Finally figured out why my Medallion is a dog.

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