The engine is a cox reedie built from the 'bucket -o- engine parts'.  It has a Golden Bee tank, backplate and case, a teflon reed from Davis Diesel, a two-bypass cylinder, and a bit of clear "small fuel tubing", and I've tried running it on two different glow heads from 'da bucket'.

The engine run sucks.  It'll start fine and run strong with the airplane in hand, but as soon as the plane is in the air the engine starts running irregularly.  Sometimes it'll run strong, sometimes it runs quite rough (and I devoutly hope that it's running rich!).  Maneuvers will change things -- if it's running rough then doing sharp up and down maneuvers will make it better; if it's running well then sharp maneuvers will make it run worse (which tends to make stunts very exciting in a rather unfortunate sort of way).

One of the glow heads is "regular", the other is high compression.  The engine runs a lot better on the high compression head, but it's still too irregular.

Does this sound consistent with any problem that you've seen (in anticipation of the wiseacres, yes, I've already gone over the "you're using a Cox engine" problem)?  I'm thinking that it may be a pinhole leak at the fuel nipple, and will try a different fuel tube soon, but I'm interested in hearing other opinions.

Thanks.

Outside -- shoulda mentioned that.  It had been on the inside lower corner of the tank (please don't ask why -- I don't know); the engine runs with it in that position were also irregular, and much shorter, too.

Check for leaks first thing.  Don't ask how I know.  Then start by varying the head gaskets.  You may be over compressed.  Running the Cox engines in Mouse racing is a challange and hate to think about flying stunt with one.  Also how much nitro and oil content.  Hope you figure it out.

I just finished building a second engine from newly cleaned parts; this should either clarify things in a hurry or confuse them hopelessly.

I don't think I'm over compressed, as both heads gave me roughly the same magnitude of problems, just against a background of significantly better performance with the higher compression head.  So I'd say that -- if anything -- it's under compressed.

I am running fairly low nitro fuel, but with plenty of Castor.  I've heard conflicting reports about what these things want -- some folks insist that they do fine on 10-15% nitro, others insist that they just won't go without 35%, etc.  I don't care about getting the World's Hottest Engine, I just want some practice on something easy to fix before I go fly my shiny new Nobler.

Tim, i think i would stay with the hi compression head with nitro that low. How much oil you got in there? Also i think i would change out the reed for one known to work well.

It's been so long since I've run any of my planes that none of my reeds _are_ known to work well!  But I can change reeds around and see if it makes a difference.

Oil is around 24% -- I win gallons of fuel at my RC club raffle, el-cheapo with 18% oil.  Then I put in 8 ounces of castor from the pharmacy, which (if I do my numbers right) should bring the oil content up to 24-25%, with at least some castor.

I use omega fuel as well. I get 25% nitro and add castor to bring the oil up to 22%. I do this because i run the same fuel in all my 1\2a engines and some of them are old and need the castor.

I had an older Black Widow giving me similar problems. I sealed the needle valve with a piece of fuel tubing witch seemed to solve it's issues.

I'll try that.  Do you just replace the spring with the tubing?

 
Check that little black washer inside.  It might be allowing an air leak.
I put a small bead of Permatex Blue RTV, the semi-clear stuff, in place
of the washer, allow it to "skin" for a while before putting on the back cover.

Also as mentioned above, a bit of tubing around the needle stops air leaks.

Now the good part;  Those old floppy disks I can't use any more make
great material for reeds.  I use the plastic one for the pattern, and
get a years supply out of one small disk.
 
       Dave

     After assembly, I screw the needle valve all the way in till it seats. I then put my syringe on the filler pipe with my finger covering the other and push the plunger down. You will probably be amazed at how many air leaks that you will find. I've had to lap the tank backs to the tank using paste compound to ensure a good fit. In addition this will tell you if that little black o-ring is leaking due to the air just leaking from the intake. This can also tell you if your needle is leaking if you apply a drop of oil around the threads as it will bubble up. I've had the threaded metal insert which is press fit into the backplate leak around its entry point. I was able to remove it and apply a little silicone around it and re-insert it. This is when you find that your backplate is leaking where the screws penetrate through. I've had the screws leak real bad in which they required total replacement. In other instances I had to apply silicone carefully under the heads prior to tightening them. All of these little leaks can surely add up to a big headache. I've always wondered why these engines run so well when new but degrade so much as they age. Well, after discovering a lot of these little problems I've been able to iron a lot of them out just air testing the engine. The black plastic backplates are the hardest to detect leaks as cracks are virtually impossible to notice. Air testing the engine will surely make them show and to date, I've found that almost all of my plastic backplates are cracked around the area the screw protrudes through. This is probably due to the abuse I've put them up through but its surely upsetting to discover they're broke. I've found no glue or sealer of any kind thats up to the task of fixing them so I discarded all of them. I was then in search of the older metal tankbacks which are also prone to cracking. I've had several that were broken in the same manner as the plastic ones. I've also discovered that the screws, regardless of condition will not always seal in the recess they go into. Sometimes new screws will quickly remedy this while other times I have to use sealer. I prefer not to use sealers as they can be problematic during disassembly. Overtightening the screws is a sure way to promote cracks in the casting which may not be present until you tighten the screws. I've had more than one troublesome engine like this in which the fuel would leak out during flight shortening it immensely. In the event the plane is showing signs of wetness around the firewall,  this and the tank seal would be my first area to look at. Ken

   Tim , Cox International sells those little O-rings  3/ $1.95 .

   http://coxengines.ca/home.php?cat=25&sort=orderby&sort_direction=0&page=3

    Hope this helps .
Bill

I tried the fuel tubing over the needle valve trick and the flight would start out good but would go bad as the flight progressed.  Come to find out, the NV was walking.  I put a spring over the NV & tubing and now things work great.  Also, 1/2 dacron line around the tank and back plate groove help w/the seal.  Good luck.

The more I futz with this, the more I'm pondering on an image from Flying Models magazine in the early '80s.  This was when they were publishing flying models, floating models, and model cars, too.  The picture was for a 1/12 scale 1/2-A racer in some class that required reedies, but clearly allowed significant mods.  The car had a Babe-Bee style tank, with enormous (1/4 inch) diameter holes in it, and a completely separate tank that actually held the fuel.

Every leak, sputter, etc...

Drilling a hole in the tank of a Cox reddie is abhorrent, sacrilege, and few more I can't type here, it's about as clever as holding the drive washer in a pair of pliers as you bash on the crank of YS 4 stroke with a hammer,whilst saying "now this is how you do it"

It's OK I’ve calmed down now     those dual vented BW tanks are getting scarce and there are still plenty product and sure start engines getting around, so the better options are still available. The peewee is a little more difficult, I guess if you have the cash then 020, and 049 teedee’s are the alternative.

Cheers.

Drilling a hole in the tank of a Cox reddie is abhorrent, sacrilege, and few more I can't type here, it's about as clever as holding the drive washer in a pair of pliers as you bash on the crank of YS 4 stroke with a hammer,whilst saying "now this is how you do it"

I think the better way is drill a hole in one of those cracked back plates.  Drill it as far from the fuel nipple as possible.  The idea is for the fuel line to make an unkinked u-turn inside the stock tank.  Drill a matching hole in the firewall and thread the fuel line through the firewall to a tank or balloon inside the fuselodge. Whatever size tank you want with no visible modifications to the stock tank when everything is assembled.

I usually use dabs of blue RTV and blue locktight in the right places to prevent leaks.  I let the assembled engine set a few days before adding fuel. I have one engine that was still running well two years after assembly.  I haven't used it in a while, but it may still run fine.