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Post  OhBee on Wed Mar 04, 2020 3:51 pm

So whats the difference that says you can't run a TD off of a regular wedge tank? Have TD'S been designed from the start to only operate on a pressure system! I havn't been in this hobby since the 60's..I didn't even know what a medallion was! They look the same to me! What is that nipple below the venturi on a Medalion for? Never see anything hooked to it in pics. I abandoned electric to get back to simpler times and engines I could keep running without having to be an electronics expert!
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Post  1/2A Nut on Wed Mar 04, 2020 4:09 pm

Originally a post to park the spring starter to. Kept if from spinning around loose.
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Post  Dave P. on Wed Mar 04, 2020 6:12 pm

Sure you can run a Tee Dee on a wedge tank.  In fact, with the stock needle valve assembly running a pressure bladder can be kind of tricky.  The needle thread is a bit too coarse for adjustment under pressure. It works fine with a plain tank.

They look similar, but they're actually quite different. The main differences between the Medallion and Tee Dee are the crankshaft, the carb body/venturi/needle valve assembly and the cylinder.  The Medallion crank has a smaller, round port which makes it a bit more docile.  The TD crank has a rectangular port so it breaths better, but has reduced fuel draw. The web is thinner on the Medallion crankshaft and early TDs than the later model TDs as well, so the crankcases may look the same, but internally, the Medallion and earlier TDs have a slightly longer front end. They aren't interchangeable.

The carb body/venturi on the Medallion are one piece plastic with the needle valve assembly basically a tube.  The TD has a separate needle valve assembly and aluminum venturi.

The TD has a #4 cylinder, the Medallion usually has a #1 or #3 cylinder.  Cox probably made some with a #2, but I've not seen one so I don't know for sure.

The Medallion isn't as powerful as the TD, but it is generally a more user-friendly engine.


Last edited by Dave P. on Wed Mar 04, 2020 6:31 pm; edited 1 time in total
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Post  OhBee on Wed Mar 04, 2020 6:28 pm

Thanks Dave!
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Post  Dave P. on Wed Mar 04, 2020 10:43 pm

About the post on the side of the carb body, as 1/2A Nut said, it is usually used as an anchor for a starter spring.  But according to the directions that came with the engines, it was a nipple that could be drilled out through the crankcase to use for pressurizing the fuel tank, allowing you to drill out the venturi for more power.  

They added in the instructions that the gains would be minimal.  I never tried it and don't know of anyone that has so I don't know how well it worked.  I've seen more broken off posts than drilled out crankcases so I have to think it wasn't a real game changer.
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Post  Ken Cook on Thu Mar 05, 2020 4:53 am

Drilling the case isn't a good idea. It can cause a lot of run issues such as the engine surging up and down in rpm's due to air leaks and the pulsing of the valve timing. The nipple was used for crank pressure, the nipple was too small and it's tapered causing the fuel line to constantly leak and fall off. The Knights Of the Round Circle had a few flyers such as Larry Renger ( Former Cox engineer ) who used it exclusively for stunt. I think it was more of a experiment than a advantage but he successfully made it work using a Foremost one way valve in the pressure line. Backplate pressure is easier to deal with and the real problem initially was the recommended hole size through the backplate and or case. You only need a pin hole for this to work effectively and the size escapes me currently which was too large. The problems faced with using case pressure is that when you turn the prop, your automatically forcing fuel up the line and into the engine, therefore a pinch off device is required to start the engine. The same type used for bladder. A bladder requires a fine threaded needle valve whereas the stock setup will work on case pressure.

I have used this on two engines in the past and the real problem is that you forget that the case is drilled. The engine will run with the hole in the side of the case but it will run poor. Until the hole is sealed, it becomes a real headache due to not being able to see it without removing the carb body. I drilled out the case and JB welded in a piece of aluminum rod to seal it.
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