I flew my new Fox 60 Eagle III on a Hangar 9 Cap using 10% nitro fuel after breaking it in on a test stand for 15 minutes. It has backfired on the ground a few times and threw the prop/spinner in the air twice. This last time it threw the prop, it sped up in a high pitch scream and I shut it down within 1-2 seconds.

After landing the plane, I was extremely lucky to find the spinner (Dubro) still held on to the prop with the washer and nut inside it. I reinstalled them on the Fox and tried to start it. It spun easily like there was no compression. It ran but only on high and died as soon as I retarted throttle. It sounded like a loud sewing machine, i.e., no compression. When I rotated it by hand backwards, at TDC it felt like there was a rough bump. I have not disassembled it to take a look. Any suggestion, help, or solution will be appreciated.

Likely you cracked the ring. It was throwing props because Fox Eagle's are not supposed to run on 10% nitro, these are basically FAI fuel only. BTW how is this a new engine? Fox is up to Roman numerial IV now.

Ah another impetious motor user. You can hardly consider a 15 minute session with a Fox motor the sum of your break in session can you? All Fox motors require considerably more then that to bed the ring and to set the running clearances. Improperly done and you'll be one of those unhappy Fox bashers that post on the boards.
The Fox 3 was an ugly looking critter that Duke Fox wanted to run on no nitro fuel as such it had a slightly higher compression ratio and a hard ring. There have been some head improvements in the interviening years since it was introduced.
You might want to send it back to Fox and have it gone over and when it is returned take the time to really give it a chance to break in and settle down before serious flying commences.
Use a fuel that is at least 1/2 castor and use short rich runs [3 or 4 minutes]followed by complete cool downs. After a few of these you can pinch the fuel line while its running to spike it into a hard 2 and then let it slip back into a ruch run followed by a complete cool down. An hour of this and then fly it on the rich side for a few times and gradually lean it out a click or so every few runs till you get to where it is holding a nice slightly rich 2 cycle. It will fly that way almost forever.
Carb set up is useless till you get some time on the motor and your engine speed for the initial runs should be controlled by the Needle valve.
Take your time with it and you should get years out of yours. Mine is 15 years old and still runs well.
Dennis

BTW, I have had a broken in engine with low compression with a lean run. It may be that it just needs to run rich some more to set the ring. Even ringed OS engines doesn't break in after 15 minutes. All ringed engines should be broken in at a rich four stroke for several tanks. Expect to run at least a half gallon of fuel before flying maybe more. Over a gallon, maybe two before it developes full power.

IMO,,,,, send it to Fox. They have the best customer service attitude I have encountered in RC! (28 years) They are very good to get it back to you quickly. I'll bet if you send it to them with a detailed problem description, you will be assured an accurate assessment and resolution. I will also expect you will receive a very fair repair cost if it is not in warranty..... I can say with full confidence, they really want your Fox motor to run well and will do all they can to help you make it happen.

Great folks to deal with.

(I was not paid to express this opinion!)


ZB

(Jimmy Woodard)

Montgomery, Alabama USA

ZB50 is correct, Fox gives great service and sometimes there is no charge for the upgrades. Send it to them and let them look at it.
Dennis

Thank you all very much for your inputs. I am guilty as charged: knowing fully well that Duke Fox designed the Eagle III to run on FAI fuel only and that Fox motors (with ring) MUST go through lengthy and controlled break-in periods. It is my understanding that Mr. Fox made a conscious business decision/prediction that nitro would be hard to come by. I bought the engine some years ago but have not run it until recently.

To opine one of own, I have owned K&B, Fuji, HB, Fox, Royal, Super Tigre, OS, Rossi, Thunder Tiger, and MVVS engines, and by far Super Tigre has the metallurgy down to an art. I have replaced rings, bearings, rebuilt carburetors, and repaired busted mufflers, etc. I have done nothing to the Super Tigre engines (3). My HB, OS and Super Tigre engines suffer none to little damage due to deadstick, or crashes. I can not say the same for other brands. I deduce that those engines that don't quit in the air and don't idle well (due to insufficient break-in) will lead to mishaps. I have replaced rings and bearings in my HB and OS engines. What impresses me the most is my oldest engine, a .60 (not .61) Super Tigre, has not lost its compression thru countless flights and years in service. I can also honestly say that every single one of the engines, regardless of brand, that I own has started and run right out of the box with some needle tweaking.

Heeding your advice, I will send the Eagle III back to Fox for repair. To answer Sport_Pilot's question, as a new engine the Eagle III did not idle well due to my neglect to break it in properly. Every one of the 8 to 10 flights I had with it resulted in a deadstick. My Fox 40 ABC did very well as a new engine. Since I treat all engine brands equally, i.e. not giving the special care and attention to the ringed Fox, my next engine purchase will be a Super Tigre.

I have observed that the newer Fox's break in considerably faster than the older ones, even the iron piston and sleeve models. There are also a few ABC models that break in rather quickly.

I am very happy to report that I my Eagle has been factory repaired. You all were right on about Fox' service. They replaced the piston, ring, mixture needle valve and apparently a brand new sleeve. They installed a new plug, thrust washer and nut and replaced all head screws with brand new one. The charge? $20 for the piston and $5.50 for the mixture needle. That's it!

The repair technician's hand written notes are as follows. Recommendations: 5% to 10% nitro Fox fuel, Fox long plug, 11X6 prop. He reported 13,800 rpm with good transition and idle. He ended with a 'thank you' and signed 'Cuong Nguyen'.

Having just got back from the field today, I will fly the engine next Friday and report back. Thank you all very much.

Be sure to break that engine in! Just me, but I like to break them in on the bench, then finish it off in the air.

I have flown the repaired engine twice: no more misbehaving. It turned 10400 with a MA 12X6 on K&B 5%, and 11500 with a MA 11X7 on Ritch's Brew 10%. In other words, nowhere near the 13800 that Fox reported in their test run. Admitedly, I have not leaned it out and could clearly hear it 4-cycle and spitting out some raw fuel. It quit when I leaned it enough to break into 2 cycle and raising the nose.

I did notice that Fox installed the sleeve 1/8" off center. Since they reported such a high rpm, I figured maybe the sleeve was intentionally installed off center. The engine turns over very smoothly, not a great deal of compression, starts easily with a quick blip of the starter, idles around 3000. I am disappointed in the power but the engine does run well.

I would center the sleeve.

What is your altitude in TX? The altitude and rich mixture would account for the differance in power. In fact the rich mixture alone would. Any engine turning a prop over 10,000 at a full four cycle is doing very good indeed. Most C/L engines turn 8 to 9,000 at a full four cycle and burst to 1,000 to 2,000 RPM higher when breaking into a two cycle. Though they are set up to turn larger props at slower speeds than most R/C engines.