Sick Engine - Cessna 150

Sick Engine - Cessna 150

Hello Everyone. I seem to have a "sick" engine on a C150G, roughly 60SMOH. I purchased the plane 4 months ago and have flown it approximately 35hrs. I flew Friday, Saturday, and Sunday this week. Friday everything was normal. Saturday and Sunday, while flying with my CFI (who is also an A&P/IA), the engine started running rough. It is most noticeable when applying power in the take off roll, with some spitting and sputtering. In level cruise things are fairly smooth but there is still an overall roughness while crusing, as well as sputtering when applying power. Leaning the mixutre improves the condition a little but by no means solves it. Application of carb heat produces no abnormal effects. On the ground, mag check reveals no obvious problmes, and pulling the prop through reveals no soft cylinders. Overall, it seemed to go from running fine to its present "sick" state overnight.

Speaking with 2 different mechanics, we have come up with some potential problems. 1) ice in the fuel line or carb that is not cured by carb heat (the temps here have not been above 32 for a couple days) and should be easy to determine except there are no warm days in the forecast. 2) fouled spark plugs/plugs with lead deposits. 3) a sticky valve. 4) some other problem with the carb. I think the plan is to pull the plugs and also do a compression check while we check these other possibilities.

I am confident in my mechanic, but it seems clear that many different factors could be at play and it seems like a good problem to keep a record of here in this forum. Any input on other stones we should turn over on initial inspection would also be greatly appreciated, or if something just seems to jump out please let me know. We should get to this later this week, and I will post again with any updates or improvements. Thanks.

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Re: Sick Engine - Cessna 150

I should have put this in the first post, but the engine roughness is not really present, or does not really show itself anyways, at low RPMs during start up, taxi, etc.

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Re: Sick Engine - Cessna 150

Things that I would check: Induction leaks, Insufficient clearance between the exhaust valve and guide, primer system leaking or missing plugs at the induction side of the cylinders, fuel screens for cleanliness, carb for security, carb heat rigging. Problems like this will definitely cause baldness, I have pulled more hair out troubleshooting these kinds of problems. The process of elimination will find the problem. One suggestion, fix only one thing at a time, as if you fix two or three things and the problem goes away you will always wonder what fixed the problem...

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Re: Sick Engine - Cessna 150

Collected some more data this week and seemed to have narrowed down the issue a bit. Here is what I have:

1. Spark plugs actually look great and suggest good burn, although #3 cylinder appears to be running leaner than others. Not so sure this is related to the engine roughness problem...

2. Engine produces full static RPM, but there is a noticeable stumble or hesitation at 2300RPM.

3. In the air, the engine runs rough in a narrow RPM band from 2300 to about 2450 - just so happens that in normal flying this is where I would spend most of the time. Above or below this RPM range, things seem to be normal.

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Re: Sick Engine - Cessna 150

Update on my problem(s):

Well, I was real happy last weekend when, during run-up, the left mag dropped out completely - dead as a doornail. I was happy because I finally had a concrete explanation for the problems I was experiencing. I was beginning to think it was a fuel-related problem, but then again what do I know.

Much to my dismay, we replaced the mag yesterday and the 2300RPM hesitation/stumble that started this whole thread is still there. It appears that the mag was a separate issue, and I am back to square one with the original problem.

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Re: Sick Engine - Cessna 150

I am having a similar problem with my O-200 that is in my 1966 150F. The problem first showed up at 500 AGL on take off where the engine would run rough, and then eventually clear up. Then the problem began occuring upon initial start-up. The engine runs to about 1000rpm, then drops significantly down to about 300 and then returns to 1000rpm. It sounds really "sick".Upon checking compression on the cylinders, one cylinder will be way below normal psi. however, it hasn't been the same cylinder on both occassions. I've been told various different things to diagnose the problem including valves sticking, carb malfunctioning, and cam being shot. Confusing and frustrating.

