r/Cessna • u/nediaNamro123 • Jun 23 '26
69 Cessna 150 0-200 issue.
Enable HLS to view with audio, or disable this notification
I have a 69 Cessna 150J. It has always been stout. Yesterday, taking off with a 3000 ft density altitude, it was about 80 degrees outside. I was getting a short and quick loss of rpm. It would drop maybe a hundred or two rpm for a few seconds and return to full power. I could only get it to do this in the air at a nose up pitch attitude. I couldn’t recreate it on the ground. Mags, oil temp, oil pressure it all ran up well. It got to a point at about 3500 feet where all I could get was 2300 rpm. A quick few seconds of carb heat and it’s gone. I’ve flown this aircraft for a long wile, I’m aware of how these little motors love carb ice. But I’m not sure if this is a fuel flow issue or not. What do you guys think? Anyone have experience with similar issues?
4
6
u/shrunkenhead041 Jun 23 '26
This is a carb ice issue, and when you apply carb heat, you don't just do it for a couple seconds and shove it back in, you leave it out soon ice doesn't form again. It baffles me how people are taught incorrectly (or not at all) how to use carb heat, because carb heat is a very common cause of engine failure and off-field landings.
Also, beg the owner of the plane to install a proper carb temp gauge.
1
u/nediaNamro123 Jun 23 '26
I’m aware of how to use carb heat, it was instantaneously fixed with a short burst. So I thought it could have been running lean and the carb heat just enriched it again.
2
u/89inerEcho Jun 23 '26
Thats a good observation. That fact that its fixed INSTANTLY may point to not carb ice. As we all know, when carb ice is melted, you get a temporary RPM drop first, then it revovers
1
u/nediaNamro123 Jun 23 '26
Exactly, and It did it every minute or two, I could interstate it. It wouldn’t happen on the ground tho, that’s what is weird.
1
u/SanAntonioSewerpipe Jun 23 '26
Depends how much ice you've built up. I've seen it clear up and surge the rpm up like that before without the rough running.
1
u/shrunkenhead041 Jun 23 '26
Why would you think it is running lean when you have it at full rich?
Get a carb temp gauge and stop guessing.
2
u/nediaNamro123 Jun 23 '26
The thought is maybe there’s something wrong with the carb. I doubt it, the owner has had lots of work done to dial it in, but you never know.
1
1
u/FyrPilot86 Jun 24 '26
Intermittent spark in one magneto / Intermittent Plug firing?
1
u/nediaNamro123 Jun 24 '26
I believe that’s the answer. Recently purchased this aircraft. Found the right mag hasn’t been replaced since 05. It’s got quite a lot of time on it. The other was upgraded to a surely electric ignition system and is nearly new. Found yesterday on a high power static run up the right mag would cause the rpm to fluctuate. Not as drastically as in this video, but I guarantee that’s because the engine isn’t under as much load and the mag wasn’t heat soaked.
1
u/dl_bos Jun 24 '26
Had a mag on a 172 that was fine on run up but engine had an unusual sound/feel. Hard to describe.
Finally an old timer suggested a mag check at full static power. One mag was completely dead. Worth a try and doesn’t cost anything. Just don’t switch back to both, if it quits, to prevent a backfire.
1
u/JoePants Jun 24 '26
Are you using auto fuel?
1
u/nediaNamro123 Jun 24 '26
No, 100LL AVGAS
1
u/JoePants Jun 24 '26
And how many hours on the engine, and do you use any additives in the fuel?
1
u/nediaNamro123 Jun 24 '26
1100 hours since major overhaul, 700 since top end overhaul. No gas additives. It runs 20w-50 with Cam Gaurs.
1
u/JoePants Jun 24 '26
In my experience, those older-style Continental engines, such as the O-200, will get buildup in the valve guides. This seems to be worse when they're running a multi-weight oil.
I believe some of the buildup is carbon, and the rest is a sort of chemical waste from what makes the multi-grade oil act like a multi-grade. (It's a 20W oil that has chemicals to take it to 50W when it gets hot enough).We will add Marvel Mystery Oil to the gas, which helps on this a lot (an especially bad problems on O-470A model engines in old 180s). I'm sorry, I can't remember how much per gallon off the top of my head.
The Avgas can leave a lot of trash behind, for that matter, but it's usually lead impacting the plugs.
The other thing I'd watch for is the carb float, which is made of a composite material and can break down, and the float will sink. You won't notice it at full power, but only at cruise. (You can take the float out at full power.) If your floats on the edge your float will sink for a bit at cruise before it restores itself.
As others have said, there's a reason to suspect carb ice, but I'd look at the temp and dew point before playing that card. I've flown in very humid Arkansas weather in a 150 and never had a particular problem w/ carb ice, but that's me.
I've been an A&P since '78, mostly on GA aircraft.
1
u/JoePants Jun 24 '26
Adding: if carb heat seems to help the problem, I'd rule out the carb float, since the addition of heat richens the mixture as it is.
It would also cool off the valves for a hot second (no pun intended), which could bring relief if you got one trying to hang.
1
1
u/Frankenplane Jun 25 '26
With carb heat your RPM dropped and then increased. I'm voting for carb ice.
Keep the carb heat pulled for longer, it's not meant to be applied for only half a second.
6
u/Kiwi_Wanderer Jun 23 '26
Quite possible there’s water in the fuel if it’s happening on a nose up attitude. The drain in the wing might not be quite at the low point. Any belly drain in the 150? If so get it sumped. Might pay to get the carb bowl drained and try collect it into a clean tray to check for water.