I have run the stock Fox 35 and a few Fox 35s with modifications.  Years ago, Fox had a deal going.  If you sent them a worn out, beyond repair engine, they would send you a new one of same type for half price.  So, I got some new Fox 35s and used the savings to get the ABC kits, hemi heads, and stuffer backplates.  As far as I can remember, all of those new parts were factory supplied items.

The only difference I noticed is that with the ABC kit, which contained a lighter piston, vibration was reduced.  I noticed no difference with the hemi head and stuffer backplate.  Power appeared to be the same.  If there was a difference with the new head and stuffer backplate, I could not see it.  Maybe too small of a change to notice.

I have no experience with any other performance mods, except for using an ST needle assembly if the stock unit proved to be a problem.

With the ABC kit, you might be able to reduce the castor content of the fuel.  But, that crank main bearing is still needing the same amount of oil.

Fuel consumption was the same.  That is to say, terrible, since so much of it went out the exhaust to help cool the engine.

I ran these modified Foxes when I got back into the hobby in the late 90s.  I did not know any better so I went with what I had.  After that, when I found better engines, I pretty much gave up on the Fox.  It is outclassed by modern hardware.  And for the money spent on modifying one, you can get a much better engine for less money.

Now, I wish I had bought another Brodak 40.  It is far better than the Fox in every respect.  Another bargain is the Magnum 36 but try finding one now.

And, with electric taking over, the Fox is pretty much a nostalgic item.  I still have them, and one 50th anniversary version that has never been run.  Maybe one day I build something for it, just for memories.

   Any of the common modifications that you can do to the Fox.35 , or combine them all into one engine, will just make the engine "better" by making it smoother and develop the power it can make for predictable. The ABC set ups will make them last longer. I like the Fox .35 and like to run them on something just for the sound and the nostalgia. If I have them, why not use them?? I get just as much fun out of those as I do out of anything else I have and fly. I am trending to smaller models as I get older and they are perfect for those. I understand the engine and it's short coming and can deal with and over come those pretty easily. One of the main things to work on is the crank shaft and it is one of the main shortcomings of the engine. They can be "balanced" to run smoother but you will never get rid of vibration from any single cylinder 2 stroke engine. For a "Super Fox .35" you would need one of Randy Smith's after market crankshafts if you can find one. The original cranks are reported to be not all that straight and they have too much counter weight. Randy's crank is much better quality and better engineered. I have one that I plane to use on a "combined" mods engine and use in a special airplane of some sort, some day. The stock needle valve was another victim of poor quality control but can be dealt with easily from what we know now. A simple , short length of fuel line over the threads seals off air leaks that cause erratic runs and takes care of most issues. Drilling the seat back just a hair or filing the flat on the needle down some helps out a lot also.  In the mean time I'll just do what I know how to do with them and enjoy them as they are. In retrospect, they were kind of a game changer in Control Line when they came out and as they improved. Gieseke won a world championship with one that was more or less a stock engine that was hand fitted, as this was before any of the current mods were known, so that is what you could hang your hat on. For it's time, there wasn't anything that was as light, or as reasonably priced that could perform like it could in the right hands. There are about a million of them out there, so why not use them?? And use them as they were intended?  Here is a video of me flying Mike Gretz's Fierce Arrow at about the 8 minute mark. It is a box stock, early vintage engine with no muffler ears. The only work I did on the engine was to add the fuel line seal and file the flat back a little. This is the run that most guys are looking for. It's pulling a 50 ounce airplane with what, over 700 inches of wing area?? And it's doing it quite nicely. I'll take this any day of the week.
   Type at you later,
  Dan McEntee

     


 

GREAT write up, Dan.
I'm still an ardent fan of the Fox 35. Although I've had a few lemons since I started using them in 1958, many of the rest have performed perfectly.
I've used the NVA thread sealing method you described with good results but my preference is still the OS Max and Enya NVAs.
I flew my Brodak Stinger yesterday for the first time in five years. It's powered by a STOCK (other than the Enya NVA) Fox 35 with a tongue muffler. It started in the first flip and ran flawlessly. Brodak 15% nitro, 50/50 lube fuel, 3 1/2 ounce single vent non-uniflow tank, 60 foot .015 lines. I'll also add that vibration is minimal. The plane has over 800 flights, was built 25 years ago and there are no stress cracks ANYWHERE on the frame.
Yes, I tried the hemi-head and stuffer backplate but saw no discernable advantage.
Bob Z.