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Re: Sick Engine - Cessna 150

Mark,
Lets try some troubleshooting techniques.
First eliminate the mags, plugs, and plug wires. At 1700 rpm, do the mags do close to the same thing, ie. rough, miss, same rpm drop etc.  If so the ignition system can be eliminated. If not trace out the magneto that is worse and fix any problem it may have.  Just a note though, make sure you are working on the correct mag by tracing out the "P" leads from the switch to the mag to be sure the right mag is connected to the right switch connection.
Second, eliminate the carburetor as the problem by using the carb heat as troubleshoot tool.  While the engine is doing its roughness apply carb heat, this enrichens the mix and if the trouble goes away it indicates a lean running engine and if it gets worse indicates a rich running engine.  Lean running indicates improper fuel level from improper float level, induction leaks or valve problems, see below. Check the primer.  Is is locked and sealed.  The first is easy, you would not believe how many pilots forget to lock it down and it leaks fuel into the induction system.  The second is to lock it and then apply suction to the engine side of the primer and see if it leaks.  Unmetered fuel through the primer is not a good thing.  Fix or replaced the primer if it leaks at all. Check for leaking at the carb bowl split, this indicates high fuel level from leaking needle, improper setting etc.  The O-200, and all small Continentals, and its carburetor 10-4894-1 have a AD associated with it.  The two piece venturi was supposed to be replaced along with the nozzle.  When it first came out only the venturi had to be replaced but it caused serious running problems.  The Precision fix was to replace the nozzle with a atomization nozzle, though this helped the problem on most C-85 to O-300 it did not eliminate the problem.  Precisions fix was to allow the old 2 piece venturi back and inspect it every 100 hours.  We have found that trueing up the course casting that the factory provided eliminates the problem. 
Lastly, the valve problem.  Newer engines especially small Continentals don't produce enough heat in the cylinder and can produce enough carbon and lead deposits to clog the exhaust valve and guide until the valve becomes "slow to close."  By this I mean the exhaust valve closes with spring pressure and can get sluggish.  If it takes 1ms too long to close at 2400 rpm it increases the valve overlap by 14 degrees.  This causes the exhaust gases back into the induction system and decreases the power of that cylinder to the point that the cylinder finally misses altogether.  If allowed to continue the valve will finally stick open and can cause breakage of many parts in the valve train.  This problem is a little harder to troubleshoot without some special tools, the best of which is a infrared thermometer with laser spot to tell which cylinder is missing.  If your mechanic is familiar with this type of problem the guide can be measured and fixed while the cylinder is on the engine keeping the cost down.
Other problems can exist, however these are the ones we see over and over.  Hope this helps.

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Re: Sick Engine - Cessna 150

John has a lot of good points, the easiest way to detect a cold cylinder is to run the engine to get the heat up and shut it down and spray each exhaust down tube with a mist of water. Hot exhaust tubes will repell the water mist immediately, however, a cold cylinder will not and this will be quite obvious...

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Re: Sick Engine - Cessna 150

Thanks for the reply John. Here is a list of what has been done over the past month or so.

1. Plugs cleaned.
2. Primer system troubleshooting. Ensured it is locked and sealing. Even disconnected it and plugged it off at the intake and problem still exists.
3. Replaced accelerator pump in carb. Along with this ensured that all carb fittings, mixture and carb heat controls, etc were in place and secure. Checked float level and condition.
4. Thorough exam for induction leaks.
5. Tried many different times (both in the air and on the ground) to isolate the problem to one mag, with no obvious differences. Tried at many different RPM settings. Also tried many times (air and ground) to see if carb heat affects the issue. No obvious improvement.
6. Passed my checkride in said airplane.
7. Did compression test - all high 70s.

