You have a small cased series V Compact 40 with small bodied Mk X carb (0.290 choke area).
The Fox carb is rather small for a 6.5 cc engine (good fuel draw).
The tank centerline should be with in 5 mm (1/4") of the carb's spray bar in the vertical plane. Fuel load should be no more that 230cc (8 oz).
Prop should be a 9x7 to 10x6 glow plug Fox long reach (non idle bar)
Seal the carb to crank case with silicon RTV (as was done at the factory). The back cover plate should have a paper gasket and no sealant applied. The head should have on head gasket to seal against the cylinder flange. There should be no need for fuel tubing on the carb needle threads if using fuel with the proper viscosity ( 20 to 25% castor oil) fuel, nitro should be 5 to 10%.

As you have disassembled the engine take a look to make sure that the cylinder sleeve port that is pointed towards the exhaust stack opens first as the piston comes down. With the carb fully open and the high speed needle open 2 to 3 turns apply glow to the glow plug and spin in a clockwise direction as viewed from the cockpit (behind the engine). She should come to life. Run the engine very rich for the first 5 minutes (even with this well worn engine, as you have disturbed the bedded surfaces of the piston and liner) now set the high speed mixture to be on the rich side of peak RPM by 500 RPM. Close the carb so that there is only a 0.5mm (0,020) gap (opening) use the idle adjustment screw on top of the carb for this. Bottom the idle mixture needle gently then open up about 2 turns. Start the engine and try to idle the engine with the glow plug lit from the driver. Set the mixture so that when you pinch the fuel line to the carb the engine stops in 2 to 3 seconds. Now tune the engine with 1/6 to 1/8 turns of the needles to get the best transition from idle to full power. Once you have this back off the idle speed screw (the thing on top of the carb, so that the carb just closes).

FYI. Some folks think there is a need to seal the needles to stop air from being drawn into the carb. Please note this is not to stop a fuel leak but rather an air leak. Air is around 6 times easier to draw (suck) than fuel. This is why it is so important to make sure you have no air leaks in the fuel system. The need to use muffler pressure has more to do with fine tuning the midrange that to help with fuel draw on this carb with its small choke area. If you need a leaner mid range use muffler pressure (this forces one to use a leaner high speed mixture hence leaning the mid range).

And last think about posting in the Fox club thread as there are a lot of folks that know how to set up these fine non asian engines! This is not an OS engine and does not tune like an OS! This is a good thing!!!



Konrad.

P.S.
Not that there is anything wrong with an OS, yeah right! They flooded the US market and now all we have left is Fox in the sport arena. I'm happy to see you try to bring back this old Fox, even if it was only Duke's answer to the OS FP. At least it has an auto mixture carb! You won't find an air bleed type carb on a Fox. All Fox carbs since the mid 60s are auto mixture carbs of some type!

First off, thanks for the reply. Second, I have the tank set properly (on my test stand, its a LST1 Monster Truck fuel tank - 170CC) and am using a balanced 10x6 prop. Carb and setscrew is already sealed, new paper gasket on backplate with a thin film of silicone - reason is one of the screw bosses was busted off so I made a new one with JB Weld. I put fuel tubing on the High speed needle because I had fuel leaking out - many others I found have had to do this for the same reason - I cannot source any all-castor fuel locally and its too expensive to have shipped - what should I do?
This is good to know. The guy at Fox gave me a more basic run-down. I set the carb at about 1/2 turn low-end and 3 turns high-end. I could not get the engine to run to full RPM with the pressure tap connected. Transition was rough and smokey with it connected or not. Had a few stalls throttling up - touch to restart because it was so loaded up. I was going to try a idlebar plug (this is an R/C engine) per Fox's recommendation and check to ensure the carb needles and/or orifices aren't fouled up.
I will check out the Fox forum.
I will also say that A) I have never ran an OS and B) I dont plan to ever run an OS, either. If it says "Made in USA" then I'm happy. I survived a K&B Sportster break-in, so I figured I could handle a used Fox. It was free, and from what many say will run 13000-14000rpms. I (some may frown upon this) have the K&B on a flat-bottomed airboat and I was going to build another airboat to put the Fox on.. I am afraid to fly - I'm somewhat crash prone with my R/C Cars, don't want to change destroying a plane. The boat is more appealing to me.

If I can get out of work early enough to have some daylight left, I will make a few modifications to the setup and verify the sleeve is in the right way. (I checked when I assembled it and the exhaust port lined up.)

