Induction leak

[Robin Hood]

On my Cessna 150 with the O-200 series engine. What caught my attention was a slight vibration at 1200 RPMs. Mag checks are smooth and about 100 RPMs on the drop so that rules the ignition out, also the primer is in and locked so it doesn't seem to be a fuel issue. I'm thinking it will be an induction leak but it is a 30 minute flight to the mechanic shop. Does anyone out there have experience with this or similar issues? Is it advisable to fly the plane to the mechanic with this apparent issue?

Feel free to ask questions also if my description is vague.

 

[Shepherd]

Just out of curiosity, what is the outside air temperature and how long has the engine been running?

 

[Robin Hood]

It was during the run-up that I noticed it and the engine had been running 15 minutes give or take. OAT was right at 90 degrees as I recall. It was also somewhat humid that day with a density altitude of 2,500 ft.

 

[Art Rose]

Don't rule anything out yet. A mag check is not a know all, tell all, diagnostic check. One symptom often observed when there's an induction leak is an after fire, or exhaust popping when you throttle back in the pattern.

Honestly, at this point, your vibration could be caused by several things. Here's just a few. O-200's are notorious for sticking valves, could be a sticky valve. When one does stick, it often happens as you throttle back when landing. You could have a bad plug, or wire, or issue with a mag, or the carb, or as suggested, icing. Ice can form in a carb throat when you'd swear it had absolutely no chance.

If it were me, I'd continue flying it close to home over less hostile areas until I could figure out if it was a real issue, or not. Some mechanics will throw parts at things like this, and you'll spend a fortune buying their kids shoes, and their wife and girlfriends jewelry and expensive vacations.

 

[Dan Thomas]

An induction leak would make for really rough idle. At idle the manifold ressure is low, and will suck air past a leak and the affected cylinder will be leaner, maybe too lean to fire.

The Continental O-200 has a carb "spider" intake manifold. The carb bolts to it and it, in turn, is bolted to the crankcase. The O-200 is different from its older A-and C-series granddaddy engines in that the spider is mounted loosely on the case studs, with special "Lock-O-Seal" washers on the studs between the case and spider and between the spider and castellated retaining nuts. These washers are metal with a rubber insert molded into them, like an O-ring. The overhaul manual says that those castellated nuts are to be turned only finger tight and the cotter pin installed. The reason is that the MSA carbs don't like being shaken, or they'll spill fuel out of the bowl vent and into the carb throat and the engine will run rich and rough. RPM figures into this, since the weight of the carb and spider and airbox, plus the varying stiffness of the rubber induction tube connections, will resonate at certain RPMs. The old Stromberg carbs weren't nearly so prissy, and those old A- and C-series engines had much stiffer rubber engine mounts so the engine couldn't move around so much. With the carb spider a little flexible on the studs, and the rubber hose connections on the intake tubes, the engine can torque-vibrate around the crank while the carb will stay relatively still. I have smoothed out O-200s just by installing the proper washers and doing up the nuts exactly as the overhaul manual says.

 

[iflyvfr]

@Robin Hood - C150's with the O200 are notorious for stuck valves because many owners are unfamiliar with the mixture knob. Do you lean religiously?

 

[Robin Hood]

@Robin Hood - C150's with the O200 are notorious for stuck valves because many owners are unfamiliar with the mixture knob. Do you lean religiously?

No I don't lean much at all. I was trained old school and they taught me not to lean under 5,000 ft so I will lean slightly on longer flights but not aggressively because I don't want to hurt the engine

 

[Dan Thomas]

No I don't lean much at all. I was trained old school and they taught me not to lean under 5,000 ft so I will lean slightly on longer flights but not aggressively because I don't want to hurt the engine

Find an instructor that knows what he's doing. You've been fed a bunch of myths about leaning. Look it up in your POH. It's the authoritative document on engine management.

 

[iflyvfr]

Keep in mind I'm an owner, not a mechanic. I'm a proponent of leaning to keep the lead out. If you join the C150/152 type club you'll get an ear full from the pilots and mechanics over there @Robin Hood. I've owned a 150 and now a 172 that uses the same cylinders as is on your plane, just has 2 more of them. I lean once the oil pressure comes up after start. Many will advise to lean it so far out on the ground that if you forget to push in the mixture on takeoff your engine will stumble. I go full rich for runup and takeoff - if I'm departing right away. If I know it's going to be awhile I lean again. Once I level off, regardless of altitude, I lean the engine.

I once met a guy who had to have the rope trick done at a fly-in in Iowa (the type club's annual flyin). He was up high and stuck a valve when he pulled the power back and fortunately was high enough to glide most of the way in. He too wasn't taught to lean and is why I asked the question. I've owned my 172 for 12+ years and have never had a fouled plug, stuck valve, or any evidence of morning sickness. Oh, I do run the REM37BY spark plugs but I don't see a lot of lead deposits on my plugs at annual.

Just a data point to ponder.

 

[chemgeek]

No I don't lean much at all. I was trained old school and they taught me not to lean under 5,000 ft so I will lean slightly on longer flights but not aggressively because I don't want to hurt the engine

You won't hurt a small normally aspirated engine by proper leaning at or below 75% power. You probably can't even get 75% power at full throttle in a C150 at most cruise altitudes, so that could save you remembering anything from the POH performance tables. Continental recommends leaning to peak rpm (airspeed) which should be safely rich of peak. Probably better is to lean until the engine starts to run a little rough then enrichen until it smooths out. (The latter will be a little lean of peak). For takeoff and climb, full rich below 5000 feet or leave it full rich as long as you have smooth operation and sufficient power to cruise altitude. Lycomings can be (and are recommended to be) operated the same way. No sense in burning extra fuel and gumming up the engine and plugs with deposits. There is no reason to be afraid of the red knob. Just follow the engine mfgr recommendations. I've had two planes and three engines with nary a stuck valve or excessive lead deposits this way.

On the ground (during taxi) you can lean very aggressively to deter lead buildup. You can't possibly hurt an engine running at 1000-1200 rpm, no matter how much you lean.

Keep in mind an engine like an O-320 will burn more than 10 gph full rich, but only 7.5-8.0 gph properly leaned at cruise. Range will be severely limited, and fuel consumption unpredictable if not properly leaned. Lean to recommendations and your fuel burn will match book numbers.

 

[Robin Hood]

Thanks for your help guys, I had the plane looked at by a mechanic and he is certain it was only a fouled plug. The engine is running smoothly now with no sign of any vibrations. I hope to learn proper leaning techniques also

 

[Rcmutz]

Interesting thread.

I just had what may be some “morning sickness” during my last engine start. Engine ran rough, and I shut down for after about 1-2 minutes. Took a look under the cowl for about 5 minutes, nothing seemed out of place. Fired back up and engine ran fine. Run up was fine, and flew for about 30 mins with about a 20 min stop at an airport. When I restarted to come home, no problem on start up or run up. No roughness on throttling back on downwind.

This is on an O-360. I generally don’t lean below 5000 ft, due to running high power setting. For the last year or so, I have been running locally with lower RPM settings, but still not leaning. On the next startup, if it happens again, I will ground the plane until I sort it out. If not, I will start leaning while at the lower power settings below 5000 ft.