The Mighty C150s

C

Crism

Thuper Member
Just a few questions for you engine gurus.

We've got a set of 2 Cessna 150s. Both 1968. One is an H model and the other is a J. Both are virtually identical externally but different internally. The H model has the push/pull switches for the lights, master, etc and also has a non-standard panel setup.

The issue with the engines is my main concern. Both are Continental O-200 engines, but perform very different.

Our H model runs absolutely terrific, however, when the power is added on the takeoff roll, it's at about 2500-2600RPM. That seems a little on the high side. When the prop is stopped and moved on the ground (mags off of course), it moves back through what I believe is the impulse coupling like butter. I can literally move the prop around with my index finger, and there is very little resistance. It's affected more by wind as well when reducing throttle in the pattern. The other engine, as you'll read below, doesn't have this problem and has some resistance when the prop is turned.

Our J model is completely different. I wouldn't say it runs rough, but it almost seems like it wants to put a cylinder through the cowling compared to the H model. Static RPM is around 2200-2300RPM. I've written it up for strong vibrations on the panel before, and it was fixed somewhat, but not 100%. It also had a problem with burning WAY too much fuel before. This was "fixed" by installing a new carburator. Our most recent issue with this airplane was a broken attachment from the engine mount on the nosegear. For those of you who are familiar with it, the engine mount is directly connected to the nosegear and directly connected to the firewall with the engine sitting above it. One of the 2 rods was snapped, and this was discovered during a 100hr inspection. A new engine mount was installed, and I think the throttle cable was replaced (which also feels terrible). Ever since then, it's been bad. The airplane flies fine, but is painfully slow compared to the H model. We used to see 85-95MPH indicated with full throttle and the RPM would never come near 2750 (red line).

I'm no mechanic so thats why I'm asking you folks if you could throw a dog a bone so I would know what to tell the MX guys. I'm not sure the time on the engines, but I'm assuming it's a lot. They sure aren't pretty to look at when the cowling is off put it that way.
 
Crism said:
...edited for brevity...
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The H model: What is the throttle position when you move it? Is the engine hot or cold? It sounds like you have horrible compression, if the engine is cold and the throttle closed. A hot engine with the throttle even half open, could give that outcome on a small bore engine like that.

The J model: I yield to more experience, I don't know.

I flew 1 c150, I don't know the specific model, but it ran around 2500 or so when rolling, not static.

One last thing to check, prop! The amount of prop twist, I don't know the technical term, and the size of the prop can drastically change the engine RPM. A climb prop will have a shallower twist and thus yield a higher RPM. A cruise prop will have a steeper/sharper twist, more resistance, and a lower RPM. Size maters too, but twist is more important with regards to the RPM you will achieve.
 
On the H, cold and idle. Another consideration as well is that the throttle on the H is about 2x the size on the throttle of the J for some reason. When you push it full forward, you've got about an inch of throttle exposed at full power when it hits the stop.

On the J you can actually feel what seems like notches inside of the throttle when you push it forward. It's not completely smooth. I'm guessing the cable might be too tight in that case.

Prop wise, both are relatively new propellers. The J was just recently replaced. It was very rough between 800-1000RPM. The fix that was given was something dealing with a balancing with the prop. It's not 100% though. Both engines ran about the same as they do now before and after prop changes.
 
Crism said:
Just a few questions for you engine gurus.

We've got a set of 2 Cessna 150s. Both 1968. One is an H model and the other is a J. Both are virtually identical externally but different internally. The H model has the push/pull switches for the lights, master, etc and also has a non-standard panel setup.

The issue with the engines is my main concern. Both are Continental O-200 engines, but perform very different.

Our H model runs absolutely terrific, however, when the power is added on the takeoff roll, it's at about 2500-2600RPM. That seems a little on the high side. When the prop is stopped and moved on the ground (mags off of course), it moves back through what I believe is the impulse coupling like butter. I can literally move the prop around with my index finger, and there is very little resistance. It's affected more by wind as well when reducing throttle in the pattern. The other engine, as you'll read below, doesn't have this problem and has some resistance when the prop is turned.

