help Fox 35 flame out

My old faithfull 35 stunt is flameing out without any reason. This motor was used lots in the 70's a little in the 90's and just now.

Motor mid 70's 35 stunt good compression hand starts easily. Well broken in. Ran on the bench 3 - 4 tanks just recently to clean it out and set the needle. Ran very smooth and steady. On the bench used SIG 10% (no extra oil) set very rich.

Fox STD long plug. RPM drops a little when driver is removed, tried 2 new plugs with same result.

Fuel Sig 5% 20% oil (50-50) with 12 oz of castor added per gallon

tank brand new smith uniflow (chicken hopper) combat tank. New lines and a filter. No pad under the tank. but very little vibration up front. Tank is on centerline of motor.

Does it with and without the muffler.

Prop old nylon TF 10-6.

Mounted on a profile shoestring. The smith tank is the only one that fits.

Motor set for a steady 4 stroke, or a fast 4 stroke. Engine quits for no reason in level flight. No warning, no going lean or rich. just quits. (always high going into the wind it seems )Runs 2 to 4 laps. Did the 4 times till I wiped out a wingtip on an upwind landing. Restarts right away. Plenty of fuel in the tank. Will run 3-4 minutes on the ground then just quit.

The only thing I can think of is the plug is going out, or there is some fuel feed problem.

What am I missing?
Not enought Nitro?

Fuel feed problems?
Fuel foaming?

Crap in the new tank?

Can I cap the uniflow line an try to run it like a normal tank?

Should I pull the motor and run it on the bench with the fuel/ tank combo?

this is making me nuts.

thanks
Dave

Dave it sounds as if you do indeed have a plug problem. The fuel you are using figures out to about 25 % oil after adding the amount you said you added so it shouldn't be the fuel unless it is old or has accidently gotten some water in it. What plug are you using ? Is it the ones that come in the redish package or the older one in the white package with blue writing?
Try an Idle bar or Some other brand of plug. I would also check the backplate screws for tightness and also head bolts.
You said that it does it on the bench and in the air and you have tried two different fuels, so I would guess either gasket leaks or plug problems, and lean toward plug.
Bigiron

I have two other thoughts.

I quits on the ground in 3 to 4 minutes. If this is a standard wedge tank, the pick up tube is half way up the side of the tank and it will "run out" at this point with a half empty tank. That is normal. You will need to tip the tank or the plane to run the rest of the fuel out.

In the air you have it at a rich 4-cycle. Then it quite high into the wind. Into the wind the engine will tend to run richer (less load on the engine). So it is possible you are simply running too rich for the conditions.

Just random thoughts to a question that needs a bit more information.

To clear some things up

The engine ran fine mounted upright on a test stand, but not with this fuel, (didn't have it)

The fuel was used just last week to break in a new motor (a new never run fox 19 bushing)and it was just fine, so it should not have water in it.

The only thing that is new is this fuel with this engine, the tank, mounting on the airplane

I will take the motor off and put it back on the stand tonight, and try it with this fuel. And check the head and backplate.

My gestion is the high oil and low nitro, could this be causing the plug to go out? And why would the motor go from running well to stop with no signs of going rich or lean? Or could the be a junk in the tank problem?

Dave: You didn't mention whether your oil addition was a fuel mix switch or an addition. (For most people, they don't have a container handy that is greater than a gallon, so they switch).

If you switched (i.e., poured out 12 ounces of fuel mix and added 12 ounces of oil, you will end up with:

27.5% oil
4.4% nitro

If you simply added to the mix (thereby ending up with a 140 ounces of new fuel mix, you get:

26.9 oil
4.5% nitro

This is a fairly mild mix, but tens of thousands of Foxes have been run this way, especially in warmer weather. It should, in most cases, sustain a satisfactory run.

Having said that, I would in fact start with more nitro, say 10, or even 15%. Go ahead and add your oil however you do it; you can see from the above that the numbers don't change much.

If Swanson (Fireball) Hot plugs are available, I would try them.

I also would, just at first, go with a conventionally vented tank. Once you get things sorted, you can go back to uniflow.

Try leaning your needle just a bit.

I'd agree with going back to 10% nitro and changing plugs. if you get an RPM drop when removing the driver, you have a plug problem. Also many people flying well with Fox stunt 35's today have abandoned the 4-2-4. Try a rich 2-cycle and see what happens.

As has been said its almost surely a plug problem. Sometime you will run into several Fox plugs in a row that have weaker elements than normal and they will vibrate and ground against the plug body. Thats a admission from Duke himself a few years ago. I am now using Thunder Tiger or Hobby Lobby #3 unshielded plugs with good results. These were recommended by GMA and work very well in both muffled or piped engines with far greater reliability.

