stunthanger.com
Engine basics => Engine set up tips => Topic started by: Rod Lamer on June 14, 2009, 08:07:44 PM
-
On the side of the plastic body for a Cox medallion, under the venturi there is a provision for a port. What is this for? I have an extra body for a .09 Med and noticed the port is there but it does not have a hole going all the way through to the crankcase. Just curious. Thanks Rod :)
-
That little nylon "teat" is for timed crankcase pressure. To use it, you have to drill through the venturi housing (with the housing OFF the engine). I think the correct size is around .008" (whatever fits the starter hole; don't make that any larger). It's generally not necessary on the Medallion, as the venturi throat is small enough to generate plenty of suction for fuel.
-
The little fitting do-dad is to be used for "timed crankcase pressure". Some engines have a screw plugging a hole under the crankshaft, opposite the venturi. ST G.20's come to mind, but others probably did also. "Timed" crankcase pressure gives more pressure to the tank than the typical pressure tap in a backplate screw hole or in the center of the backplate. Same idea, but more pressure (I suspect). TeeDees need a little pressure, but not a whole lot, or they get really touchy on the NV. The Medallions don't need pressure for normal use.
I've never used 'timed' pressure, and don't think many did. It would probably be best for a free flight install, where you'd probably use a flood-off engine shutdown. More pressure would deliver a bigger shot of fuel to flood the engine when the time limit is reached. Kinda harsh treatment, but so is shutting off the fuel and going lean for shutdown. 8) Steve
-
I've got an extra body for a .09 and just noticed this, huh. Ok, so if its of no importance I'll just leave it alone. Thanks
-
It was nice that Cox provided this capability but most never, ever use it. It is perfectly fine to just cut the nipple flush if you want but most just leave it be. It is not unusual to have one break off. So long as it was not drilled, there is no loss.
Robert
-
The nipple gets in the way of my engine mounts, so I routinely cut it off. I use backplate pressure. I bought a used TD 09 and was disappointed that it would not run quite right. While cleaning and Risloneing it, I saw a bubble come out where the nipple had been. It had been drilled and I had not noticed. I sealed the hole with a soldering iron and then engine then ran fine. HB~>
-
I used the pressure tap on a TD .049 in one of my RC ship. Worked great. Even run to the end of the tank. Only problem is starting. Needle is way to lean to start untill the prusser comes up. A high speed electric starter worked great. By the way, a prusser tap in the back plate with a check volve works the same. The big proble is keeping the fuel tubing on the nipple. I have seen several messages showing a fix for this problem. I believed thay used a 2-56 blind nut with the flange ground off.
-
Yes, a blind mounting (T) nut with the flange ground down works great. I find the size to fit the nipple (2x56 on the Tee Dee .049, 4x40 on the Medallion .049) and after shaping the flange, screw it on with a bit of epoxy or goop to assure a good seal. You have to drill through the aluminum crankcase in addition to the plastic housing. I prefer to completely disassemble the engine so I can properly clean the bore after drilling, but the Cox instructions say to just turn the crank so the port is in the drill area. I guess they figured there just wasn't that much aluminum crumbs to matter.
Once you use pressure, you can drill out the venturi for more power!
-
Reviving an old thread here.
I have a medallion that I bought on eBay that’s always been a dog. So I decided to disassemble the engine and take a look at it. Ah-Ha! There’s the culprit! It seems the previous owner had tapped the crank case for pressure. The PO had actually done a very good job of it, not a hack-and-slash job like the last one I got. As I was about to mix up some JB Weld to fill in the hole, I remembered this thread and the post by Larry Renger describing how to tap a Cox rotary valve .049 for pressure.
I dug out an old 4-40 Tee nut from the nut drawer in my stash, and carefully threaded it on the pressure nipple on the side of the venturi body.
(http://i32.photobucket.com/albums/d26/batjac/DSCN2952_zps2b10b0d3.jpg) (http://s32.photobucket.com/user/batjac/media/DSCN2952_zps2b10b0d3.jpg.html)
(http://i32.photobucket.com/albums/d26/batjac/DSCN2955_zpsac2afdde.jpg) (http://s32.photobucket.com/user/batjac/media/DSCN2955_zpsac2afdde.jpg.html)
Then, I removed the nut and ground it down until it was just a nub that the pressure hose would fit over, but not pop off.
A little hard to see, but the Tee nut cut decent threads in the venturi body.
(http://i32.photobucket.com/albums/d26/batjac/DSCN2963_zps85a1cf62.jpg) (http://s32.photobucket.com/user/batjac/media/DSCN2963_zps85a1cf62.jpg.html)
(http://i32.photobucket.com/albums/d26/batjac/DSCN2965_zpsab82052d.jpg) (http://s32.photobucket.com/user/batjac/media/DSCN2965_zpsab82052d.jpg.html)
(http://i32.photobucket.com/albums/d26/batjac/DSCN2969_zpse33c7510.jpg) (http://s32.photobucket.com/user/batjac/media/DSCN2969_zpse33c7510.jpg.html)
If you see this post Larry, tell me if I did it right.
Mark