widow giving me fits

Ron, the engine is mounted to a wood stand that I can clamp down or hold on to. The last time I ran it was about two years ago and I cant remember if I was having trouble with it then or not. I pulled it down for cleaning, gaskets, fuel hose and reed. I started the engine before I tore it down and it acted the same way. I just figured the carb was gummed up. I have had it apart now about 10 times and cant find the trouble.

Close your needle down all the way. Pressurize either the top or bottom filler pipe with a syringe while holding your thumb over the opposing one. First, listen for air leaks. If the venturi o-ring is leaking you will quickly and audibly hear it at the rear screen. You will probably find the screws leaking as well. I place a drop of oil on them and check each one individually. The tank to backplate can leak as well. Here is where a piece of dental floss carefully placed in the v-groove overlapped slightly about a 1/4"  solves that leak. You will also find a leak at tank to crankcase so putting some light machine oil on that joint will also reveal bubbles if it is leaking. In addition, the needle valve will show leaks during this test where the entire assembly is pressed into the backplate. This test is entirely irrelevant to the reed valve, just the above. If the screws are leaking, strip off a piece of Q-tip swab and roll it between your fingers into a string. Wrap this around the upper portion of the screw in the direction that will tighten the cotton onto the screw opposed to making it want to come off. This almost always fixes a leaky screw and makes it service removable and reusable. Ken

Thanks Ken! Would a leaking back plate screw cause my problem? I figured it would only cause a fuel leak. I have one screw leaking some and I sorta expected it. It bubbles a little with oil on it.
I fully expected to find the o ring leaking, but nothing. I put some oil on the screen and no bubbles, also didnt see any on or around the needle valve.
I did install a second new fuel hose tonight and havent run it yet.

chevyiron420 wrote: Thanks Ken! Would a leaking back plate screw cause my problem? I figured it would only cause a fuel leak. I have one screw leaking some and I sorta expected it. It bubbles a little with oil on it.
I fully expected to find the o ring leaking, but nothing. I put some oil on the screen and no bubbles, also didnt see any on or around the needle valve.
I did install a second new fuel hose tonight and havent run it yet.

A leaky screw sure would cause trouble. Hard as it is to believe, it messes with the pressure head and once the fuel level begins to drop, it continues to get worse.

I suppose a good analogy would be trying to  drink through a straw with a hole in it. In addition Phil, check the pickup where it attaches to the barb. If it's remotely loose this is another problem area. Not so much when the fuel level is high but as soon as the fuel level lowers it begins sucking air. The pickup tube needs to fit tightly and older ones swell.  Ken

You may have a faulty venturi gasket. That is that rubbery washer looking gasket that fits on the venturi opening inside the tank back, that mates to the venturi in the tank back. In a pinch you can slice a thin section of silicon fuel line tubing and use that as a gasket. Also check the tank to crankcase gasket, and verify you have a properly functioning reed valve. Check the holes in the back side of the tank back to see it is free of burrs and that you don't have a crack or fracture there that might leak air.

I'm sure the problem is a very simple one. If you still have problems, try swapping the back with a known good one. If that corrects the problem, then something is wrong with the back.

Get some aluminum safe sealant and apply to the rim of the tank, the venturi gasket, the crank gasket, the underside of the screw heads. No problems.

I use "anaerobic" sealant from the auto parts store, comes in a little tube.

Phil

Well, I had made up a second new fuel pick up hose last night, and I sealed my tank screws with Q tip cotton. It seemed good on the pressure test. I ran it today. The first run was a little rich and ran the tank through. I had a spark of hope. I thought the needle feels odd to me so let me fit it with a piece of fuel line. Maybe its sucking air. I fired it up and after a minute it turned bad. Then it would only run with the tank full. I thought about chunkin it in the dumpster, but calmed down. The new pick up hose I made up was silicon hose and even though its tight, it wont stay on the non barbed nipple. Man I hate that plastic back plate. The hose I normally use on my bee's would not go on the plastic nipple, so I stretched it some tonight and got it on there. I sealed my tank screws again and am ready for a re-test tomorrow.
You know, I am not a cox expert, but I am 58 now and have been fooling with these things since I was probably 12 years old and have never had a problem like this. I am not arguing or anything but I have had them leak at the tank screws and around the back plate and other than running out of fuel fast I have never had a problem before.
Phil

Use copper wire and tie it onto the fuel barb.

