Muffled Fox .35

[Christopher Root]

Getting my airplane ready for spring. 40th anniversary  Fox.35 with stock tube muffler with pressure tap to fuel tank.  What size prop?  Thanks in advance!

C R

[Brett Buck]

Getting my airplane ready for spring. 40th anniversary  Fox.35 with stock tube muffler with pressure tap to fuel tank.  What size prop?  Thanks in advance!

C R

 10-6.

    Brett

[Joe Gilbert]

10x5 APC trim best you can and then try find a better prop.

[Dan McEntee]

  What is the airplane and weight?  10-5 may be enough.
    Dan McEntee

[Christopher Root]

It’s a Stirling F6-F Hellcat, profile, 42 inch wingspan.  Not sure on weight

C R

[Gary Dowler]

I like a Master Airscrew 9-6 on a Fox 35 with factory muffler. I've got that set up on a Sig Twister. The Fox 35 has such a flat power curve you can run almost anything except certain props will put you in the vibration band.  Depending on how heavy your piston was cut you'll have to try different props. With 10" props you're hoping to stay below those rpm and with 9" props your hoping to go fast enough to get up out of it.

Motorman

Interesting. Before I pulled the Fox 35 off my Shoestring Stunter , I was running a TF 10-5 on it. Properly balanced.  Now and then on a flight  there would be a vibration that I could feel in the handle. Was weird. Had never had that happen years ago with this same engine. Could be the same things you describe? It would go away as rpm changed.

Gary

[Dave_Trible]

Interesting. Before I pulled the Fox 35 off my Shoestring Stunter , I was running a TF 10-5 on it. Properly balanced.  Now and then on a flight  there would be a vibration that I could feel in the handle. Was weird. Had never had that happen years ago with this same engine. Could be the same things you describe? It would go away as rpm changed.

Gary

This could be prop blade flutter at certain RPM with a soft blade,  perhaps a very old or somewhat oil soaked prop.  Also if the engine is pretty old and hadn't been used for a while corrosion might be eating away the fits inside.  If the wrist pin gets stuck in the rod from encrusted castor or metal filings the engine will still run with the ends of the pin rotating and hammering away in the piston which is fairly soft.  If so it's about done.  If it's spitting a lot of dark junk in the exhaust or crawling out the front shaft bushing thats a sure sign.

Dave

[Christopher Root]

Thank you!   Forgot to mention: I am using a Super Tigre replacement needle valve.  Brass one with long, thin, curved handle and fine threading.  Is that OK?

C R

[Dan McEntee]

   A Super Tiger needle valve body is bigger than a stock Fox, and chokes down your venturi. The Fox .35 Stunt venturi doesn't have much meat on the walls to allow much drilling out, almost none. If the hole in the needle valve body is centered properly or at least close, you can turn down the center portion a bit carefully in a drill press or hand drill an using a needle file. Or, I have just filed flat spots on each side of the valve body, just don't go into where it seats in the wall of the venturi. Choking down the venturi just cuts your power a bit, but will also make your fuel mileage go up. You can just fly it as it is and see how it runs and responds. If you have an extra stock muffler, you may want to expereiment with cutting off the stinger tube to open up the exhaust a bit. Since you are restricting the intake a bit, it will help to open the exhaust and let it breath out easier.
   Type at you later,
   Dan McEntee

[Christopher Root]

My ST NVA O.D. Fits  through the hole in the Venturi without issue.   The fuel hole in the spray bar is 0.140 mm.

C R

[pmackenzie]

It will fit through the holes, but blocks too much of the actual intake.
The stock Fox needle is stepped to a smaller diameter in the middle.

(Google image below, first image is of the stock NVA)

https://www.google.ca/search?q=fox+.35+needle+valve&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjXu4CzvLLgAhXp6YMKHeU3ChIQ_AUIDygC&biw=1728&bih=939#imgrc=_

File down the two sides of the NVA, you will get much better power.

[Dan McEntee]

If you file it thin and tighten it just right it will break right across the fuel discharge hole. The stock supertiger needle valve assembly will be just fine and is a tremendous improvement over the Fox assembly.


Motorman

     For this application, it may work just fine as I mentioned. But if his circumstances call for having to use the engine in a situation where he will require the extra power in a larger and/or heavier model, he is at a significant loss. If he can measure the diameter of the hole in the spray bar, I would think he has the ability to work the OD of the spray bar down a bit and not ruin the spray bar. Why give up any ability to make a little more power? A suitable substitute would be one of Randy Smith's needle valve assemblies that is made just for the Fox.35.
   Type at you later,
   Dan McEntee