No answers?  Oh well -- I'll have to give it a try, just so someone can disagree with me:

This is most likely from dried up castor oil that's turned to varnish.  The stuff can get pretty hard if it sits long enough.  Fortunately it's a good metal preservative and fairly easy to remove.

If you don't have Cox wrenches, get a pair.  There are some variations, so you want at least one wrench that'll fit your cylinder.

If you have wrenches, the first thing I'd do is to take off the head and visually inspect the cylinder and piston.  If it's a big ball of rust, or if the cylinder walls are obviously galled (a bit of scuffing is normal, I'm talking about big gouges or rough spots) then you'll need to replace the piston and cylinder.  If it's castor it may not be apparent, or there may be some dirty brownish-looking varnish in there -- that's a good thing.  If there's really light rust you may be able to salvage the thing, although you could probably earn enough for three engines flipping burgers at MacDonald's in the time you'd spend restoring the engine.

If it's not rust, try heating up the cylinder with a heat gun -- you want it somewhere between "ouch" and "sizzle" -- and see if things will turn.  If they do, it's castor and all you need to do is disassemble and clean.

Now, whether that worked or not, take the tank off (you need to do this anyway, because if the piston is stuck then the reed valve is most certainly in need of cleaning).  At this point you would like to take the cylinder off.  There's only two reasons not to: one, the piston is stuck all the way at the top of the cylinder, which means there's nothing to accommodate the upward motion of the cylinder as you unscrew it from the case, or two, the rod is stuck tight to the piston and will get twisted off as you turn the cylinder.  So look at the piston's position in the cylinder, and if it's OK then take a pair of needle-nose pliers and try gently turning the rod.  If it appears to turn, and if the piston's not too far up, then take the cylinder off the case.  Now you've isolated your problem.

If you didn't unstick things with the heat gun and you still think it's castor oil, then the best thing to do is the antifreeze/crockpot thing (Google for it, or ask for clarification).  Do the whole motor, as disassembled as you can get it.  If that frees things up, then just clean everything thoroughly, replace your gaskets, and go fly.

If it doesn't respond to a hot bath in antifreeze, or if you just know it's rust, then try soaking just the cylinder and piston in penetrating oil.  WD-40, CRC, kerosene and motor oil, or whatever your favorite is.  Give it a few days to creep into the space between piston and cylinder, then see if things move.  If it still doesn't free up, then you probably want to start thinking of measures that involve blocks of wood and hammers -- check back here before you start whacking the thing with a hammer, though!

In the end you should get the engine down to a pile of nice clean parts (I go as far as taking the crank out of the case, and scrub each part individually).  Then get a rebuild kit, reassemble, and see if it all works.

      Jim, I do have some suggestions. One I feel your going to have to take it apart for a few reasons. The reed is more than likely glued to its seat and therefore will not come unglued without proper cleaning. A new reed is probably in order. The clear plastic reeds do work great but in turn they do distort and this will give you fits unless you replace it. Then comes the little o-ring seal within the venturi. This is more than likely hard and squished and it will leak and cause erratic runs. These items are more readily available now and also by bulk more than they were 20 years ago. The other thing is that particular engine by the looks of it I'm guessing 1986-1993 era were good runners. The one drawback I've had is the plastic backplate and the connecting rod launching through the top of the piston. The piston conrod joint needs to be reset to the proper clearance because the connecting rod will go through. Use caution tightening that backplate because they will in fact crack if its not already. Prior to taking it apart you should close the needle all the way down put a syringe on the fill pipe while holding your finger on the overflow pipe and pressurize the tank. This will tell you if the venturi o-ring is leaking by hissing out of the screen area and also if the backplate is leaking. Sometimes air will also leak around the needle valve as well. ALL LEAKS NEED TO BE ADDRESSED. I've heard it more than once, this engine ran great than one day it just stopped working correctly. They are very temper mental at times but its all the small factors that add up. The other problem I've had is the crank gets glued to the case and when running, it almost acts as a brake and just doesn't allow for the engine to turn up. I dismantle my engines and do a bit of polishing on the crank but this is not all that necessary. A good soaking and cleaning will also break the gooey gunk down and a little oil and your back in business. The other problem is the fuel pick up tube. More than likely its hard as a rock and its in the incorrect spot with the end that attaches to the backplate all swollen up at the barb. This should be replaced. I don't use the stock fuel tubing as I find it troublesome and it just doesn't work as well as silicon tubing. I'm not suggesting that the engine won't run in the condition you have it now. I'm just stating that in order for it to work like it was new again, some of these steps may be required. Ken

