continues for a couple of minutes, and then fixes itself and resumes flying?

Right now I'm not buying either one until somebody can explain what theoretically is happening ... SPECIFICALLY.

Jim
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The OP called it a misfire. That implies ignition and some vibration. Now he says it's a smooth sag.
It could be something rare like a loose carb heat cable clamp. But without much better description of symptoms, nobody here will have any worthwhile advice. He'd better get a mechanic to look at it.
BTW, I've seen mags intermittently fail and cause an RPM sag.
Why it would do it on a schedule like the OP's is another thing.
 
Spend the money and have a professional look at the the fuel and ignition system. The next flight in your 150 may be your last.
 
Brad Smith said:
Spend the money and have a professional look at the the fuel and ignition system. The next flight in your 150 may be your last.
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Best comment yet, along with post #4.
 
I appreciate you all taking the time to post and share your knowledge/advice. The plane will be going to the mechanic this week. Hopefully he will get it figured out.
 
btp44 said:
I appreciate you all taking the time to post and share your knowledge/advice. The plane will be going to the mechanic this week. Hopefully he will get it figured out.
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Please let us know what turns up. We like playing told-you-so :)
 
BTDT! When you say misfire, do you mean a cylinder is skipping or it just starts running crappy?

Tom D on this forum who has already responded is very knowledgeable about these engines and a virtual encyclopedia is available by searching Fly Baby Harry Fenton. Harry has written a long document with a massive amount of well indexed information. He also will answer emails and is generous with his time and knowledge as is Tom. He has answered several questions in that document that relates to what you describe.

Hope this helps.
 
Time for the envelope and the winner is....

Do we have a word on what the problem turned out to be? btp44 ?
 
Carb Ice???
The mechanic did multiple test and said he couldn't find anything wrong with the plane. The plane runs great on the ground. He thinks the plane is building carb ice during ground operations and that is causing the stumble shortly after take off. I have flown the plane once since he looked at it. I used carb heat from start up to take off and the plane did not stumble. He also added a fuel treatment to help with a potential stuck valve.
Thoughts???
 
Probably dumb questions but here goes:

1. Do you have pretty long wait times from startup to takeoff. For example, its always busy and you are number 7 to takeoff. Or maybe a very long taxi for takeoff.

2. No matter how much you taxi, how aggressively are you leaned prior to takeoff?

3. Are you leaned during the climb? If so how much? When it happens have you tried going rich again?

4. Are these flights taking place at times when you can make carb ice (eg. visible moisture, high humidity and air temps in the "sweet spot")?

5. What density altitudes are flights being done at?

...if I over lean our continental during climb it can result in a small little "Wake Up!" but it isn't right away it might be 3 or 4 minutes into the climb. I just enrichen it a bit and it goes away. The cylinder head temps is okay. The MP is not changing enough to be ice, just for altitude.

Its seems weird in your case that it would be ice each and every time. It seems more like a Mag or a few plugs don't jive with your climb out mixture settings and then it quickly starts firing everything again after burning something off or warming up again, etc.
 
I remember a A-65 that stumbled a bit and the pilot aborted the takeoff. I walked over and there was water dripping out of the carb box. It had iced up.
 
Don't need visible moisture for carb ice.

The O-200 is famous for oiling the bottom plugs at idle.

The O-200's carb is mounted to a carb "spider," a small manifold with four outlets. There are supposed to be Lock-O-Seal washers against the top and bottom of that spider on both studs, and the retaining nuts tightened FINGER TIGHT and cotter pinned. This lets the engine move without shaking the carb so much. Shaking causes fuel to slop from the carb bowl into the carb throat thru the bowl vent and makes for rough running and big rpm drops. We chased that issue for weeks on one 150 until I found that little paragraph in the overhaul manual. Installed the washers and it all cleared up.
 
Answers to the question that Sanistar asked:
1. Typically 20-25 minutes from start up to take off
2. Leaned as much as possible during ground operation
3. Not leaned during climb
4. High humidity
5. 2500-3500
 
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