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I have spent the past five months tracking down a strange problem. It was a problem that confused me, A&Ps, and lots of very smart people.
It was a problem that sent me down a path of inspections, repairs, experimentation, and lots of test flying.
It was a problem that was ultimately solved by two cents with of zipties.
When I pulled back the throttle on my 912ULS powered (550 hours on the engine) 701, the engine would start to get rough around 5200RPM. At 4900RPM the engine would be uncomfortably rough, and here is the kicker, pulling back on the throttle would start to increase the RPM. With this came a little whiff of fuel.
Everything would smooth out again at 5100RPM (while pulling back), then the RPM would decrease again.
Adding power from idle in flight would get the engine bogged down at 4900 for a while (about 1/4" of throttle advancement) then the RPM would increase again.
These symptoms would only happen in flight. Never on the ground.
My first thought was carb sync. Nope. The engine ran a little better and the symptoms decreased a little but it still happened.
Next was a carb rebuild. The needles were very worn and the floats sat a little off from each other. Replaced those parts. The symptoms got a little better, but it still happened.
Then we took the carbs apart, checked the sliders, cleaned them up. No luck.
Maybe the carbs were getting too much fuel? So a pressure regulator was added to prevent that.
Next came the gear box inspection. The springs were very worn. So they got replaced and the gearbox was shimmed.
Once again the engine ran a little better, but the symptoms persisted.
At this point I was very much resigned to having to spend the rest of my flying life in the pattern testing minor tweaks and spending money on replacement part after replacement part.
A good clue came when I noticed that the symptoms were the least pronounced when I raised the nose to reduce throttle. The symptoms got worse when the nose was lowered.
I finally had a breakthrough on the cause by being a bad pilot. On one flight I skidded the plane with too much rudder while turning onto the downwind. The engine RPM decreased by a few hundred RPM. Putting the ball back in the center brought the RPM back up.
I skidded in the other direction. The RPM stayed put. Skidding back the first way brought it back down.
What could be doing this?
I had talked to lots of people about this, and called around alot. Everyone was stumped.
Finally I called Rotech Research to pick their brains. After describing the problem they asked me two questions, then told me the solution.
It turns out that a few months prior the overflow tubes from the carbs were getting really yellow and looking ready to crack. Now of course wanting to be safe, the tubes had been run down through the cowling, away from anything hot, and vented out the bottom.
The tubes were replaced almost exactly.
The new tubes were just a little bit longer. Maybe just a few millimeters. This was enough to put the output into the slip stream.
Fuel was being sucked out of the carbs by vacuum.
Routing the tubes up to vent by the air intakes and using two cents worth of zipties fixed the problem. They now had nice static air not sucking the fuel out.
So there it is. Five months of mystery passed on in case you ever see the engine RPM increase when you pull back on the throttle.