Is it just me, or do all .35-S's love to run away?

 I've just spent the last two days fighting this problem...again.
 
 Now I do know that the .35-S isn't necessarily a fan favorite, but I've got some of them that I could be using. I've also heard of ones that have run beautifully for their owners. I've messed with four different ones off and on over the last few years, and every one of them follows exactly the same pattern no matter what I do. Two of them are near new BTW. This experience has also been on three or four different models so I can't really believe it's an airplane/engine harmonic thing like I have seen before.
 
 Here's the routine: I'll fire the engine(s) with a soft blubbery rich needle setting and then start sneaking up one click at a time to approach that "happy" spot just before the break. It will seem like I'm coming up on it, but the engine is a long way from it's peak yet. Then I'll go just one more click and the thing goes CRAZY! By this I mean to the point where the whole plane and engine is vibrating and buzzing madly with the engine in an absolute scream. When the engine(s) take off like that it usually takes backing the needle down a good 1/2 turn just to get it to settle down again.

 I've tried countless tank and prop combos. All props have been checked and balanced. I've tried the stock C/L venturis as well as .286,.282 and .280 Jim Lee versions, all of these while using the stock O.S. "springy" NVA. I've tried stock FP, Big Art and Hobby Fastener tongue mufflers, all with and/or without pressure. Fuels tried have been Sig 10 and/or 15% nitro with 1/2 syn and 1/2 castor oil, Sig 10 and/or 15% nitro with 20 and/or 25% castor and also Powermaster 10% and 15% nitro, I forget the oil content on that one.

 What's the magic combo???

Sorry, Wayne, I can't help you on this one.  I love the OS .35S (6) that I have and have no run issues at all.

Hot Plug
5% nitro-29% castor (Powermaster GMA blend)
10-5 Top Flite paddle blades
Everything else is "box stock".

I also have a Big Art tuned OS .35S and use the same set up.

Big Bear

I'm flying an OS35-S in a T'Bird.  It is sensitive on needling, for some reason.  I would imagine a needle setup with a finer thread would help.  In any case I think it important to use at least 28% all-castor fuel.  Steel piston and steel sleeve requires it for long life, and cooler running.

I still remember the horror stories that GMA used to relate about destroying engines without enough castor oil.  But George did tend to be a bit dramatic!

Floyd

What Pete says. The PA or an ST g21 valve. The ~ 0.280 vent should go with either valve. The stock Max valve body could be one of the old 0.135 dia and if so you need to find one of the 0.265 vents if you stick with  the small valve body. The PA and Tiger valves also have nice long tapers and fine threads so the valve responds pretty well. Good luck.

Thanks guys.

 I forgot to mention above that another thing I've tried is running up to three head gaskets, still didn't help.

 I usually like to only change one thing at a time to keep track of what works, and what doesn't. I have been thinking recently that my venturis are just to big in this equation.

 How about this? First I think I might try going down to something like a .255 to .260 venturi, still with the stock O.S. needle, and see what that does. I say that only because mine all have the O.S. NVA in them already. If that didn't fix things, then I would try a PA or ST NVA, still with the small venturi.

 What'cha think?

Hi Wayne,

I think it is a good idea to use smaller venturies.  Some people don't like the OS NVAs but I leave them in the engine if they are already there and so far no problems.  Steve Wooley used to put the OS NVA in his Foxes and that's also what Big Jim Greenaway always used in his reworks.

I would add to make sure you have enough castor in the fuel.  Some guys get away with running lower oil content with less castor, but castor DOES help to remove heat from the engine.  (and heat can help cause runaways)

Your predicament is odd since the OS .35S isn't known to have problems with "runaway" like the .35-.40FPs do.

Big Bear

Back in the day, I ran OS35s with the stock needle valve, .281 dia. venturi and K&B 100 fuel.  First without mufflers and normal venting, later on with mufflers and muffler pressure.  Never had a runaway.
 

 I did find and install an ST type NVA, a nice fine thread jobbie. Whatever brand it may be, it's the same diameter spray bar as the stock O.S. one I'd been running. I put it in with a Jim Lee .280 venturi, the smallest I have right now. It's also the same venturi I've been running for a while during this wrestling match. I put the tongue muffler back on too, and plumbed it for pressure. Next chance I get we'll see how it works.

   The small venturi is the way to go. The larger of the two it came with was extremely prone to exactly what you are describing.

      Brett

Ran all mine with Adamisson tube type mufflers and ST NVA's. A wide bladed prop 10 X 6 helps sometimes and Randy fuel! Nary a problem! A chip muffler with two rows of 3/32nd holes is likely choking your engine and causing overheating. (That causes runaway...)

BTW you could probably add some rubber bands to your tank hold down. A vibrating tank will cause bubbles in your fuel. Guess what that causes? Are you using a fuel filter?

One last "wild one"...You could be setting up a harmonic vibration with your engines. That is, the OS's "happy" RPM may just cause the nose on that airplane to shake uncontrollably. I had a Firecat that would would vibrate unreal with any Fox .35 I put on it. Switched to an OS 35S and it was happy forever! Really!

Ward-O