Caught by an engine problem by Phil Chaddock

Attached my story of a wee adventure enroute to/from Oshkosh ’95

The flight to Oshkosh was going well. Late morning found us at Scotts seaplane base on Crane Lake, just south of the US-Canada border in Minnesota and west of the Boundary Waters Wilderness Area. The trusty old PA-12 with its upgraded 150hp engine had transported me from my home in northern Manitoba about 500 miles to this point. The weather was fine and US customs had been helpful and courteous.

The takeoff run to the north and east was a bit longer than I wanted – likely just didn’t hit the sweet spot on the step just right. Made a boat load of fishermen nervous as we headed in their direction. Climbed out and turned to the SE for the next leg to Round Lake, Wisconsin. Throttled back on reaching 4000 ft and the engine abruptly ran a mite rough. Thought the temperature and altitude might be making things rich and tried leaning the mixture. The engine settled down to its normal rythym. As the mini-adrenalin rush ebbed away I tried to figure out what was happening. It was a warm day by my northern standards, about 25 C, but within the summertime range I normally encountered with no roughness. The humidity seemed higher in these southern parts. Maybe temp + humidity + altitude was a condition I’d not seen before, one requiring more aggressive leaning. The engine was running just fine now; I slowly relaxed.

The leg to Round Lake went fine. Felt a bit tense flying over 15 miles or so of Lake Superior, and landmarks were harder to find as we got further south out of the heavy duty lake country. But Round L. showed up on schedule and the ’12 settled gracefully onto the water. After taking on fuel and chatting a while we taxied out of the lagoon and lined up for takeoff. Seemed to take a long time to accelerate to flying speed on the step. Hot and humid – yes, that always extends the takeoff run. Finally we were off the water to the north and climbing and turning for Oshkosh! Throttled back on reaching 3000 ft and the engine ran a bit rough again. Not as bad as at Crane L., but definitely not smooth. Leaning cleared the roughness up once more and I started to lull myself into the notion that this novel southern environment just took a little adjusting.

One farm looks like another to me, but nav was fairly easy on a SE course. Just point the nose evenly between the N-S and E-W roads and the heading is near perfect. The farms and forests of Wisconsin went rolling by. Not long after the mini-mountain at Wausau the waters of Lake Poygan came into view and we lined up to go north of the city and get low over the water and avoid the Oshkosh airport traffic. Crossing the shoreline of Lake Winnebago I was down to nearly 100 ft and turning south to seek out Vette seaplane base through the haze. A few minutes listening to the Osh approach control convinced me of the need to stay low. Excitement grew as my dream trip neared a close.

As I neared Vette base I enriched the mixture in preperation for the climb to make my circuit for landing. The engine coughed and ran rough immediately the mixture control pushed in. A moment of near panic passed as the engine smoothed out – once my frantic hand leaned the mixture again. We started the climb with throttle open and mixture lean. Soon we were in position and gliding for a bumpy landing in the waves of the lake. The engine roughness was forgotten for a while in the excitement of arriving, unloading, registering, and setting up camp.

Once I’d gotten my bearings at Oshkosh and got over the initial rush of “Oh Gosh” as a first time visitor to the convention, I returned to worrying about the engine roughness. I found the building where volunteers tried to sort out maintenance problems. Was introduced to a helpful gentleman, their expert on engines. I related my story. He took it all in. He said ” The engine is getting too much fuel and the mixture is too rich. It is possible that the density altitude required a leaner mixture. If it is mechanical I can think of two things that would cause the problem. Either your carburetor float is sinking or gas is leaking past your primer. If the float is sinking you will notice gas leaking from the carb when you have a look. Very soon it will have sunk completely and you won’t be able to even start the engine.” He seemed very sure of himself.

I thanked him for his help and promised to check things out. I felt a lot better having an expert opinion. I wished it was easier to get a mechanic to check my aircraft, but at least I had a starting point. I went out to the plane at its mooring and pumped the floats and gave the engine compartment a good look. No leaking gas. Everything looked to be in its place. Mixture cable still attached correctly. Ditto throttle cable. Carb heat cable and flapper seemed to be Ok. I guess that leaves the primer as the likely cause. Density altitude sounds lame given the roughness at low level but might be a contributing factor.

As much as I enjoyed my week at Osh, thoughts of engine roughness still troubled me. Considered asking a mechanic to come out to the sea base, get the plane towed to the dock and have it given a once over. But I feared all I would get is a big bill, maybe a rebuilt carb and several days delay. And the problem might still be there! I decided to make a test flight before making the decision to head home.