Ray, Ken, John, Tim,

Thanks for taking the time to offer insight and guidance.  I do appreciate it.

I'll see what I can do this week-end.  I had read elsewhere about using an old crock pot and ATF to unstick old engines, but those recommendations seemed targeted toward older, larger engines with no plastic in the system.  I have been concerned that the mixed materials in the Coxes might have problems with that approach.  The "use fuel" and "heat gun" ideas seem benign.  And, I'll go back to the CoxInter. web site and see about picking up some replacement reeds.  I don't remember changing out reeds in my late '50's and early '60's vintage BabyBees, but I didn't worry about that too much back then.  Taking steps to have consistent runs and easy starting seems like a good thing to do.

Thanks again,
Jim Howell
Huntsville, AL

I'm glad to hear that you got it unstuck, and that it's in good shape!

I would hope that the plastic they use in those engines would be OK with antifreeze or ATF (I've only used antifreeze), but you want the motor fully disassembled anyway; at that point there's no problem in leaving the plastic parts out of the pot.

Let us know if it runs.

           The majority of those engines came with a clear mylar reed. I'm calling the see through clear reed mylar. That reed provided a major performance boost over the copper style. I prefer this style reed. That particular reed will suck down into the venturi hole causing a dimple and not providing the proper seal. I keep a lot of them on hand as they're prone to this. I've tried the Davis Diesel style teflon reed but I found it not so effective for me. That reed is a bit softer and seems that it too would be prone to distorting. I haven't used it enough to find out. I had something happen with that reed that prevents me from wanting to use it again. This reed is white in color and you can't see through it nor can you tell if two of them are stuck together. I put the reed in not knowing two were together and the spring clip felt as though it was in its groove. This was my best running engine until the clip departed and went through the side of the case. It came out of the case sideways. I just can't imagine the force that was behind it. While were on the topic of case, its always a good idead to flatten the mating surface of the tank to case area. Placing a piece of 400 grit or finer paper on a piece of glass with a little oil will flatten the surface if you move it over the paper in figure eights. I sand until the anodizing is removed and clean well.   The reason I posted this was to suggest not cleaning the reed simply replace it. Take a peek at the screws that hold the tank on as well. There is probably corrosion on the screws which can also cause problematic runs. I've managed to clean these with a soft wire wheel. There are replacements now that weren't available back then made of stainless. These screws are inexpensive compared to the time required to clean those old screws properly. Using the method above of pressurizing the tank, I also check the screw heads with a drop of oil on them while pressurizing. More than once I had the firewall paint blistering off only to realize the screws all had been leaking. Not only is this giving you a short tank run but its sucking air causing poor fuel delivery. I've found many of the metal tank banks to be cracked in the corners where the screws seat. This is the case with plastic tank backs as well. I've been able to seal the backplate to screw seal simply by just replacing them. I haven't been able to successfully fix the backplates that were cracked . I've  tried silicone, epoxy, and also j-b weld.  Each repair eventually failed and leaked.  I've also had the tank leak where the v-groove is as well. Its tedious but a small strand of dental floss carefully place in the v of the tank will resolve this leak. I love these little engines and tried for years to improve on some of the problems I had with them. Ken