So I got a brand new, still in the box, 40th Fox 35 to replace my worn out one on this Cobra Paul gave me last fall. Something I could not diagnose was going on with the old one, costing me most of the practice time I had this summer.  So I replaced it.

New engine suffered terribly from the Fox burp. Old one never did this to any appreciable degree, so this was new to me.  Spoke to Paul about it and he described stuffing a shim in the intake port.  I found a great post on here about doing this, and set about the task.

Got it done. Test runs showed it needed a much more open needle setting, otherwise it seemed fine. Got out today with it and……..WOW!  THAT is how a Fox is supposed to run!!!!   Better power, flawless 4-2-4 run, absolutely zero burp or hesitation.  It was amazing.  It ran like all those Fox’s that won all those contests way back when.

Happy fella here!

Gary

Isn’t it a side-mounted problem mostly? L

Can't talk for others but I ran Fox 35's from '61 till '76 when I switched to OS and I had only one that didn't burp.  It was one reworked by Fox in '64.  I also can't remember one quitting after a burp.  Just got used to it. 

Ken

 Most will not like this answer. I never had the burp until i started to use uniflow tanks. I stopped using them and proublem went away

Isn’t it a side-mounted problem mostly? L

This one is partway between side and inverted. Cylinder points down to about the 4:30 position.

Isn’t it a side-mounted problem mostly? L

   Yes. Of course, queue up the "it never happens" stories, the "it's the tank" stories, and "it's supposed to do that" stories. I have seen it burp, crash, destroy the airplane, and still have the unfortunate owner come off the circle with a paper bag full of balsa raving about how great those Fox 35s run. Telling someone their Fox 35 isn't the greatest engine of all time (and would still win every contest if the judges weren't biased against it) is goring the ultimate ox. Also queue up questioning the patriotism of anyone who says otherwise - even if you won the NATs with an engine made 514 mile due south of Fort Smith.

   It works unmodified *just often enough* to provide the positive reinforcement that there really isn't a problem.

    Of course, there are *other* issues that can cause engines to quit, particular with a giant venturi and no compression, that explains most of the upright/inverted issues. Engines that quit unexpectedly from multiple undefined reasons are not "the greatest engine ever made".

     Still, correcting the very obvious defect, as Gary did, hurts nothing and causes otherwise no problems, so, no reason not to do it and remove that as a source of problems.

      Brett

The remainder were junk.

Not all, just the ones from the hobby shop.  I had the fortune to have Duke Fox take notice of troubles I was having getting a decent run at the 1964 NATS.  He asked if he could have it for a bit and see if he could make it run better.  Whatever he did made that motor run like a well oiled burp free sewing machine for the rest of it's life.  From 1975 through 1979 I flew with Gieseke several times a week.  I don't think he got his from the Hobby Shop either.  When the OS35s became popular, I got two and even they outperformed Bob's Foxes but he never changed while I flew with him.  Personally, I think the reputation of the Fox came from the select few that got them directly from Duke.

Ken