Pull the glow plug,  remove backplate care fully so hopefully you won't ruin the gasket.   Douse it real good with fuel.   Let it soak for a bit.   Mount a 9-6 prop and after soaking see if it will move.   Some say use a heat gun, but I don't like that.   Brand new Fox 35 Stunt are usually very stiff at the start.   Do not use an electric starter on it.   Get a heavy glove to use once it starts to move.  also make sure the wrist pin in the piston gets plenty of lubricant.   If you are lucky it may not take much to get it moving.   Of all the Fox's I have each one has been different.  Just don't get too forceful with it. 

Now the experts can tll you how to really do it.

    All Fox engines were supposedly test run at the factory before sale, and this one is of the vintage where I believe that they really did that. It was just a short run of a minute or two just to make sure things percolated, then wiped down and boxed for sale and shipment. It is just stuck from congealed castor oil. If you have a monkote heat gun, put a prop on it and just heat it until you see some of the oil starting to smoke a bit, or gets too hot to hold. Keep the heat gun moving around the engine to make sure you heat it evenly. When you think it's hot enough, try turning the prop, and as Doc mentioned, don't force it too much. It will probably move hard at first and then get more free. At this point, I like to use PB-Blaster penetrating oil and give it a shot where ever you can to lubricate things, or use some automatic transmission fluid. both have some detergent and cleaning properties to them. Pretty soon it will loosen up quite nicely. If you have a plug in it, see what the compression is like and if it will go "flipity flipity" for you.  This same process can be done after heat soaking the engine in the oven at 125 to 150 degrees or so. When cool, you can take it apart and clean things if you wish. I just flush with some fuel, put a good, hot plug in it and test run on a bench stand or in a model. For a really stuck, crusty used engine, the crock pot routine works well and has been covered numerous time and a search wil get you the info for that, but I don't think you will need it for this engine.
  Good luck with it,
   Dan McEntee

Ok here it is. I just picked up a new in the box fox 35 stunt for a price I couldn’t pass up. The only problem is the piston seems to be stuck. The guy I purchased it from said it was never ran and it appears to be true  there is no evidence of old fuel residue anywhere and the needles are still perfectly in the box. So what is the best way to try to free it up without damaging it anymore than it is? And what is the best way to brake it in once I get it free? And dose anyone know what year this motor might be from?

   73-74 or so, I had one just like it, in the same style box and heavy thrust washer.

   To loosen it up, remove the plug, put some light machine oil or even something like WD-40 in the cylinder and intake. Put it in the oven on the lowest setting (like 170 degrees), leave for 10 minutes or so, upright, so the oil stays in the right spots. Take it out of the oven with oven mitt, attach wood prop, and turn it over, until the oil is distributed all around and the engine is nice and 'flippy". Put in some real oil, any household type like 3 in 1, air tool oil, or my favorite, Singer Light Machine Oil, lots of it, to rinse out all the old crud. Then take it out in the next day or so to run it for break-in.

     Brett

     Hi:

     I bought a Fox .35 Stunt in the early 80’s that was so tight that it was impossible to turn over as it was so tight.  It could not have been run at the factory! I returned it and received a free one in return.

     Frank

I use ST NVA, but you have to file them down to the same width as the stock one or you lose too much power. (Either that or open up the venturi)
I think Randy Smith sells something that will work without mods

  Use the stock needles/spraybar, don't replace it unless you have a problem.

    Looks like you have two of them, one might work better than the other. The flat spots in the otherwise cylindrical needle are what create variable-size orfice, and it is actually a relatively clever attempt to support the needle in two points, the threads on one end, and the inlet side of the spraybar on the other. Whether a particular needle works depends on where the flat ends up relative to the holes, which seems to be random.

   Note that you cannot (or at least do not want to) replace the needle with the later conical needle without also replacing the spraybar with the later steel spraybar. The brass spraybar will very rapidly wear out if you use the conical needle, because the needle will vibrate due to the lack of support on the inlet side. The steel spraybar also has much longer threads, and is harder to begin with, so it can handle all the loads only applied to the threads.

    Brett

   I will add that I have had engines/needles of this vintage that did not want to adjust rich enough for me and the needle more or less fell out!  When I get this, I take a flat file and make the flat spot a bit bigger and longer. On all Fox.35 stunt needles I put a short piece of fuel line on the threaded portion so it will seal the threads. About 1'4" or so is enough that it will seat against the spring ratchet wheel and the spray bar body.
   Type at you later,
   Dan McEntee

   If that is the case, I would prefer drilling the seat deeper rather than filing the needle. Part of the problem - which I didn't ever get when I was running them, but was mentioned in a post by someone else either here or SSW - is that the metering seat is nominally drilled *right next* to the cross-holes. That's why it is so sensitive to where the flat winds up. If you drill it deeper, you not only get the needle to be richer at a particular position, you also straighten the flow before it gets to the outlet, which would be expected to reduce the sensitivity to where the flat is aligned. 

   I don't know what size drill to use nor do I necessarily recommend doing it, but I would explore it myself if I had that problem.

