Spent a weekend doing formation flying at a clinic in CA. Flew back to Seattle in marginal weather, picked up some light airframe icing. On downwind I pulled the power back for landing and engine was running very rough - RPM was low and uneven, touch the throttle and RPM would surge. I immediately put power back in and engine became smooth again. After landing, I was unable to shut down using mixture control, engine would keep on idling. Had to kill it using mags. I had assumed I picked up heavy carb icing, which narrowed the throttle body and cause excessively rich mixture, and that I couldn't lean the mixture enough to kill the engine.
Fast forward to the next flight, weather was good, no icing, but same problem - unable to turn off engine. Yikes, there's something going on here.
Take it to the shop. Mechanic says unable to replicate problem, but found that idle was set too high (I thought 700 RPM was normal) and adjusted idle down to 500 RPM, but otherwise throttle and mixture linkage is in good condition. I told them they have to keep looking. Mechanic did a prolonged ground run, got engine temps up, and was able to duplicate the problem.
Lots of head scratching ensured. Mechanic noticed that primer line to cylinder #1 was broken. Ordered new primer line, turned out to be a foot or two too short, so mechanic removed the primer line and plugged it at the port. I now only have primers to cylinders #2 and #3. We're not sure how an open primer line contributes to excessively rich idle mixture, but we fixed something and run up was good.
Next flight, same issue again - can't turn engine off. In addition, I had a hard time getting my RPM above 1ooo during the taxi unless I leaned the idle mixture all the way out. Engine seemed fine during take off and cruise. Yes, I was a bit nervous taking off with a questionable engine but I had a 10,000ft runway. Leaning in flight at altitude seemed normal. So this means I have excessively rich mixture but only at low RPMs.
Mechanic suspect fuel is leaking through primer line. Mechanic replaced O-ring on primer pump. Pump has always been really sticky and hard, now it's silky smooth. Took it for a flight today - idling, taxiing, take off, cruise, and engine shutdown seems normal. However, engine still runs unevenly during descent when power is pulled back. Throttle response is nonlinear and engine surges. Meaning if I pull the throttle back, RPM decreases gradually to about 2000RPM, after that it drops precipitously, and if I keep pulling throttle back, RPM will rise a bit, then drop. I am also hearing more popping noises (backfire? afterfire?) than normal during descent.
Any ideas what is going on here? Is engine running unevenly at low RPMs during descent normal? Hoping your collective experience here can save me shop hours troubleshooting this problem. Thank you in advance.
Fast forward to the next flight, weather was good, no icing, but same problem - unable to turn off engine. Yikes, there's something going on here.
Take it to the shop. Mechanic says unable to replicate problem, but found that idle was set too high (I thought 700 RPM was normal) and adjusted idle down to 500 RPM, but otherwise throttle and mixture linkage is in good condition. I told them they have to keep looking. Mechanic did a prolonged ground run, got engine temps up, and was able to duplicate the problem.
Lots of head scratching ensured. Mechanic noticed that primer line to cylinder #1 was broken. Ordered new primer line, turned out to be a foot or two too short, so mechanic removed the primer line and plugged it at the port. I now only have primers to cylinders #2 and #3. We're not sure how an open primer line contributes to excessively rich idle mixture, but we fixed something and run up was good.
Next flight, same issue again - can't turn engine off. In addition, I had a hard time getting my RPM above 1ooo during the taxi unless I leaned the idle mixture all the way out. Engine seemed fine during take off and cruise. Yes, I was a bit nervous taking off with a questionable engine but I had a 10,000ft runway. Leaning in flight at altitude seemed normal. So this means I have excessively rich mixture but only at low RPMs.
Mechanic suspect fuel is leaking through primer line. Mechanic replaced O-ring on primer pump. Pump has always been really sticky and hard, now it's silky smooth. Took it for a flight today - idling, taxiing, take off, cruise, and engine shutdown seems normal. However, engine still runs unevenly during descent when power is pulled back. Throttle response is nonlinear and engine surges. Meaning if I pull the throttle back, RPM decreases gradually to about 2000RPM, after that it drops precipitously, and if I keep pulling throttle back, RPM will rise a bit, then drop. I am also hearing more popping noises (backfire? afterfire?) than normal during descent.
Any ideas what is going on here? Is engine running unevenly at low RPMs during descent normal? Hoping your collective experience here can save me shop hours troubleshooting this problem. Thank you in advance.