Our J model is completely different. I wouldn't say it runs rough, but it almost seems like it wants to put a cylinder through the cowling compared to the H model. Static RPM is around 2200-2300RPM. I've written it up for strong vibrations on the panel before, and it was fixed somewhat, but not 100%. It also had a problem with burning WAY too much fuel before. This was "fixed" by installing a new carburator. Our most recent issue with this airplane was a broken attachment from the engine mount on the nosegear. For those of you who are familiar with it, the engine mount is directly connected to the nosegear and directly connected to the firewall with the engine sitting above it. One of the 2 rods was snapped, and this was discovered during a 100hr inspection. A new engine mount was installed, and I think the throttle cable was replaced (which also feels terrible). Ever since then, it's been bad. The airplane flies fine, but is painfully slow compared to the H model. We used to see 85-95MPH indicated with full throttle and the RPM would never come near 2750 (red line).

I'm no mechanic so thats why I'm asking you folks if you could throw a dog a bone so I would know what to tell the MX guys. I'm not sure the time on the engines, but I'm assuming it's a lot. They sure aren't pretty to look at when the cowling is off put it that way.
Click to expand...
Fascinating post. I'll make some WAGs tomorrow...my wife wants me off the computer tonight ;)
 
If the engine had low compression it wouldn't get anywhere near redline RPM.


My guess is that one has a climb prop and one has a cruise prop. As to the vibration I can't really say, but a broken tube on the nose gear support is almost always a wheelbarrow landing and it happens all the time.
 
trafficinsight said:
If the engine had low compression it wouldn't get anywhere near redline RPM.


My guess is that one has a climb prop and one has a cruise prop. As to the vibration I can't really say, but a broken tube on the nose gear support is almost always a wheelbarrow landing and it happens all the time.
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If the H model has a climb prop, why is it faster in cruise than the J?

If the J model has a climb prop, why does it get lower static than the H?

It could be an issue with the new carb-either the mixture or the throttle (or both) is not rigged/calibrated properly. How does the J model behave when you lean the mixture at high power?

As for the easy-spinning engine, it's entirely possible to have completely lousy compression when the engine is sitting still and still make good power-the pressure from the combustion gases gets behind the rings and pushes them against the walls, sealing them.
 
Roger said:
It could be an issue with the new carb-either the mixture or the throttle (or both) is not rigged/calibrated properly. How does the J model behave when you lean the mixture at high power?

As for the easy-spinning engine, it's entirely possible to have completely lousy compression when the engine is sitting still and still make good power-the pressure from the combustion gases gets behind the rings and pushes them against the walls, sealing them.
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When I bring the mixture back it gets to a certain point where nothing seems to happen then anything beyond that point in travel is a rapid loss of power. No EGT gauge, but with the old carb I used to be able to literally pull the mixture full out (other than about an inch), and the engine would run perfectly fine. I'm guessing it was way too rich. I used to sometimes hear an INCREASE in power on some days. I used to take off with only 1/2 mixture because I knew it was running very rich.

Are there any questions I should be asking the mechanics or anything in particular that I should be having them look at other than the carb+throttle/mixture?

The J model is the same engine that we would joke "only developed 75HP" because the airplane was SO slow and climbed at 200fpm max gross. We've flown in a string of 6 airplanes once. The J took off first and always ended up dead last landing....
 
Roger said:
As for the easy-spinning engine, it's entirely possible to have completely lousy compression when the engine is sitting still and still make good power-the pressure from the combustion gases gets behind the rings and pushes them against the walls, sealing them.
Click to expand...

Just want to ensure I am reading this right. My assessment on low compression could be true? I always associated compression with the ease of spinning the prop. Not something I talk to students about, but just in my experience with engines in and out of aviation.
 
Crism said:
When I bring the mixture back it gets to a certain point where nothing seems to happen then anything beyond that point in travel is a rapid loss of power. No EGT gauge, but with the old carb I used to be able to literally pull the mixture full out (other than about an inch), and the engine would run perfectly fine. I'm guessing it was way too rich. I used to sometimes hear an INCREASE in power on some days. I used to take off with only 1/2 mixture because I knew it was running very rich.

Are there any questions I should be asking the mechanics or anything in particular that I should be having them look at other than the carb+throttle/mixture?

The J model is the same engine that we would joke "only developed 75HP" because the airplane was SO slow and climbed at 200fpm max gross. We've flown in a string of 6 airplanes once. The J took off first and always ended up dead last landing....
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well.... when they rate the horsepower they do it with no exhaust... so your 100hp engine probably only makes 80hp ayway ;)


How much of an increase in power are you talking about? was there any other indication of an increase besides what you heard?

