Reviving a 1950s Fox Stunt .35

I forgot to mention that the vertical scoring is normal on these things. I have a very old Fox manual that says to expect it and not to remove them or you will lose compression. I think it's just indicative of the metals used way back when. If you look close you can see them on the Cox pistons as well.

Ken Cook wrote: I had mentioned that in a prior post that some of the early Fox's had brass end pads. I hope you did indeed place them back in there. That's exactly what they're intent is, however they can wear grooves in the liner. This is why I suggest high castor fuel for that particular setup. Dan had made some Delrin replacements for me in one of my older wide bypass Fox .35's.

I hope you get a chance to run it this week. Ken

The ends were hollow to a depth of about 5mm. I didn't see any removable brass end caps or plugs. I didn't lose any parts either. The tip ends look brass colored in the crappy picture I posted, but I would have to take it back apart to be sure. If a brass part was missing from the pin, there were no stepped reductions in the circumference where brass caps would have fit over the ends, and if brass plugs went into the hollow ends I would have seen them if they fell out. I think it's like it's supposed to be.

cribbs74 wrote:Sorry about the chip Rusty, I had no way of knowing though. The guy said low run time and it appears to be so.

Did you find the chip? Or was it missing from the get go?

Ron

No I didn't see the chipped out piece, but I bet I did it with the allen key I was using to pry the rod from the pin. I don't think it will affect anything at all. There is no raised metal around it. It looks and feels like a good engine to me. Like I said above, I think the wrist pin has brass coated ends. I'm looking forward to firing it up tomorrow. My birch ply Thunder Tiger mount looks like it will fit it just fine.

Wear ear plugs......... A video would be cool if you're up to it.

Thanks, I wear ear muffs when I bench run. My ears always have crickets from a long life of Hard Rock music, model engines and skeet shooting. And I already have the camera on its tripod. I'll try to get it done so it can be on the Tube before too late this evening.

More investigation is needed. I searched for "Fox .35 wrist pin pads," among other terms, and found reference to hollow ended pins that had brass inserts. My pin looked solid until I started cleaning it and found the hole in each end. Upon probing with a pick, I dug out green sludge to reveal hollow ends. Maybe this was the remains of brass pegs that used to be there. I'm not sure if I'm going to run it or not before I get more info. In my search I did hear of free-floating pins. I'm off to the shop to take another look.

It's all good. They are tiny round brass eyelets pressed into the ends of the pin. The window is still open for a run today.

The cylinder requires the brass eyelets. The wristpin is made out of material which is similar to drill rod. It's much harder than the liner and will indeed cut grooves into the cylinder walls. I'm somewhat curious about the nick on the piston now. Did a piece of the brass get jammed in the port of the sleeve?


Seeing I'm late on this response, great fire it up. Ken

I see no damage to the brass eyelets that looked like it could have caused a chip like that.
In any case the engine runs great. Mercy it vibrates! If y'all hadn't talked about it at other times, I would have been alarmed. As soon as it cranked my fuel tank started dancing, the shim and all the padding danced out from under it within the first 3 seconds, and then the fuel line fell off like Ken said it might.

I began with 5 turns on the nv and it was obviously lean, Finally between 6 and 7 turns was right. It cranked and ran at a fast 2-stroke so I turned it out richer to a solid 4. I could not turn the stiff nv lean again, but I found I could get short lean runs by pinching the vent tube. I had an old wide MA 9x5 prop, and its highest RPM was 13.5k and it 4-strokes steady in the 11s. The camera cut off in mid run so it missed most of the taching. I restarted it and tried to get some good readings but by then it was low on fuel.

I'm off to make the movie now. It'll be posted here later.

LOL that's funny,

The harder you mount them the better they do. It will be fine on the plane. Sometimes they resonate like mad and sometimes they are smoother. I had a really bad vibe a while back, the flight just before it was fine and had the same prop. It actually hurt to touch the NV.

My guess is they hit a resonant frequency that makes it shake itself to death.

Ron

The video should be available in an hour and 15 minutes from the time of this post.

Over an hour????? I need to hear that bark now!!! This just won't do Rusty.......

Refill your popcorn, it's live now.
BTW, I went out to the shop for a minute and counted the turns on the nv. 8-1/2 turns was where it was running.

