Try running it on another airplane or the test stand. If it runs OK, then the problem is something about the airplane; excessive vibration, some sort of tank issue, etc.. If it never runs to your satisfaction, try the usual suspects, like different fuel, different glow plug, checking the head bolts and backplate bolts for tightness. With essentially nothing to go on, suggestions are unfortunately rather generic. Good luck!

Thanks for the tips guys

             The .15X likes to turn up as well. I prefer wood props because they're lighter and I use a 7x6. I found even though it can swing a 8"prop, it prefers the 7". Air test your fuel tank and make certain you have no leaks. Use a large syringe, plug the vents and inflate it as if your trying to pop it underwater.  I make certain that none of the crankcase is touching the engine bearers aside from the lugs. As Dan mentions the needle needs to be sealed and this has a large impact on engine run because this could certainly be your issue. If you already aren't using one, a fuel filter inline can also prevent unwanted air bubbles in the line which can make a engine run erratic.

 Just so you know, A Super tiger needle valve fits in a .15X and it's throat diameter as it's the same diameter as the stock one.

             I've been using Powermaster 10/22 in my .15x's for nearly 25 years. I achieve impeccable runs doing so. I couldn't be more satisfied with how it runs on this fuel. The same fuel I use in my stunt .35's.  One of the big problems I feel with this engine is that some of them came with a overly thick head gasket.  I have a huge bag full of them. When removed and the thinner one is in place, it increases the power and compression. The thick gasket was offered to be used until broken in and it's at least 3-4 times thicker than the head gasket that Fox offered in their replacement envelopes.

        That's the difference between an 15x and 15xx, I think.

     Brett

Hello A few more differences between the 15X and 15XX besides compression, such as the machined rather then cast rod, lightened piston and better main bearing material in the case. The lower head is a giveaway but only if you have the stamped XX on a lug. See:
https://sceptreflight.com/Model%20Engine%20Tests/Fox%2015X%20and%20XX.html

I have found performance of a standard Fox 15 and X can differ a large amount over the many Fox 15's me and my family have used.

Regards Gerald

That being said, I see really no substantial run quality difference between the rod made by a caveman and the nice anodized rods. I'm certain as to which one would fail first.

   It was never intended to improve the performance, it was to keep the explosions down at 20,000+ rpm.

      One casual examination of 5 different engines, some marked XX and some not, I found no correlation between the internal parts and the markings. Of course, then, as now, everyone considered themselves expert engine men and randomly swapping parts was common. There is also the factory running changes/modifications and using parts on hand (like putting machined rods in 15X engines because that's what they had that day).  So, aside from the old Aeromodeller reviews, I don't think at this there is any good way to tell what is supposed to be in any particular engine.

        Brett

Similar situation with a lot of Fox engines , which makes them interesting . As cheap engines for stunt or combat we stripped a few for parts and a few mongrels were made.
Especially when it was easier to pick up a whole engine rather then parts here in NZ.

Regards Gerald

PS when I was a Fox agent nearly 30 years ago, parts were a real pain with so many versions of the 35 and 36 engines and most of the engines I sold were the 35 Stunt .

   That's interesting, because we had the opposite problem. There were so many parts available either from hobby shops or the factory, it was very common to just mix/match parts. You could call the Fox factory, get someone, say "I need a crankshaft for a Fox 35" and it would be in the mail that day, you send the money later. And it was cheap, I think in the mid-80s a Fox 35 crankshaft was something like $6.   Crash and destroy the engine, you could send it in and either get it fixed or replaced for 1/2 price. You can say what you want about Fox as a business, or Duke as a human being, but service in this sense was always stellar.

   As mentioned (repeatedly...) almost everyone figured they knew which parts were "special" and were more-than-willing to start swapping in "the good wristpin", etc.

    Brett

  Another Fox revelation!! I never knew about the XX versions. I have quite a few Fox .15s and I'll have to dig those out and go through them. I haven't really used any in a long, long time but have some small models I would like to build that they would be good for. The ones I used years ago as I started to fly stunt ran OK as far as I remember but I know more now than I did then, so it's worth revisiting them again.
  Type at you later,
   Dan McEntee

               Sadly, we lost Bill Mohrbacher a few years ago. Bill's knowledge of the chronology of Fox engines and the different makes and models were unbelievable. Bill would travel with his cases of Fox engines were just the coolest thing. If you Google Fox engines, most of the time it's Bill who wrote the timeline and pictured the examples. While this is off topic, it's somewhat fascinating that Duke went to great lengths to produce certain examples. Duke made the .15 BB using the .36 Combat Special crankshafts. Bill had one of these examples and I really enjoyed looking at it.