The closest I can get to full castor fuel is SIG Champion 20% oil (10% castor 10% synthetic) I would rather run full castor in all of my airplane engines but nobody will get it for me and I cant afford a whole case. I'm not sure what I should do.. I think I can get FAI fuel - all castor but I kinda like the idea of having some nitro in it..

Good position about the "Made in America".

I'm concerned that if this engine has a broken lug, and that there may be some distortion in the crankcase. This can case extra drag and often results in sealing problems with the crankshaft and crankcase. Now I fear that the most likely damage is that if this engine was crashed ( broken lug) that the low speed needle might has been jammed into the low speed metering orifice (found in the spray bar) If this has happened the damaged low speed fuel orifice will not allow for proper mid range operation.

I'm concerned about the 1/2 turn low speed needle setting. I suspect that this is still too tight and might be your fuel restriction at full throttle. Is this 1/2 turn set with the carb just barely closed?

Can you define "full RPM"? I don't think a 10 x 6 will spin 14K on this engine. Do you get a high speed needle response from rich to lean?

The fuel is not an issue for getting the engine to start. It is more an issue with longevity.

All the best,

I dont think the broken lug was from a crash. I think it was from being disassembled for some time sitting in my buddy's garage in a box with a bunch of other r/c stuff... The threads on the needles are in good shape and the needles themselves are straight as an arrow, and the ends look good.

I did a bunch of googling trying to find a base setting on the needles and the 1/2t/3t low/high settings were what I found on a aircraft info site. (cant remember the name off-hand) Chris at Fox Mfg. said to set the low speed to 1-1 1/2 turns and high speed 3 turns. I haven't had time to try this yet, but will in the next day or two.

Well, Full RPM = WOT. When I ran it last time, It seemed to be really loaded up and wouldn't clean out to run full WOT. With the line disconnected, it would open up all the way. I did not tach it as I was just running it for the first time. I would guess I was getting about 8000-9000rpm with the line connected and 10000-12000 with it disconnected. I think something is amiss in the carb, and I want to try an idlebar plug. Chris at Fox said they have a new carburetor out for the fox .40's that (in his words) will outperform all previous carburetors. Downside to this is the cost of the carb ($49.40) is about the same as a new engine using their trade-in program.

The bit about 14k on a 10x6 was read on another engine forum - I think it might have been the Mecoa Forum even.. I am gonna grab a 9x7 prop too and try both of them out when the engine is running better. I have a feeling the carb is junk though; I just havent ran it enough to determine if this is the problem or not.

The small case 40 turns a 10x6 at around 12,500 rpms. So your actual RPMs are in line if you around 12,000 rpms. A lot of factors can affect the RPM readings.
If the engine is loading up, then it is probably too rich on the low end causing it to load up.

One thing is to check the muffler pressure fitting. One can use a small 4/40 thread fitting with a small pressure vent hole and it won't provide as much pressure as the larger 8/32 threaded size pressure fittings that have a large vent hole in them. the 6/32 threaded fittings are sort of in between in size.

Now what muffler are you using? I ran across a muffler on a Eagle I 60 engine where someone used a lathe and made a new rear cap with a smaller outlet on it. So I thought I would check to see if yours had been modified any or is it stock. My best friend's son had put a snuffler unit on one of my small case 40 engine mufflers too, as well as a lawn mower muffler onto the end of a large case 40 muffler.

What fuel are you using? What is the nitro-methane percentage? Many Fox engines do not like high nitro fuel and do well with little to no nitro in the fuel. Fox wasn't a big fan of running the engines with higher nitro percentages in the fuel so his engines tend to have higher compression ratios and run good with little to no nitro in the fuel. Granted some of his higher performance engines would love higher nitro in the fuel.

I have had glow plugs flake out real fast, especially on new engines and not just with Fox engines either. Over the years i accumulate old used plugs. That is where you suspect a plug might be bad, but it still seems to work and glow nicely, so you save the old plug and put in a new one. Later I use those old plugs on other engines when bench testing to run the engines. I had a non-Fox engine once eat several plugs for a few tanks of fuel. After that it was good and stopped destroying plugs (weird).

I have some small case 40's, I plan on using a couple of them in a twin later, and when I tested them a couple of weeks ago, the carbs were acting all screwy. So I need to clean the carbs out good as they probably got all clogged up with congealed castor oil. I haven't had a chance to less with them since then though.