It sounds to me like you might have more than one problem. One being the plug & the other being a possible problem in your tank.
Here in the NW nearly everyone I know that runs Fox .35's uses a Thunderbolt long plug. Idlebar or no idlebar doesn't seem to really matter. (I run Thunderbolts in my ST's & everything else as well.) I'd try a different tank if the Thunderbolt doesn't fix the problem. It's possible that you have something in there plugging the feed line part of the time. I've scooped little flying bugs & bits of grass into my uniflow tube before. The next time I fueled up the yuck blew right on into the tank. One of the things I like about plastic uniflow tanks is that you can see what's going on in there. When I see floaties in the tank I pull out the stopper & flush out the whatever.
Nils

Maybe the plug. Maybe not. Nearly every plug will drop a bit when you remove the battery--Thunderbolts, Enyas, everybody. It is when you get a major drop that you know you have a problem. As for the Fox plugs, I have found these to be one of the best and most reliable (not to mention inexpensive) around. Thunderbolts are also good (and inexpensive) but I wouldn't rate one above the other.

the RPM drop is not great, but you can tell. It is not huge. the drop seems larger than when I had it on the bench before I had the extra castor. When I did this years ago, I used dukes, fox idle bar plugs and any old wedge tank, and rarely had an issue.

I have never had a motor just plane flame out like that. You could always had some warning. A blocked fuel line would go lean, as would a cracked tank. A blown glow plug would leak around the plug and not want to restart hot. This goes from steady running to just stop. If I didn't know any better I would say the tank isn't venting properly.

I'll get some 15%, a hotter plug, replace the filter, flush the tank, pull the plug off the vent line and and try that.

thanks for the ideas

Dave: Once again, go with a fuel with more nitro. And a standard tank for the time being. Currell

Currell, here is an area where I don't agree. I see no reason why a Fox 35 needs high nitro just to keep running. We have run them on 5% nitro for years. The uniflow tank will only affect the fact that it won't speed up nearly so much towards the end of the run. It certainly won't cause flame out (unless there is "junk" in the tank or a cracked tube or some such.) If you have a uniflow tank, simply do not block off the overflow tube for one flight and you will have a standard tank. Then you can very easily compare the difference in the run to see which you prefer. You don't have to put on a new tank just to check.

Not disagreeing with anything you say, Len. Sometimes, to cure a problem (and this philosophy is not restricted at all to the world of CL), it helps to overcompensate in some areas. You want to be beyond reproach, as it were. The All American Way, as it were: If two of something is good, then seventeen is real, real good. And twenty three is even better. (But don't take pills this way).

If I run, say, 15% nitro in my Fox, and also a standard, no frills tank, I'm not going to lose any sleep over my fuel mix or choice of tank. If I didn't recommend these, there would be, to use an Alfred Hitchcock movie title, a "Shadow of a Doubt" regarding these two variables. I can quickly eliminate these two as factors if the problem persists.

Just my way of looking at the world, and I wasn't trying to pull your Uniflow chain here I know that they're right up there with Jennifer Lopez as far as you are concerned.

But; yes: Somewhere in the world, in some 5th World country, where nitro is as rare as flyable balsa in a Berkeley kit, guys are happily flying their Foxes on FAI fuel, with zero go juice. More power to 'em (literally and figuratively).

But that guy isn't me.

Who's Jennifer Lopez?

We always have a "square one" that we go back to when things go wrong. When we make inprovements, that becomes the new "square one." Sometimes muffler pressure can be a problem, but rarely uniflow. The only problem I have seen with uniflow is when the tank was not set up properly, or where there were leaks. If you want to check, just pull the plug, as it were. Don't cap off the overflow and you are running straight suction. However, if square one was with uniflow, this is no longer square one.

Now, if you don't have a "square one". Try things until you find improvements. Then try other things one at a time to see if it makes it better or worse. But make sure you don't jump to conclusions until you have tried the other. That's what I like about uniflow tanks. Just pull the plug and you can try the other.

>Currell, here is an area where
>I don't agree. I
>see no reason why a
>Fox 35 needs high nitro
>just to keep running.
>We have run them on
>5% nitro for years.

That's certainly true. While 15% really wakes them up, a Fox in good condition should be able to at least run on 5.

Unless it's contaminated. I've had several instances of inexplicable, "like you flipped a switch" flameouts and it was always fixed by either using a different plug, or by using fresh fuel. Rossi plugs are what got Ted at the 91(?) NATS and Fox occasionally do this, too. The Rossi is very repeatable.

> The uniflow tank will
>only affect the fact that
>it won't speed up nearly
>so much towards the end
>of the run. It
>certainly won't cause flame out
>(unless there is "junk" in
>the tank or a cracked
>tube or some such.)
>If you have a uniflow
>tank, simply do not block
>off the overflow tube for
>one flight and you will
>have a standard tank.
>Then you can very easily
>compare the difference in the
>run to see which you
>prefer. You don't have
>to put on a new
>tank just to check.

I agree, there seems to be as much bizzare mythology about uniflow tanks, and tanks in general, as there is to 4-2 breaks. Tanks have about 4 possible failure modes: they leak fuel, they leak air, they clog up, or they fall off. That's about it, uniflow or no. As long as those failure modes are not present, the tank is almost certainly not the source of the problem!

The sole difference with uniflow is that the pressure stays constant, and it's always at the same pressure that a suction tank has near the end of the run. That's it! If it can run out a tank on suction, it should run out a tank on uniflow.

