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"Cox Tee Dee .010"



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Finally figured out why my Medallion is a dog.

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Finally figured out why my Medallion is a dog.

Post  batjac on Wed Jan 07, 2015 1:03 am

Well, I think I finally figured out why one of my Medallion .049’s is such a gas guzzlin’ dog.  I had a seized TD .049 and a gummed up Medallion .049 that I needed to set right, so I disassembled and cleaned them well.  Both are now clean, free, oiled up, and good as new.  So, since I was doing rotary valves this evening I figured I’d tackle the disappointing Medallion I got on eBay a few months ago.  Since it seemed to suck down a lot of gas and give poor performance, I thought maybe some dirt or gunk had gotten in the needle body and was necessitating opening the needle more than it should need to be open.  Pulling the valve and looking at it, it was clean.  So I went ahead and completely disassembled the engine.

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.  There was no indication of the tap from the outside, so I never even thought of checking.  Lesson to myself on all further purchases.  In fact, I’d better check all my TD’s and Medallions now to make sure none are tapped without my knowing it.

So, even though the PO had done such a clean job, I decided to go ahead and block the port to get the original performance back.  As I was about to mix up some JB Weld to fill in the hole, I remembered a post by Larry Renger over on the 1/2A section of Stunt Hanger from a couple of years ago.  They were talking about the merits of pressure tapping the crankcase on TD .049’s, and Larry commented on his experience.  Larry said that it could be marginally useful if done right.  He described how he did his, and I figured I’d give it a try.  I could always go back later and seal the engine back up if I didn’t like the results.

Larry’s method of pressure tapping a case is to drill out the tap per Cox’s recommendation.  Then, for a TD to use a 2-56 Tee nut, or for a Medallion to use a 4-40 Tee nut as the hose barb to keep the hoses in place.  I reassembled the engine and made sure it flipped over smartly and had a new glow head on it.   Then 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.





A little hard to see, but the Tee nut cut decent threads in the venturi body.




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 of, as is usually the case with trying to keep a pressure tap hose on the smooth nipple of the engine.  Then I threaded it back on and made sure that it was nice and secure.





Now to just find a plane to put it on that could use a pressure tap for the tank.

The “Feelin’ the Pressure” Mark
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Re: Finally figured out why my Medallion is a dog.

Post  Kim on Wed Jan 07, 2015 3:36 am

Wow ! THAT is some cool work! It'll be interesting to see the difference it makes. Have you run it on a stand?
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Re: Finally figured out why my Medallion is a dog.

Post  Ken Cook on Wed Jan 07, 2015 4:32 am

The reasoning for case pressure is typically due to the engine's inability to draw fuel due to a very large venturi. Seeing that the Medallion doesn't suffer from this like a TD does, I question the need for it. Boring out the venturi on a Medallion would gain little as the crankshaft and the single ported liner is the limiting factor to more power on this engine.  Timed crank pressure on a TD and I can see the Medallion being no different, usually ends up with the engine surging back and forth. You might be happier revisiting your original idea and filling that hole. If running pressure is something that you would still require, doing so via the backplate is a far superior method. Ken
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Re: Finally figured out why my Medallion is a dog.

Post  batjac on Wed Jan 07, 2015 4:41 am

Kim wrote:Wow !  THAT is some cool work!  It'll be interesting to see the difference it makes.  Have you run it on a stand?

I just finished it a little while ago. I doubt the neighbors would appreciate me test running it at 11 o'clock at night...

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Re: Finally figured out why my Medallion is a dog.

Post  batjac on Wed Jan 07, 2015 4:43 am

Ken Cook wrote:The reasoning for case pressure is typically due to the engine's inability to draw fuel due to a very large venturi. Seeing that the Medallion doesn't suffer from this like a TD does, I question the need for it. Boring out the venturi on a Medallion would gain little as the crankshaft and the single ported liner is the limiting factor to more power on this engine.  Timed crank pressure on a TD and I can see the Medallion being no different, usually ends up with the engine surging back and forth. You might be happier revisiting your original idea and filling that hole. If running pressure is something that you would still require, doing so via the backplate is a far superior method. Ken

I wouldn't have done this myself in the first place. But, since it was already done by the PO, I'll still give it a try. I realize that this is more of a TD thing, but what the hell...

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Re: Finally figured out why my Medallion is a dog.

Post  ian1954 on Wed Jan 07, 2015 6:28 am

Whether it is necessary or not, I like the way you have effectively made a nipple for the fuel tube to be bound to.

The venturi pipe for pressure "tapping" is delicate at best and easily snapped off. It is even more delicate with a hole drilled through it. I have broken a few while cleaning.

I have no experience on running TDs or Medallions with pressure tapping but it does seem an odd place to apply a tap but Medallion venturis aren't easy to come by as replacements.

A pal of mine experimented with pressure tapping (I supplied him with a tapped backplate). He fitted this to Medallions, TD 049s and 051s and he said that the rear tap worked well in all three but I never saw the set up, tank or even the plane he was using.



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Re: Finally figured out why my Medallion is a dog.

Post  RknRusty on Wed Jan 07, 2015 10:40 am

Most people seem to agree that backplate pressure tapping is far superior to tapping the front end. I have never tried either.
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