Good! That 4 stroke with little blips into 2 stroke is just where you want it on launch. Remember that sound.

The darn thing was shaking the vice to pieces. Glad you got it running. All that's left is the Yak.

Ron

One thing for certain, the Fox needle valve leaves a lot to be desired. If your using one, it has air leaks. These air leaks will cause the engine to run quite bad and vibrate. Counting the turns on a Fox needle valve is out of the question. You could take an identical needle to replace it and it may require two more turns in or out. I've had some barely hanging into the spraybar, they're just that inconsistent. They do work, and with a little fiddling, they can be made to work quite well. Installing a piece of tubing over the threads and up to the knurled knob is a must. The problem is, finding the correct size tubing. Some fuel tubing fits better than others, and cutting it square so it seats well on the spraybar is in order. Here are two pics of the different needles. Both needles fit but the taper is the replacement and most currently available. It fits entirely different than the older flat sided version. Many didn't like the flat sided style, but I actually prefer them over the taper.

I've done this in the past and made my own from a 3-48 screw. Grinding a taper on the end.The needle doesn't need to come to a sharp point works well and generally they fit better in the spraybar. I believe the threads on the Fox are rolled rather than cut. This is only speculation and might explain the poor fit in the spraybar. If you have an engine that just won't needle and the needle is loose in the spraybar, it should be replaced. Randy Smith http://www.aeroproduct.net/ makes the best replacement available today. No other needle is a direct replacement or as high of quality as the Randy Smith PA needle. Randy's needles don't require any shimming to center the spraybar and this is a major plus. They're the same needles and spraybars used on the PA Stunt engines with the exception that they're turned down in the center to the Fox spraybar diameter. For the additional dollars it costs to purchase one, it amounts to less than $10 more than a Fox replacement. It will forever put an end to the indecisive needle setting the stock unit gives you.

In fact it looks so nice, it should be worn as jewelry so purchase two. I've used the Tiger needles which do work, they're equally susceptible to airleaks which can be tough to nail down, but they also utilize a larger diameter. This will compromise power. Some feel it's not enough to worry about. Only one could tell by flying. The Tiger needle can be turned in a lathe or chucked into a drill press and filed down, it can't be turned down to the same diameter as the Fox. I'm talking a true Tiger needle. They will snap directly at the hole. I can't speak of Hobby Products Tiger needle which Eric Rule offers from RSM. I have one of them and never explored the option of turning it down. If you choose to use the Tiger needle, I would strongly suggest filing flats on both sides as opposed to turning it down. Ken

Your needle position is in the same ballpark as my brand new stunt 35. I want to say 6 1/2 - 7 turns out is my start point. I got nervous backing out that far the first time.

I bought a Fox remote needle kit with spray bar and it is much nicer than what is on stock, but the super ringmaster I am putting the engine into is set up for the stock needle assembly so I am not going to swap over.

Phil

My needle is so stiff I can barely turn it, I'll clean out the threads again and double check to see that it's not bent. I will clip it, as Ken suggested and put a j-bend in it. While the engine was running I was on the verge of slipping and jamming my finger into the prop. I could turn it out, but I couldn't turn it back in.

Anyway, I think the engine sounds healthy. I mixed up 6 ounces of fuel, so I'll run it again and use it all up. The clunk was rattling so badly it was bouncing out of the fuel half the time. I need to try to isolate the tank from the engine better. I hoped that birch ply would hold it tighter than that. It was smooth as ice with the Thunder Tiger bolted to it. Maybe next time I'll set the vice on my 19th century concrete bench instead of a plastic yard table.
I'll be asking questions about bolstering the beam on the Yak when I start on that fuselage.

I have rigid tubes in my test stand fuel tank. I got lazy and didn't install the clunk. I don't seem to have excessive vibration, I did balance the prop first (and I am confident you did too) My test stand is a 2x4 with a aluminum test bracket, this gets clamped to an old sheet metal machine base (from a relatively modern table saw)

On the off chance you didn't balance the prop as good as you could have rotate the prop 1/2 turn relative to the piston position. If you did a very good job balancing there will be no difference.