Or some of the soft wire inside of paper twist ties.

I use 3/32 medium silicone like on larger engines.

Phil

I actually prefer the plastic backplates. They're lighter, they seal better and they're less prone to cracking at the screw bosses when they're new. I had several metal backplates that were faulty or had casting issues resulting in pinholes not to mention the entire casted area of the screw cracking out. The metal certainly delivers more punishment in regards to ground pounding, so keep it out of the ground. The barb on the plastic backplates is slightly larger than the metal. I've tried to remove the tubing on metal backplates  only to have the entire barb snap off. Each one has their own share of problems. If your screws are leaking, it's an issue. If you have leaks in all the areas described it's an issue. If the engine isn't needling properly I would replace the backplate. The needle seat might be damaged inside or a broken point of a needle is left internally. The best solution is to use a piece of aluminum tubing bent into a horseshoe joined with a small piece of tubing to the fuel barb. The stock tubing is horrible and 9 out of 10 times it doesn't stay put.

Many other factors can determine if your engine is even in good enough condition to run up  properly. Do you know the history of this engine?  If the case itself has been sitting for a long time it could be  varnished  with gooey castor this will also make your engine run go south as it heats up acting  like a brake on the engine when you try to lean it up. Dismantling the engine and cleaning the crank and case thoroughly is in order. This stuff will not break down and remove itself it must be done manually. Is this the correct piston for that cylinder? Swapping piston to cylinders can also tend to go south if they're not fit properly. The engine begins to sag in short order.

Your statement in regards to running these when your 12 is very similar to some of us as well. The difference however is that we also didn't know what we know now later on after gaining 40 years experience.
Some of the symptoms you describe in regards to your engine run follow a pattern which is similar to bad fuel. In order to diagnose I would take all simple factors out of the picture. GLow plugs can do the same thing even if they appear to be glowing properly. Issues like you describe are common to this engine. In order to figure out why, all factors need to be eliminated. When that doesn't work , wrap the shop rag around it and fire it off and set it free. Ken

been reading this , all good stuff , my question would bee are you using a spring inside your fuel pick up hose so it don't pinch and are you cutting the hose pick up end at a slant ? Keep at it its a good learning experience

This indeed has been an interesting discussion, have really enjoyed reading the responses. Concurring with Ken, my gut level feeling is that it may be a defective or bad back. Also, I was having an aversion to using a plastic back, having never had one. I didn't know there were merits to using them. The metal ones are sufficiently strong and have survived crashes without tearing off the lugs. Of course all my flying was done away from paved surfaces. And yes, Ken, I have done exactly as you, ruining a metal back by breaking off the metal barb fuel line connection.

Regarding the spring inside the fuel line, I've got to hand it off to the ingenuity of Leroy Cox. Personally I believe that spring, also used in a few short lived Cox RTF's with the 290 postage stamp engines was to ensure positive fuel flow. The spring acted as a wicking agent to draw fuel in. This is perhaps another factor that helped Cox dominate half-A flight for over 50 years. Even the Pee Wee's have that little spring.

Again, if the test flights prove futile, swapping out the back with a good known one even a metal one will help verify that it is the back assembly that is bad and not another issue.

pkrankow wrote:Get some aluminum safe sealant and apply to the rim of the tank, the venturi gasket, the crank gasket, the underside of the screw heads.  No problems.  

I use "anaerobic" sealant from the auto parts store, comes in a little tube.

Phil

Phil, do you have a manufacturer's brand and trade name? I went to a couple of local auto parts stores and asked for anaerobic sealant, and they looked at me like I was talking Swahili. I tried reading the backs of the tubes they had for sealant, but nothing looked right.