The muffler is unmodified stock that came with the small .40's. The pressure tap was missing when I got it - I put a 6/32 one into it. I have a feeling the needles were reversed. The idle needle is the short one, right?

I was running 5% Sig fuel. I bought a gallon of Byrons Aero Gen2 fuel in 15% nitro and 18% oil (80/20 synth/castor) but I think I am going to return it and get something else. I cant find an all castor fuel with nitro in it. (i have 2 K&B .65's that need castor fuel too but I think that they should have some nitro in it.) IF I can get all three engines to run decent on FAI fuel, I'd do that because I think the FAI my LHS carries is all castor. I just dont know if no nitro will work worth a darn in these K&B's...

Yes that is correct the low speed needle is the short one and the high speed needle is the long one.

I have taken two different fuel jugs and mixed them together before. You could take the Byrons fuel and mix it with the FAI fuel to get something like 7.5% nitro fuel with a mixed higher percentage of castor oil in it.
But yeah the K&B engines may like more nitro in them, I forget at the moment. I have a old K&B 65 I got years ago. I bench ran it for a while using 5% nitro fuel and it was running OK. But I never used the engine after that though.

Anyway, it doesn't really hurt to run the K&B engines with FAI type fuel and see what happens.

I would put it on a stand and see how it runs. Sometimes, these engines are pretty tough.

I have it on the stand right now, I just couldn't get it to run for crap last time. I think 1/2 the problem lies in the needles - The carb was assembled when I got it, the rest of the engine was apart. The needles were in the wrong spots to begin with, so when I took it apart and put it back together, I put it together the same way it came apart. I switched the needles and I want to check the sleeve to make sure its in the right direction. (there is no locating pin on this sleeve) and try to run it again.. maybe tomorrow.

I am gonna stop and see if the LHS has any FAI fuel left and see what it costs for a gallon.

I broke the K&B in on 5% nitro fuel - about 1qt and then tried 10% nitro. It seemed peppier with the 10% in it, I grabbed the 15% because they say they will handle up to 15% and I figured the fox would run okay on it too. I have castor oil I can add to it. If I add 3oz of oil to a quart of the byrons fuel, I figure I would end up with a final mix of 13.7% nitro (all numbers by volume) 25% oil (47% of that is castor and 53% being synthetic) I know thats pretty oily, but it would get me by with what I have. Fuel is pretty expensive and I think my wife is about to pull the plug for awhile.. (lol)

I'll test run it and if I get it to run decent, I'll make a short vid and post it in the Fox forum and see if I got it right or not.. I'm still pretty new to airplane engines.. Some of these old ones run forever but can get finicky at times I've noticed.

Thanks for all of your help.. if anyone else has anything they'd like to add, feel free..

Just caught your post about your FOX engines. I have several, you may check the liner to see if it is installed the proper direction, some did not have a pin to make you install only one way! If it is installed 180 out it will run but of course the port timing is all wrong.

The cylinder liner was in the right way. The exhaust port lines up perfect. The carb needles were backwards, so I switched those back. I had it running a little bit yesterday, but not very much - It started and ran for a little bit, but I couldn't get it to run long enough to tune the high speed needle. Seemed to run out of fuel and stall out. The fuel tank was full, and at the proper height in comparison to the spraybar. After it ran for about 5 minutes it would just stall out. Each restart attempt would get it to fire only a few revs before it would quit. The pressure line is connected and It ran best on a regular hot plug - didnt run for crap on the idlebar plug. The head screws came loose, so I think that was the problem towards the end. I'll have to clean the screw holes out so I can loctite the head screws so they dont come loose again. (The head screws are phillips head, not hex screws)

It seemed like it wasn't priming well, either.. I blew into the pressure line to the tank to prime the engine and it wouldn't even pop. I put a few drops of fuel down the plug hole and it fired right off and quit but eventually fired on its own.

From what I've been able to gather, some of these engines had a head shim in them and some didn't. Mine doesnt have one and I don't know for sure if it is supposed to have one.. Can someone tell me if its supposed to have one?

