Question for Paul McBride, the General Aviation News engines expert: We have a Lycoming IO-360 A1A fuel-injected four-cylinder engine on a 1966 Mooney M20E that has a slightly rough idle. The engine only has about 400 hours on it.
We have checked the timing, replaced the intake gaskets, cleaned spark plugs, and cleaned fuel injectors, fuel lines, and fuel screens.
We borescoped the cylinders and valves and see some staining, but not too bad. The engine runs smooth after about 1,000 rpm.
We get hardly any rise in RPM when pulling the mixture to idle.
We have tried increasing the rich setting with no luck. We have not tried going all the way to a lean setting and then all the way to a rich setting, just increased the rich from where it was.
This has been an ongoing problem.
We would appreciate any ideas you might have.
Walt Burger
Paul’s Answer
Let’s see if I can offer some suggestions to help improve your rough idle issue, Walt.
It appears that you’ve already done the usual things to get closer to finding a solution to the problem.
I’d like you to go back and try adjusting the mixture star wheel on the throttle body again.
Making sure the engine is at normal operating temperatures, try moving the mixture control slowly towards the idle cut-off position and record any RPM rise you see.
Then turn the star wheel about two or three clicks towards the rich side and repeat the above.
It’s probably a good idea to run the engine up to about 1,200 rpm to clean it out before making the next star wheel adjustment.
Again, take notice to any rise you might see and record it. If you see a little rise, then I’d continue to make tiny adjustments to the star wheel until you see a 25 to 50 rpm rise while coming into the idle cut-off position.
You might also check the manifold pressure gauge, which at 650 to 700 rpm should show a MAP of around 10 inches. If it’s higher than 10 to 11 inches, you may have an induction leak.
Even though you’ve changed the intake pipe gaskets, you may have a leak at one of the induction tubes where it enters the sump or, possibly, just a loose hose clamp on the connecting hose between the sump and the intake pipe.
Also, there might be a possibility one of the intake tubes has come loose where it’s swagged into the sump. This wouldn’t be that unusual for an engine this old and is probably worth checking.
You could check this by using the old farmer’s method of putting some soapy water around each pipe at the sump looking for any indication of leaks.