To be honest, if you came to me and said "look, one airplane is a dog compared to the other one and there's no logical reason for it." I would investigate it... but if I didn't find anything obviously wrong then what could I do?
 
True, just wondering if there COULD be something wrong with it and where I could possibly point the MX guys.

And the increase in power was very subtle but it was like climbing to 7000ft with the mixture full rich then leaning it for best power but without gaining that much RPM.

Depending on the TBO, is it possible that we should start looking at a new engine?
 
Crism said:
True, just wondering if there COULD be something wrong with it and where I could possibly point the MX guys.

And the increase in power was very subtle but it was like climbing to 7000ft with the mixture full rich then leaning it for best power but without gaining that much RPM.

Depending on the TBO, is it possible that we should start looking at a new engine?
Click to expand...

It sounds like you are over analyzing it IMO. Have you checked what kind of prop it is? You didn't tell me, was the engine hot (just run) or cold (sitting) and where was the power when the prop was so easily spun.

As for your mixture, leaning most smaller GA engines gives 50-100 RPM increase. That changes with altitude. At a higher altitude the RPM increase will generally be less, at least on non turbo/smaller bore engines.

An overly rich engine isn't a bad thing, wasteful, but not dangerous. It would shock you how much fuel needs to be introduced into the engine before it starts bogging down from being too rich. As long as you can reach full lean before idle cut off, the mixture adjustment, IMO, is fine.

Side tip: Find a local RC flying field that uses gas engines. They are often two stroke engines, so they are less complex. However, the leaning procedures (low, high, and idle sets), spark operations, and a magneto are very similar. You can learn a great deal from some of those guys, you might even be lucky enough to see some fully ripped apart.

Simple leaning procedure: Keep spinning till she sputters and move it back to smooth ops. Have a landing site in mind before ever leaning an engine.
 
Crism said:
True, just wondering if there COULD be something wrong with it and where I could possibly point the MX guys.

And the increase in power was very subtle but it was like climbing to 7000ft with the mixture full rich then leaning it for best power but without gaining that much RPM.

Depending on the TBO, is it possible that we should start looking at a new engine?
Click to expand...


yeah, if TBO is close or passed a couple hundred hours ago it might be that time.

How is oil consumption?
 
shdw said:
It sounds like you are over analyzing it IMO. Have you checked what kind of prop it is? You didn't tell me, was the engine hot (just run) or cold (sitting) and where was the power when the prop was so easily spun.
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That was cold. Not sure on the type of props. I'm away from the airplanes for about a week or so now.

I'll fly them a bit when I get back and get you guys more info.

Oil consumption, if I remember correctly, was a quart every week or 2 on the J maybe a little higher than normal. I think the H was a trooper on the oil though.
 
Crism said:
That was cold. Not sure on the type of props. I'm away from the airplanes for about a week or so now.

I'll fly them a bit when I get back and get you guys more info.

Oil consumption, if I remember correctly, was a quart every week or 2 on the J maybe a little higher than normal. I think the H was a trooper on the oil though.
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If your brave enough, you could venture across the field and compare to a third 150. Not that I'm saying it would help at all, but just an idea. :D
 
Crism said:
That was cold. Not sure on the type of props. I'm away from the airplanes for about a week or so now.

I'll fly them a bit when I get back and get you guys more info.

Oil consumption, if I remember correctly, was a quart every week or 2 on the J maybe a little higher than normal. I think the H was a trooper on the oil though.
Click to expand...

Interesting, that does seem odd if the engine was cold with the throttle closed or almost closed.

Oil consumption is an interesting thing though. Engines have, what I consider, an optimal oil level. If you put more in than the optimal, the consumption is very high. Max out the oil in any aircraft and watch how much faster the oil is consumed than normal. I don't know why this is, maybe one of the A&Ps can shine a light here for me, but it just happens. I call anything I can't explain magic.
 
They both seem fairly happy at 4 quarts which is where we usually keep them. Once they tip under 4, we'll add another quart in.

When I get back I'll fly each airplane and see what I find. I'll have the MX guys take a look at them as well. Low compression on the H, and throttle/mixture/carb on the J. I'll also check the engine times.
 
:rotfl::rotfl:

I'm sorry, I saw the thread title, and just had to laugh.:rotfl::rotfl:

A good trainer, for sure. Mighty, maybe in mouse terms.
mighty_mouse.jpg
 
Found a photo of the J prop for you guys. The H is in the back. Let me know what you think.
 

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