     That was a *great* insight that would have saved me great pain had I known that in 1978.

     Brett

     I had never known that about the seat in the spray bar, but will tuck that info away for future reference. I have a couple of good sets of number drills that can help figure out the size of things. This always seems to happen at the flying field, and most times the only file is on the pocket tool I always carry on my belt. You don't need to file it much, just a few strokes, enough to tell that you took a bit of metal off, and it has always improved things. The next time I'm into my Fox Box I'll look to see if I have an older model with that brass spray bar assembly. The big problem I find with that spray bar is that there is no barb on the fuel line nipple, as the nut to secure it to the venturi goes on that side. The pink Prather fuel line holds better than typical fuel line, and I double up with another short piece stretched over it at the nipple to help secure it. It's easier to change the needle valve assembly but some times you just want to run things the way they are. The last one I dealt with is on the Fox .35 in a Fierce Arrow that Mike Gretz built and I got from his estate. I have had some fun and learned some things just trying to make things work the way I found them and not butcher up the model. It's a 50 ounce airplane, and I NEVER would have thought that the little .35 would have pulled that thing the way it does, but it flies a LOT better than I thought it would and does a pretty respectable pattern and gets better each time I take it out. Right now the addition of 1/2 ounce of nose weight and a Rev-Up 10-6EW have made things even better the last time I had it out.
  Type at you later,
   Dan McEntee
 

Run the flat needle before you start changing out things. They really do work but aren't as user friendly as tapered ones. If you can't get it dialed in, turn the needle ONE FULL TURN in or out and then tweak. Keep doing this untll it hits the sweet spot and adjusts like any other needle. Once I learned to do this the flat needle was easy. I also run a piece of fuel tubing on the nv to keep it from leaking and vibrating. Ran tapered needles in the old brass spray bars without issues...and the 4 well used brass spray bars I just checked have a ridge for the fuel line...these are from the late 50s and early 60s. So it appears (based on other posts here) there are variations in the Fox nva and you have to find out what works for yours.

Greg,

You have a lot of good info and advice about freeing it up, and dealing with that vintage NVA, but no one seem to recall that you also asked about break-in. Ideally - to me anyway - do it on a test bench. (if anything is going wrong, you can pinch - or pull - off the fuel line and at least stop it from possible suicide. Here's how I did it...

 Fox used to make at least a short run before packing one for sale. It was done with a flywheel and Superfuel - 29% castor! Castor can dry over time; that may be why yours wouldn't turn over. The suggestions you got work well!

For break-in, use a light prop, 10-4 or 9-4 well balanced, for first runs, and fuel with plenty of castor. I prefer a few short runs slobbering rich to polish-in the rod bearings. Next, short runs a bit leaner. After each, check how hot the engine is. Let it cool to the touch before next runs. Run rich, but easing toward 2-cycle over about ten short runs, run it leaner  but not full 2-cycle over the next several. If it isn't getting VERY hot and holds setting nicely continue, but pinch the fuel line two or three times a minute to help mate the piston and cylinder.

Figure taking total running time of about a half hour so far... If it feels nice and loose and quickly drops back to the needle setting when you unpinch the fuel line, do longer runs until you can run it set just below 2-cycle for at least 3 or 4 minutes at a time. Keep checking for overheating - of course, it will be too hot to touch comfortably, but should not be seizing or loading up.

Somewhere in this phase, switch to a light 10-6. If all goes well over at least several runs, it should be safe to fly it in the rest of the way. It may keep getting better over the next hour. You don't want to run it screaming! That's too hot, and you can't shut it off when it's 60' away going ~50 MPH...

By this time it should be safe to go to a more available fuel - one with at  least 25% total oil, mostly (if not all) castor. That should be the minimum oil %. Some synthetic (detergent) oil% is okay, but castor mainly!.

On every new engine, I loosen the head bolts a bit - just enough that the bolts turn freely.. Check how freely the engine turns over. If that feels good, turn bolts in until they just touch evenly. From there, I 'torque' them down in a triangular rotation - prevents distortions. How? Call the front bolt 12 0'clock. Take a fraction of a turn in. The next bolt  is at the 4 o'clock position, then the next at 8 o'clock.  Then cross over the head to the 2 o'clock , then 6 o'clock, then 10 o'clock. Continue. The sequence 'walks' around the head. After each 6 bolt circuit, check for binds or dragging, so it turns about as freely as when they were loose. Make several passes in the rotation until you have them very tight, evenly - ideally with the same free turning. If a bind comes up, back off at least one 6 bolt cycle, until again free, then resume.

When you first run the engine, the bolts will loosen a bit. Retighten them after first run, same torquing sequence. Check and retighten after about 5 more runs. Maybe not  needed, but good practice. Further occasional checks through the benching runs? Yup.

BTW, Fox and British PAW 2-hole spraybars are cross-drilled slightly off dead center, on purpose. They are to be installed so that neither  jet hole is visible from the venturii entrance side: the holes are "just downwind" of the spraybar max diameter in the airstream. ...