Well it was during this compression test we discovered that the #3 cylinder is running VERY lean. Plugs are ash gray and even some yellowish color on the insulator. So that cylinder is running lean and hot. This seems to rule out a carb problem because the other 3 cylinders are getting a good burn. So you would think induction leak, but we have been through the induction system now 3-4 times with a fine toothed comb and there is nothing there. So the remaining possibilities seem to be limited and also not good (although I quit caring about the costs a while back and now Im just itching to throw some money at it). The only logical things I can think of are:

1. Valve problems - as mentioned above these are common issues on the O200. However, you would think a STUCK valve would be really obvious (bent pushrod, big power loss, etc), while a STICKY valve would be intermittent. I also have a hard time seeing how this would make the cylinder run so lean. John, can you see any conection there?

2. A crack in the cylinder itself. My understanding is that these can exist even if a compression test says everything is normal. Also, this all started on a very cold day when then the difference between the hot and cold side of the cylinder would have been at a maximum (the uneven temperature distribution maximizing the potential for a crack) . And on this engine the "cold" side of the #3 cylinder is hanging right out there in the breeze, so to speak. These cylinders are also old - unfortunately they were reconditioned instead of replaced during overhaul by the previous owner.

Based on these somewhat frightening possibilities, I am no longer flying it until I know what is going on. I was flying it under the assumption that it was a bothersome, but non-progressive fuel or ignition related issue. Now it seems more like a time bomb.

But then again, what do I know. I thought I "had it" at least 5 times during the past month. Strange thing is - I have no regrets. I know my plane a lot better than I did a month ago. I also know all of the things that are not wrong with my plane...

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Re: Sick Engine - Cessna 150

A few things not mentioned yet:

1. Weak / broken valve springs may cause a RPM drop.Usually @ higher RPM

   Harmonics may play a role here.

   NEW parts can break too.


2. Pressurize the Intake & Exhaust System by hooking a Vacuum

   (pressure side) or leaf blower to the exhaust stack.

    CLOSE the throttle & spray a soapy solution at all possible places.

    Bubbles will alert you of areas to be checked.



3. Loose baffles in a muffler will obstruct the outlet & cause a

   On/OFF power loss.


4. Could your Throttle, Mixture, Carb Heat control be slipping

   & changing the adjustment?

   Wouldn't be the first time.

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Re: Sick Engine - Cessna 150

Mark,
OK, here are the next in the list.
As I mentiond before the slow to close valve will not come and go.  The reason for the slow to close valve is accumulated sludge from the combustion process.  Lead, especially from 100LL and oil combine to form a hard sludge that will eventually squeeze the valve so that the spring can't keep up with the speed of the operation.  The entire combustion process from compression stroke to compression stroke at 2200 RPM takes about 56 milliseconds. Exhaust valve and the intake valve are both open for a time in almost all engines.  This allows the exhaust to be evacuated and the intake charge time to get completely into the cylinder.  This "overlap" on a O-200 is about 24 deg. If the exhaust valve takes one extra millisecond to close this equates to about 14 deg extra overlap.  I have actually measured this with a scope, 500 psi transduce installed in the cylinder and run at 2200 rpm.  The result is the combustion of that cylinder is less than the opposing cylinder and this gives a vibration.  As the valve gets more and more sludge build up it will finally cause the valve to stick altogether and thats when the fun starts.  We are in the process of overhauling a Lyc. O-360 from a stuck valve that bent the pushrod and broke the end of the lifter off.  As I said before if your mechanic is familiar with the technique you can drop the valve into the cylinder, measure the guide and hone all of the sludge out without taking the cyinder off.  A technique outlined in a Lyc. service bulletin.
That took a little more than I thought it would.  The second problem I would suggest is to look at the upstream side of the carb venturi.  The original two piece venturi was made smooth to the carb top.  The new two piece venturi has a lip on it and causes problems with air and fuel flow and only happens at just below full throttle to about 2100 RPM.  I had a Tcraft with a O-200 and put the new venturi in it and thought the thing would quit every time I reduced power until I figured it out.  If you true it up on a lathe it will work perfect.  We do this to every Marvel carb we work on and have had very good success from C-85 to the O-320 which all take different venturis but exhibit the same problems.
OK so I didn't intend to write a book but hope this help.  Troubleshooting is part art and a big helping of knowledge.

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