On Aug.2, the night before home time, I taxied the ’12 out of the lagoon and prepared to take off. As the throttle opened the plane reared back in response to the thrust and the back elevator and tried to climb on the step. We ploughed along, unable to get on the step. Once I was convinced that waiting would not help I tried leaning the mixture. The rpms came up and we climbed on to the step, making an otherwise normal takeoff. A quick circuit and we touched down once more. I was frustrated and puzzled as we taxied back into the lagoon. Opened the cowl and looked the engine over once more. No sign of anything out of place or abnormal. Closed the cowl. Secured the plane for the night at the dock and headed for bed. What to do? Problem seems to be getting worse. Can’t find anything wrong. Seems to work fine when leaned. Convention all but over now – how would I get a mech to check it out? Would a mech have any better luck finding a problem? I *do* want to head home tomorrow. If the engine sounds worse on the way I can get help at Round Lake or Scotts. If I get all the way to Red Lake, Ontario I’ll have my choice of seaplane maintenance bases. Guess I’ll try it in the morning and see how it goes. Settled into an uneasy sleep.

Thursday morning the weather was lousy. Thunderstorm went through. Steady light rain, low ceilings. Maybe inproving by lunch. Conditions better to the west and north. Hung on, biding my time, thinking of the weather. And the engine. After lunch conditions were marginal but a bit better. Decided to head out. Used leaning to get airborne, climbed couple hundred feet and snuck past Wittman field and Oshkosh. North and west of the city climbed to 500 ft and settled in for the ride. All seemed well. Weather not great but doable, and hope for improvement in a while.

Somewhere near Waupaca, maybe 50 miles from Osh, the engine instantly went from smooth to rough. Tried more leaning. Maybe a wee bit better, hard to say. Starting to lose altitude. Increase power, very slowly, engine sounds like there is not much more to have. Just about full thottle and just holding altitude. Getting rougher.

As things slid downhill we had turned north to a small lake, maybe a couple miles away. As we got closer I could see buildings on the south shore, one fairly big. Circled to the north and started descending. Lake had lots of reeds so likely pretty shallow, no time to be choosy. Hope we can get back out again. Will have to get a mech to drive out. Major delay. Major expense. Darn! At least we’ve got somewhere to land.

Set down OK, engine still running, and idled toward the shore and the buildings. Couple and a wee dog coming down to the beach as we head in near their dock. Apologize for “dropping in”. Introductions. Explain problem. Open the cowl. Nothing seems to be wrong. But I *know* something is wrong, finally. Out of frustration I lay hold of the carb air box and give it a wiggle. IT MOVES. I can wiggle the heat box and carb bowl back and forth. I can see the safety wired screws in the top of the carb wiggling with the bowl as it moves. WOW! The safety wire prevented the bowl from coming right off the carb, but it did *not* prevent the screws getting loose.

Was worried the screws might be stripped, but after cutting away the wire found I was able to tighten the screws up just fine, all four of them. Used snare wire from my survival kit to rewire the screws. Not the greatest wire but I’ll be checking at every stop. Sure hope nothing else is wrong. Thank my hosts for their harbour and their help. Fire up and taxi out. Big question now is whether this lake will let me go. It’s big enough if I have normal power, but I don’t know these waters and could go aground. The folks said another cottage owner flew on floats out of here so there is hope.

The engine snarls to full throttle and rpms are normal with the mixture full rich! A quick run on the step and we’re off to the west and climbing well. Circle and wag the wings for my new friends. Throttle back to cruise with no problems. As if in approval the weather slowly lifts and the return adventure starts to become fun. Round Lake, Crane and Sandpoint Lake, Red Lake, Gunisao Lake, and home to Thompson, Manitoba, all went well. A great trip, and VERY educational.

A lesson learned, I guess. Looking back I can see the evidence of a problem accumulating, starting at Crane lake. A sluggish takeoff, roughness at cruise. And it only got worse thereafter. I kept trying to convince myself there was not a serious problem. I was reluctant to hire a mech while away from base when the problem wasn’t well defined. I was too eager to rely on the well meaning opinion of an expert who had only my verbal describtion of the symptoms to go by. I failed to learn from the last and loudest warning on the test flight the night before leaving. Then I compounded the risk by heading home under a low ceiling, giving me little altitude to play with if (when) things go sour.

I was lucky that there was a useable lake nearby when the engine threatened to give up on me. Further along I would have been forcing it into a farmers field. Still further and I’d have been cutting down trees with the old PA-12. Not a nice thought.

Still don’t know why a loose carb bowl will make the engine run rich. Seems like the air leaking in above the bowl will disturb the venturi flow and should make the mixture too lean if anything. Perhaps the air leaking into the induction was atomizing fuel as it passed over the bowl and thus enriched the mixture. Anyway, it was too rich. If the bowl had gotten loose enough the engine would likely have stopped dead. Cheerfull!

My mechanic had worked on the bowl at the last annual. When he safety wired the screws he had not brought them tight enough, and may not have had the right threading direction through the holes to maintain the screws in their tight position. It is not easy to do working in cramped spaces. When I got home I did it myself. And did it right.

A carb security check is part of my normal preflight now. It’s easy to open the cowl on a ’12 and I do it anyway each day I fly. That’s one gremlin that will never catch me again!

Hope this little story helps someone else avoid a bad experience.

Phil Chaddock