Most, if not all, problems that are *apparently* fixed by switching tanks are actually fixing vibration issues, that are altered by by changing the mass and spring rate of whatever's vibrating with a different weight tank.

You're probably right about the tank not being the problem. But, in your own words, the Fox could stand some "waking up". Hence the suggestion to go with 15%.

It just sounds to me like a combination of the fuel and the plug not able to support continued running. His bench run was OK since it was running hotter there in a stationary mode.

When I came back into the hobby last spring I was having all kinds of problems with engine runs on profiles. Seemed as though it didn't matter what engine, tank or glow plug I used it would either quit before it was out of fuel or not run consistent. Most of the plugs I tried, including Fox, showed a significant reduction in RPM when I removed the battery. I felt like I was cursed and fought this crap all summer.. Finally, after much experimenting and following suggestions I found the combination that works for me.

5% Fox Fuel, Thunderbolt RC Long plugs and GRW Chicken Hopper tanks..

I am using this combination on everything from an old OS 30S to the FP-40 on My TwistMaster and all of my engine run problems have went away.

Now everyone has told me not to run 100% castor in the FP's because it will gum up the works so I went by Fox in Fort Smith and bought 4 gallons of Super Fuel and one gallon of 5% nitro 18% all synthetic. Going to mix it all together and if my math is close I should end up with 5 gallons of 26% oil of which about 4.5% will be synthetic. I am hoping this will be a good combination for all my engines so I only have to deal with one fuel mixture...

Your mileage may vary....
Bob

Bob, all castor will gum up the works only if you let it dry out over a period of time. If you run them every day, or every week, it should be no problem. If you are going to store them over a period of time (like over winter) add a bit of after run oil to make sure they don't get gummy.

Castor can leave a varnish, however, if you run the engine hot. thus some engines should not be run on all castor for this very reason. 4.5% synthetic oil is not enough synthetic to prevent this condition, however. Generally a 50-50 mix is recommended in these situations. Do not go less than 50% castor, however, unless your engine manufacturer specifically tells you to (not says it is OK.) Even in those situations I seriously question the sanity of doing so.

The Fox 35 and OS 30S and 35S engines need a lot of oil and a lot of castor. The FP will run fine on all castor or on 50-50, and 20% to 25% oil content should be sufficient. We have run them at both extremes and have settled on about 22% as a good compromise. Just remember to use more oil with the Fox 35 or OS 30S and 35S.

Leonard,

Thanks for the info.. If I am understanding you, then my goal to come up with one fuel mixture that will work for both old and new engines is unreachable. Some how I was under the impression that it didn't take that much synthetic to keep the innards of the engine clean.. Live and learn..

PS, did you find my FP-35?

Thanks,
Bob

I haven't seen the possibility of engine wear presented as a possible basis for the flameouts. Is there any chance that this engine might be showing enough wear to reduce compression and combustion temperature, leading to intermittent flameouts? I thought that another symptom of a worn engine is the inability to perform at a set needlevalve setting, and more-than-usual time is spent finding a decent running mixture. Maybe this helps make the flameouts understandable? I've seen engines in other applications behave this way, and a subsequent comparison with a good example of that engine suggests that the worn engine exhibited symptoms like these. Thanks.

"help Fox 35 flame out"

I'm sorry, I must have missed something. Isn't that what they all do?

See Ya

He says it has good compression, but what about the crank bushing?

OK time for my two cents worth. This is an old engine and like things that get old sometimes they have a sore joint. I'll bet if you take this engine appart you'll find that the wrist pin is stuck and the engine is overheating. Other then that you are doing everything right. SCOTT

Dave;

Had the exact same problem last fall with my older .
35S.(Fox and McCoy), But mostly great dificulty getting them to start
and stay running. (never had thes problems 30 years
ago with the same engines)!!

I was running 5% nitro with synthetic lube! switched to
15% nitro with castor(Heli fule), Engines started on first flip
and ran like a bat!!!!!!!

Try a fuel switch with castor and higher nitro%! just might
work, Good luck!

G. Man

Second that. Currell

Hi Dave. I hope I'm not too late with my 2 cents worth, but my Fox .35 stunt of 1967 vintage did the exact same thing, shut off like a switch was thrown. I tried everything EXCEPT remove and clean out the needle valve barrel. It seems a small amount of castor varnish would move with the fuel flow and block the needle valve fuel port after 4 to 6 laps. Of course it was so pale yellow, that unless I squinted down the tube and ran a wire down it, it just didn't show. After I cleaned it out, it ran on 5% nitro and 4 oz to a gallon added castor and ran just fine. And I swore I really cleaned it out after sitting for 25 years in a box. Clean out that needle valve assembly or replace it with one from a ST .35.

Thanks for all the ideas.

But for wahterver reason, going to a fuel wth more nitro fixed the problem. I could mix 10% 50-50 with the stuff I had and it would work just fine. I don't claim to undrstand why, as My Fox 19 runs just fine othte other mix. But bringing the nitro above 5%, and doing nothing else corrected the problem.