H ANDS ON HINTS FOR HOMEBUILDERS
A classic stuck exhaust valve: Notice the rocker arm is not touching the valve stem, and the valve is stuck in the open position. This open position kills the power in that cylinder, and that is 25 percent of the power of the engine, which makes a lot of difference in a small four-cylinder Continental.
Stuck Valve Fix BY BARRY DAVIS, EAA 389226
SOMETIMES CALLED “MORNING SICKNESS,” a stuck valve is nothing to fool around with. Usually the first signs show up in the morning or the first crank-up of the day when everything in the engine is warming up at different rates. Sticking valves never get better and won’t cure themselves. A classic stuck exhaust valve will remain open, killing the power to that cylinder. There are a couple of ways to fix this, one of which is to take the cylinder off and have it overhauled. Another is to pop out the valve, clean the guide of built-up carbon, and then replace the valve. It is easy to do and can be done quickly.
With the spring compressor in place and the bunched up rope holding the valve in place, compress the springs and remove the valve keepers and springs.
74 Sport Aviation June 2011
First take off all the spark plug wires and all top plugs so the engine cannot fire, as you will be in close and moving the prop. With that cylinder on top dead center, slide the rocker shaft to one side and remove the rocker arm. Remove the spark plugs from the stuck cylinder and turn the prop so that the piston is farthest away from the valve. Start packing small rope into the cylinder (it could take 12 feet or more), then gingerly turn the prop so the rope is jammed up against the head of the valve. This will pop the valve out of the stuck position and keep the valve from moving in while you are compressing the valve springs and removing the valve keepers. Don’t force anything! I fabricated a simple tool from 1/4-inch threaded rod and some angle iron I had lying around the shop. Thread the rods into the screw holes normally used for attaching the valve covers and use a couple of nuts to keep the angle in position. Use an oldfashioned valve spring compressor and compress the valve spring to remove the valve keepers. Remove the valve springs and set aside, keeping the orientation in the correct position.
PHOTOGRAPHY BY BARRY DAVIS
At this point, move the prop and relax the pressure on the rope inside the cylinder so you can remove the rope. This is when you begin to sweat, thinking that the engine will never run again, but be assured this is just another step in the process. Tie a 24-inch long piece of nylon or strong cotton cord to the end of the valve and position it inside the valve groove so the cord will not slip off when pulled. I spray the valve with Kroil or any other good penetrating oil and prepare the valve to be pushed into the cylinder. Putting the valve into the cylinder will probably take a little force. I use a brass punch and a small hammer, carefully keeping the punch away from the walls of the valve guide, and drive the valve into the cylinder so it falls to the bottom of the cylinder. Feed about 12 inches of the cord into the cylinder through the guide so if the cord is frayed, you will still have some strong cord inside the cylinder away from harm. Using a gun cleaning kit (a .45-caliber brush works great) and the chemical that comes with the kit for cutting carbon, clean the valve guide very well, but not too much. This chemical will attack brass, so spray through and around the guide (inside the cylinder as well) with carb cleaner. Don’t spare the cleaner, and clean it well. When you are sure that all of the cleaner is washed out
After 10 to 12 feet of rope is inserted into the cylinder, turn the prop so that the rope bunches up behind the face of the valve and pops it unstuck. Notice the valve is no longer in the stuck position and is even with the intake valve.
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H ANDS ON HINTS FOR HOMEBUILDERS
A gun bore brush is used to clean the valve guide.
The valve is worked back in position and ready to reinstall the spring and keepers in the opposite sequence of rope and compressors.
76 Sport Aviation June 2011
of the bottom spark plug hole, spray on some penetrating oil. With the string tied to the valve and by using a screwdriver through the bottom spark plug hole, line the valve back up into the valve guide. It will slide back in like new. Now reverse the process. Move the piston out, insert about 12 feet of rope, tighten up the rope, install your special angle tool, reinstall the valve springs, and compress. Install the valve keepers, remove the tool, and reinstall the rocker arm. You will have to press in the rocker arm about 1/16-inch to align the rocker shaft. I use a big blunt screwdriver against the valve spring keeper and press with my chest, keeping my hands free to guide the shaft through the arm and head boss. That valve should not give any trouble for a long time after that. It works great to have a son who is an A&P mechanic that can look over my shoulder while this repair is taking place. Start to finish (removing the cowling, cleaning the valve, and replacing the cowling so it is ready to test fly) should take only two to three hours including making your angle tool. That is a lot better than having to remove the exhaust system and cylinder just to clean the valve and the valve guide.
PHOTOGRAPHY BY BARRY DAVIS
