There are all kinds of possibilities but you said a new Fox 35.  They take a lot of break in.  I like to put at least an hour of bench time on any engine before it goes into a plane.  If I were bench running a Fox, I might consider adding up to another hour of bench time.  The Fox 35 also likes all Castor, at least 25 to 28%.  Failing those, try replacing the tank with a known good one and install it with the tank outlet tube pretty much in line with the NVA.  Make it adjustable up and down so that you can re-position the tank if you are not getting essentially the same engine run both right side up and up side down.  I would not move the tank much over 1/16" while making those adjustments.

You have probably heard about the famous, or infamous, Fox burp.  That does happen with some, but no all Foxes.  You can search these forums and find a lot of information about the burp but from what you wrote, I don't think that is the problem. 

If these "hints" are not on target, add some additional detail about engine, fuel, tank, mounting etc etc and I am sure you will get a lot of help here.

Hi Joe,

Your statement of "negative G" just screams Fox Burp.  There are many posts here that deal with it.  The fix is simple, but takes a disassembly of the engine.

Search for Fox Burp.  It is a fairly common problem of profile mounted Fox .35s.

As Michael said, tank adjustment is also a critical item.  Generally, a Fox .35 on a profile needs the tank raised about 1/8th".

 BIG Bear
RNMM/AMM

Joe,
First thing to check is the plug. Sometime a plug will start the engine and run but on any upset condition (i.e. an outside maneuver with a side mounted Fox) it will flame out and just quit like the switch you described. Next, the Fox 35 with the muffler needs to run a lighter load prop then open face setups. I fly mine on the old Top Flite 9x6. With this prop it launches in a fast 4 cycle, breaks at the tops and on 60' lines does 4.9 lap time. I run 5% nitro with 27% (25/75) castor/sync but SIG 5% (50/50) works to just add 2 1/2 oz of Klotz per gal.

The burp is something that on profiles can be a problem even with a good hot plug. The simplest fix is the "stick in the bypass" which is just a piece of 3/16" hardwood about 3/32" thick that is fit between the cylinder and the case in the bypass. There is a small lip on the bottom of the bypass that the stuffer is held in by. You can also fill 1/2 the bypass with JB Weld, will do the same thing.

Best,          DennisT

About a year ago I plans built 2 Ringmasters. One I put in a brand new Fox.35 the other I put in a Torpedo .19. When I try any genitive G maneuver the Fox just stops it doesn't go lean or act like it starves for fuel it is like you shut the switch off. It dose inside loops just fine but no chance for a figure 8. Also it stops just going up and down the instant I put in down elevator but it has enough momentum to start again. At first I thought it was a defect in the tank I took off the back of the tank but found nothing. I tried putting extensions on the tank vents to get some ram air pressure, didn't help. I tried running it rich to 4 cycle but it doesn't have enough power to take off. I run it as rich as it will fly the airplane. I saw a thread about turning the spray bar so the hole is just out of site. didn't help. I saw another thread about putting the prop on so it is vertical up to compression. No change. Dose any body have any other ideas I could try?         

   This is a very well-known phenomena, with a well-known solution, search for "fox burp fix". The solution is to put a piece of wood inside the engine, in the bypass, to take up space. There may be other things wrong, too, but this is almost universal, and almost always happens to one degree or another. Ignore anyone who says it doesn't happen or that this is not the fix, even though they will descend on this thread to talk you out of it

     Brett

I put a peace on maple in the bypass and that seemed to fix the sudden stoppage. Thanks again,  Joe .

  What would happen if the engine were mounted "inboard" instead of outboard? Would not throwing fuel on the plug help any??

    Read Frank's original article. The thought that, too. Then, when he tried it, it burped on *insides* instead of outsides.

    The mechanism is NOT "throwing fuel on the plug". That's why the all the supposed plug-related fixes, including Fox's, do not reliably fix it.

     Brett

Philly Fliers run lots of Fox 35s. No stick in the ports. Nadda. Powermaster 50/50, 5% or 10% fuel. Hot plug. Piston fit often an issue. Don't over prop. Bad vibes can be difficult, especially on profiles. Tame the vibration as much as possible. Don't run slobbering rich. George's 2/4 break wasn't that.

Philly Fliers run lots of Fox 35s. No stick in the ports. Nadda. Powermaster 50/50, 5% or 10% fuel. Hot plug. Piston fit often an issue. Don't over prop. Bad vibes can be difficult, especially on profiles. Tame the vibration as much as possible. Don't run slobbering rich. George's 2/4 break wasn't that.

  Right. I see this all the time. People have all sorts of problems including crashing inverted when the engine quits or burps, then they pull it off the circle and tell us all how they never have any problems.

   Brett

I believe Danny came in third at the NATs in Classic flying an Old Time Galloping Comedian. Won many contests with that bird. The "Winningest Ringmaster of All Time" (won or placed in something like 20 Old Time Contests) is powered by a Fox 35. I just retrieved the oil soaked travesty, still wet with goo. The competition record written with magic marker on the monokote wing.  Danny's Galloping Comedian had vibration issues early on. The mounts are kind of suspended in the barrel fues, not a very rigid mounting. Took a while to get that right. Also, a custom muffler hanging off the Fox, a C shape made to drape within the Aluminum cowl, magnified the bad vibe. Fox cutouts, in my experience, are usually a function of over tight piston/liner fit, fuel leak somewhere, or vibration. Foxes do burp when run at certain rpm ranges, usually this means an over rich setting or less than optimum fuel. In an case the burp is considerably less intense than the simulated buurrrrupppp of PW's current stunt war wagon. Besides TRUE Fox aficionados applaud a healthy eructation now and then. Ken Cook's Oriental has a fab running Fox 35. Lots of sport birdies in the Philly Fliers do their shapes powered by the 1949 designed Fox.

Hi Steve. Yes. Exactly. Dan gets the engines to run without putting a stick in the port. We follow that example.

That's an interesting aspect of Foxes, not always appreciated. They can be run hard and fast for years. Honestly, the way some of them are abused by our club, I don't understand their durability. Dan can also run the Foxes at a more stunt friendly speed. A recent Ringmaster/Fox 35 combo that Dan put together is a point it put it where you want to plane. A very nice predictable model. I believe that Ringmaster was a flea market buy, already assembled. Most of our Ringmasters have a similar history. It's possible to write pages and pages about Fox 35s. They are idiosyncratic, can be a hobby within our hobby. Machining quality varies engine to engine. There have been as well significant factory modifications over the years. Over tight piston cylinder fits happen in 40th anniversary and other late engines, engines which when right can be great running Foxes. Hand lapping might be needed, hand lapping by a person with the feel. The thin casting needs care when reassembling. Torquing the head correctly, critical. And so on.