Basic Cox Cleaning and Rebuilding
You need to get the Cox really clean inside. Little particles of congealed oil will cause fuel metering and reed issues. In particular, corrosion will occur due to the dissimilar metals used in the tank. Steel screws, plated pot metal for the backplate, beryllium copper for the reed (if yours is old enough), aluminum case and tank parts, and so on.
Be careful what you soak your parts in and what you soak in the same container. If you dump all the parts into a cup and pour in the wrong things and leave them sit, when you come back you will have a gooey congealed, dissolved, corroded mess. And will be hitting the swap meets for some more engines. For example, brake cleaner will do damage if used for soaking. Kerosene won't. I haven't used WD-40, but assume that someone recommending it has actually done this without doing damage.
Be very careful when removing the cylinder. You need a Cox wrench. The issue on the older engines is that one tool engages the exhaust ports in order to remove/install. Obviously, distorting the cylinder means your engine may never run right afterwards. Later engines had wrenching flats cut in the top fins of the cylinder.
Once clean, the tuneup should include: new O-ring on the venturi inside the tank, new fuel pickup tubing with anti-collapse spring, new case gasket as a minimum. (0verhaul kit: 49OHKBB) Be sure the reed is undamaged and will rotate freely when nudged around with the tip of a toothpick. It has to float freely under the retainer clip. All the different style reeds work. Some guys have a preference for one versus the others. There is the star shaped beryllium copper one, a racetrack shaped mylar one, a racetrack shaped stainless steel one, and a racetrack shaped Teflon one. The only one that I think is a bit tricky is the Davis Diesel aftermarket reed made from Teflon. It seems to take a bit of running to seat. I also had a bit more difficulty getting it to float freely under the clip.
Check the needle valve to see if it is straight. If not, it will not give a nice linear rpm change when you turn it. That is annoying and will result in some cooking flights sooner or later. The spring does two things: it keeps the needle from rotating under engine vibration, and; it seems to help reduce air leaks. Air leaks into the tank thru the needle, thru the backplate v-groove seal, thru the case gasket and thru the backplate screws will make the engine harder to set. When you have your engine running, cover up the two tank vents and the engine should lean out and quit. If not, you have a tank leak. They will run ok with a small leak, depending on where it is. If you choose to try the larger tank with the over and under stunt venting that Tim showed, it is really handy to shut down the engine this way. The other way is to flip the plane around so that the pickup comes out of the fuel. Most other “techniques” will wear out or damage the engine. Tim also showed a photo with a standard mod that a lot of guys find effective. Remove the spring on the needle and save it. Install a small flat washer over the pointy end of the needle and then add a short length of medium silicone fuel line over the threads. It takes about a quarter inch or so. Push the needle down to get the threads to engage and start turning. This seals the needle for sure, and will solve many setting and run issues.
Check the glowhead for distorted element, for a frosty look, and for a decent glow with a 1-1/2 volt battery. If you can see it glow it will run. It may run even if you can't see it, but might be hard to start. New glowheads are available, as are tuneup parts from several sources. I have had good service from Cox International in the great white north, eh? See link below. Be sure you have at least one head gasket installed.
These engines use a ball and socket connection between the rod and piston. This will loosen up either during break-in or after some running time. When you have the piston out, check the fit. There should be a little bit of end play but not much. If you feel a clunk-clunk it is too much and continuing to run the engine will start to push the rod up thru the top of the piston. The solution is to reset the rod. There is a standard tool to do this. It takes a bit of feel, but after you've done one you will totally get it. Unless of course, the only tool in your toolbox is a hammer. In that case, just lay all the parts on the bench and use about 4 or 5 good strokes each.
Do not use anything that is abrasive on the piston, cylinder or case. Polish the crank journal and pin if you think they need cleaning up. You are not trying to remove material, rather, you are trying to get any corrosion or semi-permanent gunk off.
On reassembly, do not overtighten the backplate screws. The backplates have a tendency to crack around the bosses. You can get new ones, but they will be plastic. They work, but have different issues.
One thing to watch is the screw for the prop shaft. The early configurations used a stud that you could bottom out in the crank. This reduced the tendency for the front of the crank to break off at the relief cut for the drive plate knurling. If you use the nice little aluminum spinner and standard screw, you are more likely to pop the crank here. Of course, a heavy wrench hand and hitting the ground has something to do with it too.
Another thing to be aware of is the case bearing. This journal bearing relies on a constant film of oil to avoid damage to the case bore surface. The fuel you choose has a significant impact on the life of this surface. The case is made of an aluminum alloy extrusion that supposedly has bearing properties, but let’s face it, aluminum on steel has low galling resistance. Cox mitigated that with a “special treatment” in their later engines. Anodize for example, helps by increasing hardness but will also wear thru. I’m still not sure what the black coating is on some of the engines I have seen. Sure doesn’t look like anodize and their marketing claims were purposely obfuscated what it was. So knowing this critical wear area, now think about the comment above about polishing the crank. It helps. One way to do this is to put a stud in the crank and chuck the exposed stud up in a drill press. Use something like SemiChrome polish on a towel or just use crocus cloth.
When you go to run it, start out with the needle set at 3-1/2 turns. This should be a little rich. The precision that these engines were made with makes this a pretty certain starting setting. They like to be port primed, usually with each burp until running. To avoid over-priming, close the port and put a drop on the side of the piston. That is enough. Use a good battery. More starting frustration comes from a weak battery than probably anything except trying to run an engine that was put away wet and hasn’t been run or cleaned in months or even years. The non-contacting spring starter will eliminate the problem of repeated starts backwards. You don’t need to wind the thing up, a half turn on the starter is all you need and is safer for your fingers. Half A props cause more minor finger cuts than any other size, it seems. Be careful since blood doesn’t easily come off of a model airplane finish and will remind you each time you see it. Of course, if you are racing your Cox engine, the blood proves you must be going all out to win!
These are amazing little engines, and kept clean, will perform really well.
I hope this helps get you going,
Dave Hull
ref.:
https://coxengines.ca/ http://mecoa.com/