The Questing Mark

Mark, "anaerobic" is all the Loctite stuff. i.e. becomes hard with the absence of air.

I certainly won't contradict Rusty's video in regards to the capillary action of the spring inside of the pickup tube. However, my best runner to date has a piece of K&S aluminum pickup tube with no spring present and never has seen an issue. I'm just stating my experience but in the event I use the plastic or silicone tubing, I first burn the spring over a flame to rid of contaminants and I install the spring into the pickup tube. Ken

Lol, that video was one of my early classics. I still get views on it occasionally.
But, my tach race BW this year had an aluminum tube with no spring. Cranked easy enough and turned over 21k in my shop. I don't recall how fast in Kim's shop, but it wasn't quite as impressive. It was sealed with permatex or something. I've had metal backplates that just won't run right. I quit using tanked Bees for that reason. They work one day for me and not the next.
Rusty

Hey Phil, I hope you've had some luck. Looking through my Babe Bee back-plate stock, I found one with a deformed barb/nipple. This would possibly create a headache.



Here's what it should look like..



While I was in the mood, I made up a fuel pick-up using med. size silicone line with a 7/32" compression sleeve. The pick-up is a 1/8" copper tube with a notch filed across the opening. I'm also using the internal coil-spring.




The sleeve is a tight fit. Coating the line with a film of petroleum jelly where the sleeve slides over the nipple would make it easier to install.

I don't like the plastic replacement tank-back; in that the nipple has no barb. There's gotta' be a way to improve on that design.

Rusty & Ken, I think the difference regarding fuel flow still working in spite of no spring basically has to do with, you know what you are doing. These Cox tank engines were purposed by all types of people, and people need a fuss free engine that will reliably start and run in about all situations they'd put them through.

Leroy Cox made sure of that, plus he made engines with the then modern equipment of the time so that he could get better fit and with finer machined surfaces requiring a relatively quick and easy break in period.

It is no wonder why Cox dominated the small flight arena for many years. Who'd want to go through the arduous labor of short heat treat cycle break in with accumulated times of up to an hour or more, when they could pull a Cox engine out of the package, run a tankful or two through the engine, then fly?
These little Cox engines are jeweled wonders.
Happy Memorial Day, all veterans.

I want to thank everyone for the great advise and info. It has been very helpful! I know there is nothing wrong with my engine itself because I have built two engine assembly's so far for it, and with a plastic carb backplate both run great. I even ran an ounce of fuel through it once and it holds about 15500 rpm until your sick of hearing it, if thats possible. Today's test was another failure. I have a real good fit on the fuel pick up hose with spring and I have no leaks of any kind that I can see anyway. The reed checks out good, no gasket leaks or tank, or o-ring leaks.
There is a possibility that this carb assembly has been this way since new. I had a lot of real bad stuff going on in life back then and I just cant remember. My new thoughts are that there is something wrong inside the backplate. Something isnt lined up right or drilled right.
Can the spraybar be pulled out and re-installed? does anyone have a picture of one? Does anyone have a list of hole sizes for the plate and spraybar? Its ether that of there is some junk in there that wont soak and blow out.
Phil

Well guy's, I finally fixed this thing. I win! I was looking for air leaks and restrictions until I was sick of it and thought maybe it aint a restriction or air leak. Maybe the fuel draw is week, and what would cause that. I figured it had to have something to do with the reed. It tripped me up because the engine ran the same way before I tore it down. I fitted a new plastic reed wile cleaning the engine and it was to big to lay down flat in the retainer. I sanded one edge until it fit and with the reed and retainer on the tank I could spin the reed freely with my dental pick. Also the suck and blow checked good. Anyway, I installed a retainer off a spare carb and put the steel reed back in and it runs great. Man this thing about drove me nuts. Taking it apart so many times, one by one all the backplate screws busted through and one by one I sleeved them all. I aint taking it apart again but I really want to know what is wrong with that reed. It looks good to me.
Phil

Finally! Good for you on sticktoitiveness.
Now, if it will run tomorrow, we can deem it a success, Lol
Rusty