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.
My money is on the sniffle valve. It’s a small blue 90 degree AN fitting on the very bottom of the sump pointing towards the fire wall. It opens at engine shut off to drain any excess fuel. Mooney modified a standard AN elbow by adding a nylon ball that sucks back into the fitting sealing it up when the engine is running with vacuum pressure, but at very low power settings it may not be sealing due to dirt or wear of the nylon ball and/or fitting and causes a small induction leak.
Remove the drain tube that attaches to the lower cowl and the small aluminum flare tube from the sniffle valve and squirt a generous amount of WD-40 inside to clean it. Reassemble and start the airplane, if your rough idle disappears then you found your problem.
If it comes back, and it probably will, displace the exhaust in order to remove the sniffle valve and clean it in an ultrasonic cleaner with Hoppes #9.
If the problem persists. Lycoming came out with a much better modern version of a sniffle valve that can be used to replace Mooney’s ancient version, it’s Lycoming part # 75444 and you will need a 45 degree elbow to thread into the sump, and slightly larger fuel hose.
Good luck,
Billy – Mooney owner.
One more thing… grab the fuel servo with both hands and give it a shake. If it’s loose, you probably have a blown gasket where the servo mounts to the oil sump.