Phil

My 3/8" birch ply may not be firm enough for the Fox, but the plastic table is probably the worst culprit. That's an old front brake rotor from a Grand Am the vice is bolted to. I can come up with a better mount, or at least isolate the fuel tank somehow. It wouldn't hurt to recheck the prop. I'm pretty particular about balancing.

I watched the video all the way through this time. Some carpet pad under the fuel tank might be all it needs.

I start my Fox Stunt 35 by NOT flipping (initially), but instead pulling the prop through and not releasing it. When I get the engine primed and ready it starts 4-cycling, I feel a bump (hold on tight!), then it passes tdc without a bump, then it bumps. After 2-3 times through this I pull it through the no-bump rotation and then flip it. If it no-start I pull it through till it is bumping again, set it up and flip.

Much easier starts this way, works almost exactly the same with my OS40 (it bumps every time though, no skips), I do similar with smaller engines too, but reedies don't act quite the same.

Phil

pkrankow wrote:I watched the video all the way through this time. Some carpet pad under the fuel tank might be all it needs.

I noticed the same thing.

A little padding should help cut down on the clunk jumping around, and foaming of the fuel.

Since I more or less finished the Yak-9 I figured it would be a good idea to run the engine and make sure she's all set. The last time I ran it was last May.

The fuel tank is the one that gave me trouble on the Shoestring when I first flew it, sucking up air bubbles from the uniflow vent. I put it in a pan on the stove to heat it and get the back plate off, and there was a blob of solder almost blocking the two vents. I don't think it pooled up when I heated it, but can't be sure. The feed and uniflow tubes were soldered together and also to the wall. I wanted to shorten the uni pipe an 8th inch, but even with my 375 watt Weller gun I couldn't get the pipes to separate. So I chamfered the top of the uni pipe to give the bubbles an earlier exit. Back together and pressure tested, fixed one small leak on the rear seam, and strapped it on the plane with it set for uniflow mode.

I covered the whole plane with a 40 gallon trash bag with just the nose sticking out, what a site.   I wanted to protect the new decals which haven't been sealed yet, and also the paint where more of them will go. I filled it with some 10/25 that Wayne uses in his Foxes and commenced to priming and flipping. It didn't want to go, and I kept priming and finally got some noise. I had forgotten it likes a whole lot of turns on the needle, probably 8 or so, I didn't count, but it finally lit off, and needed some more turns. BTW, I put an OS8 plug in it. It was all I had for a spare, but it lit brighter than the OK plug that came to me in it.

It leveled off just south of a 2 stroke, popping 2 now and then. If I lifted the nose up it would fairly reliably break lean, though not quickly. I didn't worry too much about that since I haven't sleeved the needle with fuel tube yet. I will before I run it next time. Sitting level, mostly 4 stroking I tached it at 11.6K with a 9x5 APC prop. I didn't notice an alarming amount of vibration, and the fuel didn't seem to be foaming. There was an occasional bubble or two passing through, but my other tank does it too, and I think that's normal for uniflow wedges. It ran at a steady pace until it sucked air, and there was only about 1cc of fuel that I could draw out after it quit.

So it seems okay for Sunday's maiden, and I'll run a couple more tanks through it before then. I'll also try it on Powermaster 10/22 and see how it does. I don't know if it needs more run-in or not, but that's all it's going to get. I'll just be careful and try to keep it rich. With the uniflow running right, I don't think it'll run away during flight. But if I have to choose between saving the engine or the plane... guess which one I'll pick.  

Just my morning report.
Rusty

I am willing to bet that the slow transition from 4-2 is due to the oil content. 10/22 will most likely alleviate that.

I wouldn't worry about a lean run. Your Fox can handle it.

Your engine looks to be a 1958 version.  This was the first year for the re enforcing pillars between the fins under the head bolts.  The needle with the flat side (excellent picture Ken) was Duke's own design and was first used in 1952!  They kept that design until 1998 when they went to the round tapered end.  Fox used the hollow wristpin with the brass eyelets until 1968 when he started to use the solid pin.  The NVAs and pins were all the same style on the 19, 201, 25, 29 and 35's.  The Needle with the funny spade tip was first used for the 35 combat special.  It is a different thread (4-48) and the spraybar is slightly larger  in diameter.