Rick Cantrell:
Two days ago I started the engine; she had a slight cough for 5-10 seconds, then ran just a little rough. After about one minute she ran fine. It may have been there before and I just did not notice it.
My questions are:
Is this the sticking valve problems I’ve read about?
How can I tell for sure if the valves are sticking?
If valves are sticking how many hours labor for A&P?
Do I repair now or wait till the annual in December?
Engine O320-E3D, carb, 1200 SMOH in 92-93, Comp mid 70’s
Bob Steward, A&P-IA:
“Morning Sickness”, as it is commonly known, is the result of carbon build up around the valve stem that jams it into the guide.
The test to determine if you have the problem (or if it’s coming), is to do the Lycoming SB 388B “Valve Wobble” test. The specified interval is 400 hours.
The cause for valve failure in Lycomings is stuck valves (followed by the piston HITTING the valve while it’s stuck open), or broken valves from excess play in the guide. Excess play allows the valve to dance around on the seat; which in turn flexes the head slightly back and forth, causing micro-fractures at the head to stem junction.
The Valve Wobble test, when done during an Annual Inspection (when the engine is already un-cowled), is about 3 hours for all 4 cylinders. I also check the “Dry Tappet Clearance” during this inspection, and continue to find them out of spec.
So long as you aren’t FLYING between now and the December Annual, be my guest and put it off. However, you have already seen the symptoms, so you are going to be challenging fate if you fly with it like this.
As for the repair, it’s just about 1 hour per cylinder if you already have the engine opened up for the valve wobble. You drop the exhaust, remove the springs from the exhaust valve (using the “rope trick”– Read the SB), and then push the valve back into the cylinder and get the valve stem to slide out the spark plug hole to be inspected, measured and polished. Then the correct reamer is run through the guide, cleaning out the excess carbon, and everything is washed off and reassembled.
Certainly NOT owner preventive maintenance, but no question there is plenty of grunt work to be done, so helping your Mechanic is a cost savings.
I’d do the Service Bulletin:
http://www.prime-mover.org/Aviation/Bulletins/sb388.html
And then decide how many cylinders need to be worked on.