The guy at Fox said they have a new carb for the .40 that outperforms the old ones any day (his words) but its $50+shipping and I almost wonder if it would be worth it to buy the carb or if I should just send this engine to them and get a new one.. The cost would be about the same using their trade-in program. (not to mention If it is supposed to have a head shim, I don't know if it would have enough compression to run right - then I would need a new piston and sleeve set and at this rate, a new engine would be the better choice based on cost.) I got this engine for free and really planned to run it as is if it will run right - I don't want to sink a ton of money into it. New Fox .40BB Deluxe R/C is $120 factory price. They sent me two price lists - one for current engines and one for the engine I have. The cyliner/piston kit is $18 on the old list and looks like $50 on the new ones, the Carburetor is $50, and I dont see a head gasket/shim on any of the lists.. So even to put just the carb on it is half the cost of a new engine.

Maybe it just gets cleaned up and put on the shelf? (I dont like shelving my engines - I like to run them as intended)

I don't think the new carb will help. Please remember that the engine ran great with Mx X carb and doesn't need the improved carb to run. Remember that these toy engines run great with fixed venturis. It sounds like you are having a fuel draw problem. This is he opposite of the way you discribed the problem earlier. The glow plug shouldn't be an issue if you are leaving the glow plug drive on. I have to ask since the needle may have been reversed could the carb's spray bar be installed upside down? The cutout (fuel discharge port) should be pointed towards the engine.

The new 2600 carb is an improvement with regards to midrange and transition. There is no improvement as far as how strong the engine runs or for ease of starting.

I'm still pretty new to the aircraft thing. The problems I was having before were the opposite of what I have now because the needles were reversed before. The spray bar is in the right way too. That was never taken out of the barrel. After the engine ran its first 5 minutes and started acting up, the crankcase was flooding out pretty bad. I had to dump fuel out of it twice. It is also not running "forward" on startup, either. It almost always starts in reverse (clockwise) even though I am flipping it counterclockwise. In that first 5 minutes of good running, it would correct itself and switch directions. After that, it would start in reverse and run for a few seconds and quit. Sometimes (just a few) it would start in forward, then switch to reverse and once or twice back to forward and/or stall. (forward being CCW and vice versa.) My K&B .65 would start backwards during break-in occasionally, but flipping the throttle a few times righted it and kept it going. <shrug> I don't know.. I will have to keep putzing with it. Any thought on the head shim deal? should there be one or is it supposed to be metal-to-metal fit?

The K&B engine is of little relevance to the Fox. Most of my Foxes use a head shim. They are often an aluminum, so there is always a metal to metal contact in the head sealing area. If you don't have piston the head contact the head gasket or lack there of shouldn't be an issue. It isn't an issue as far as fuel draw.
You should be able to get the engine to "run" if the crankcase is properly sealed. Power, over heating and throttle response are all issue we can work on after you get the engine to run.

The only reason I mentioned the K&B is in reference to starting in reverse. I know its common, but the fox doesnt want to "right" itself as well as my K&B did. That aside, I have a hard time getting it to "run". The Backplate is sealed, and the carb is sealed. The only thing that isn't totally sealed is the head. The piston does not hit the head, obviously. Since the weather is shaping up, I'll try running it again this afternoon. With a little luck, it will start and run in the proper direction long enough to get it running good and proper. I'll post back with my findings.

Well, all of this doesnt matter anymore. The engine is junk. After the last run yesterday, the glow plug melted down and a chunk got between the piston and liner scoring the liner enough to lose over half of the compression it had. The engine will pop a few times but no more. Evidently it was a lost cause from the beginning. Its not worth putting a $50 piston and sleeve kit in it, IMO.

Thanks for your input, advice, and help. I was hopeful I could get it to run. As the Mythbusters say "Failure is ALWAYS an option"....

Thanks again.

Sorry to hear this.
Fox does have a buy back program. Order a new engine with the 2600 carb. And proudly show the imports up!

All the best,

Konrad

I had a Fox about like the one in the photos. A guy gave it too me, he could not get good steady runs on it. I replaced the goofy carb with a Perry Carb....boy did that engine run good then. I think some of the fox carbs was the worst I had seen. The engine was built good and had very good compression...just a bad carb! capt,n

The carbs have been great. The issue is that you had to read the manual that came with the engine. They do NOT tune like an OS and this is a good thing (they often need less expensive low or no nitro fuel than the OS to run well) now the "odd" carb may have had the butterfly barrel if so the low speed mixture control also held the high speed fuel jet. this meant that the low speed mixture needed to be set first. as any change to it would result in a lot of cross talk with the high speed needle. Now the later Mk X and Easy Adjust carbs had you set the high speed mixture first which is the convention today on most carbs.

All the best,

Konrad