Cross Post
My 74 Warrior PA 28 151/160 started running on after idle shutdown with full lean over two flights. I could only stop it with the mag switch. Also gave me the feeling that something was not quite right in flight, but the gauges looked ok. Leaning seemed to be somewhat erratic but I put it down to the turbulent air I was flying in. The mixture lever was also "walking" back towards rich.
Research turned up several likely causes but when I got to my A&P he first opened the left cowl panel, reached inside and motioned me over. He showed me that the upper and lower halves of the carburetor were not sealed tightly but had opened up slightly, developing some play at the juncture. Despite the locking castle tabs still being in position, the seal was clearly open and the flex throwing off the delicate mixture control, allowing fuel to bypass the control. The engine would run on so long fuel was still in the float bowl (could be a long time if the fuel selector was open).
My A&Ps theory was that the gasket between the two halves of the carburetor shrunk over time, opening up the joint. The carb had about 500 hours since overhaul at the normal place and had not been disassembled. Apparently this is not an unusual problem.
The carb had to be removed from the engine, gasket inspected, carb reassembled and retorqued with locking tabs reset. Pain to go through all the disassembly and reassembly involved but immediate cure for the problem. Idle shutdown with full lean now works, leaning in flight now back to normal and the mixture lever stays put.
One caveat, reassembly of the carburetor requires care to ensure that the mixture shaft, which rotates in a tube controlled by a bellcrank at the bottom of the carb, is properly inserted in the tube. If it misses, there is no mixture control and you get to start over. This insertion is tricky as you can't see the mating taking place very well. These are my terms, not sure what the proper names are.
My 74 Warrior PA 28 151/160 started running on after idle shutdown with full lean over two flights. I could only stop it with the mag switch. Also gave me the feeling that something was not quite right in flight, but the gauges looked ok. Leaning seemed to be somewhat erratic but I put it down to the turbulent air I was flying in. The mixture lever was also "walking" back towards rich.
Research turned up several likely causes but when I got to my A&P he first opened the left cowl panel, reached inside and motioned me over. He showed me that the upper and lower halves of the carburetor were not sealed tightly but had opened up slightly, developing some play at the juncture. Despite the locking castle tabs still being in position, the seal was clearly open and the flex throwing off the delicate mixture control, allowing fuel to bypass the control. The engine would run on so long fuel was still in the float bowl (could be a long time if the fuel selector was open).
My A&Ps theory was that the gasket between the two halves of the carburetor shrunk over time, opening up the joint. The carb had about 500 hours since overhaul at the normal place and had not been disassembled. Apparently this is not an unusual problem.
The carb had to be removed from the engine, gasket inspected, carb reassembled and retorqued with locking tabs reset. Pain to go through all the disassembly and reassembly involved but immediate cure for the problem. Idle shutdown with full lean now works, leaning in flight now back to normal and the mixture lever stays put.
One caveat, reassembly of the carburetor requires care to ensure that the mixture shaft, which rotates in a tube controlled by a bellcrank at the bottom of the carb, is properly inserted in the tube. If it misses, there is no mixture control and you get to start over. This insertion is tricky as you can't see the mating taking place very well. These are my terms, not sure what the proper names are.