   Any of the common modifications that you can do to the Fox.35 , or combine them all into one engine, will just make the engine "better" by making it smoother and develop the power it can make for predictable. The ABC set ups will make them last longer. I like the Fox .35 and like to run them on something just for the sound and the nostalgia. If I have them, why not use them?? I get just as much fun out of those as I do out of anything else I have and fly. I am trending to smaller models as I get older and they are perfect for those. I understand the engine and it's short coming and can deal with and over come those pretty easily. One of the main things to work on is the crank shaft and it is one of the main shortcomings of the engine. They can be "balanced" to run smoother but you will never get rid of vibration from any single cylinder 2 stroke engine. For a "Super Fox .35" you would need one of Randy Smith's after market crankshafts if you can find one. The original cranks are reported to be not all that straight and they have too much counter weight. Randy's crank is much better quality and better engineered. I have one that I plane to use on a "combined" mods engine and use in a special airplane of some sort, some day. The stock needle valve was another victim of poor quality control but can be dealt with easily from what we know now. A simple , short length of fuel line over the threads seals off air leaks that cause erratic runs and takes care of most issues. Drilling the seat back just a hair or filing the flat on the needle down some helps out a lot also.  In the mean time I'll just do what I know how to do with them and enjoy them as they are. In retrospect, they were kind of a game changer in Control Line when they came out and as they improved. Gieseke won a world championship with one that was more or less a stock engine that was hand fitted, as this was before any of the current mods were known, so that is what you could hang your hat on. For it's time, there wasn't anything that was as light, or as reasonably priced that could perform like it could in the right hands. There are about a million of them out there, so why not use them?? And use them as they were intended?  Here is a video of me flying Mike Gretz's Fierce Arrow at about the 8 minute mark. It is a box stock, early vintage engine with no muffler ears. The only work I did on the engine was to add the fuel line seal and file the flat back a little. This is the run that most guys are looking for. It's pulling a 50 ounce airplane with what, over 700 inches of wing area?? And it's doing it quite nicely. I'll take this any day of the week.
   Type at you later,
  Dan McEntee

     


 

  I typed my reply in a bit of a hurry this morning and forgot a few things. I have been testing some various hemi heads on a Fox.35 mounted in a Ringmaster. I see no real improvement when running 5 and 10% nitro. In fact, I think a stock head ran better where I live. It's not until you get up to 15% that any change is seen. The best way I can sum things up is that none of the modifications I know of and use really has any affect by themselves, but when executed correctly in a combined effort, you get a better running engine. One of the things I do , even on a new engine, is chase all the threads in the engine case with a 4-40 tap. Just chase the threads, no need to try and go deeper or anything, and you will be amazed at how much metal comes out of the holes. This means that you are not getting equal toque applied to each screw when assembling the engine. Cleaning up the holes with a good tap helps a lot. The Fox Burp often comes up in these discussions. I generally use a 5 inch pitch prop on my Fox.35s and just needle the engine up more, and the extra RPM creates more vacuum in the crank case for better fuel draw, and I have never had a problem with the burp. I have stuffed several Fox.35 by pass ports as described in the engine section and that helps the crank case vacuum issue. If you have never seen the Fox. 35 bypass in comparison to a more modern engine like a ST.G-51 you would be amazed! I think, though that stuffing the back plate changes the run a bit. I don't think it causes any negative effects, it just sounds different to me. Stuffing the bypass is a performance mod to add to the list. The stuffer back plate works in conjunction with the bypass port. This is what a stock back plate should fit like anyway. If I were tp ick one feature of the Fox.35 that is really bad it is how the back plate was designed and fitted with that cheezy cork gasket!! It's an air leak waiting to happen!! And again, these things done by themselves most likely would go un-noticed, but done in concert with each other wit some attention paid to piston ands cylinder fits, and you will have a better running engine engine.
  Type at you later,
   Dan McEntee

You can tune up a Fox 35 with allot of fancy hardware but one thing you can't fix is the squareness of the cylinder to the shaft. If you get a good one the engine is a joy but, most of them are way off.

Motorman