         

   I will get back with other information later, but I would definitely check the backplate for leaks, or rather, I can tell you without even knowing anything about it that it IS leaking. I actually cleaned up one of the Hurl Engines (the one that set the high score in 2022) and *ran it*, but even before that, and even with it lapped flat with a brand new gasket, the backplate would not seal. The assembly lube (Singer light machine oil) seeped right through it just sitting on the bench.

      Brett

  p.s. OK, I have it working reasonably well now. I sealed the backplate with Permatex Ultra Red, because that's what I had. I was again reminded that this stuff sticks to everything, gets absolutely everywhere and makes a huge mess. I cleaned the backplate with brake cleaner and applied the sealant to it, leaving some oil on the crankcase mating surface. I faced off the crankcase with my diamond lapping plate. There were numerous pits and lines straight across the sealing surface, and those had to go.

  Also, the two lugs where the screws go through were bent and the effect was that there was no pressure applied to the actual mating surface. I filed a wedge on both lugs to hopefully make it easier to pry it back off again in the *extremely unlikely* even that anyone ever takes it apart again.

   For those who don't know what we were talking about above, I will have some pictures later, but head uses a "spacer" between the top of the liner and the head gasket. This is an aluminum ring that sits on top of the flange on the liner. It is no less than ***  .059" ***, that's right, almost 1/16". Remove that, and the head would sit normally on the flange and with the gasket, the piston clears it. The gasket itself is .011"



   This spacer *very drastically reduces the compression*, a typical 40-60 sized engine, we adjust by .003 at a time. The effect is to make it easy to flip over, but very difficult to start unless you are running high-nitro fuel, like the recommended Missile Mist (25%). Having used up my last can of Missile Mist in about 1978, I used Cox Racing Fuel (30%) which ran pretty OK but nothing to write home about, powerwise. In fact, it was super-wimpy.


    Remove the spacer, and you have all the performance of a 15XX, which is a pretty big difference, maybe 2-3000 RPM. This engine has the 15X cast rod, so I would expect it to blow up at high RPM when the rod lets go.

   Other interesting points - I compared the broken off end of the original needle (probably out in the grass at the Napa Hurling Grounds) to the "new" one. The original was very nicely machined and as near as I can tell, exactly concentric. The replacement was lopsided just like my "new" 35 needles. Overall, while you can tell it is a Fox, the internals seemed to fit appropriately and it did not seem fussy about how you torqued the head or anything like that. Overall it looked WAY BETTER than any of my newer Fox engines dating to the 70's, at least on the inside. Neither the spacer nor the top flange of the liner were properly finished for a sealing surface, both look kind of like a raw lathe cut surfaces complete with tool marks. There is no gasket or any other seal between the liner flange and the spacer. Outside, classic Fox, casting flash all over and not cleaned up, generally crude overall.



   I also found that if you use regular medium silicone tubing instead of the needle valve spring, it seals up and marginally improves the steadiness of the run. Most of the problem was due to the leaky backplate, that made a huge difference.

    Powerwise, I got about what Aeromodeller got based on the old reviews. I was a bit surprised because this engine seems to have been run A LOT with a large amount of carbon deposited on the top of the piston. And, as noted before, had been issued by Hurl Technical Services as a official Hurling Engine and has been Hurled several times.

   As Aaron and The Commissioner found, it is brutal to start on regular stunt fuel. It runs weakly, like Arden 09 weak, on stunt fuel if you ever get it started. I resorted to priming it into the cylinder with Cox Racing Fuel, which made it a lot easier.  Without the spacer it starts and runs OK with OK power on stunt fuel, and on a par with other 15s like the Max I/II/III and the Sportsman 15, at least as far as I was willing to test it - in the parking lot - with no muffler - on an otherwise quiet Sunday afternoon. The vibration seems OK, but it was bolted to the same test stand I use to test RO-Jett 61s, and even a Jett 88 for the brief time I had it.

     So, if someone was willing to tempt fate and run it on an airplane - in blatant disregard of Rule 2.0 and the formative principles - with a fair bit of work, you could get it to work.

   BWB

WHAT KEN SAID!!!
I use the Permatex Anaerobic gasket maker in all my engines. I don't like paper or fiber gaskets.
It not only cures in the absence of air but it also expands and can fill gaps as large as .025.
I've used it with glow, Diesel and gasoline fuel.
Never had a problem and some applications are more than twenty five years old.
HOWEVER: I have never tried it for a head gasket - I use silicone for that.

Bob Z.