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Cox medallion pressurization

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Old 10-20-2017, 02:08 PM
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ochsnm00
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Default Cox medallion pressurization

Can anyone tell me if the port on the bottom of the plastic carb housing is for pressurization and how it is plumbed. This is a brand new .09 with muffler throttle. This is an interesting story, years ago, a former WW2 pilot from here built a Dynaflite 1/2 A with this engine. He is 91 yrs old and would like to see it fly. A fellow club member and I are making it flyable. I test ran the engine today.

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Old 10-20-2017, 03:04 PM
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Yes it is for pressure. Backplate pressure is easier to adjust needle settings. The tubing slips off the plastic piece. Pressure is really only needed if the carb is opened up from the stock size. A small hole needs to be drilled through into the aluminum with the crank removed. I always cut it off flush and don't use it. It is hooked up to the vent line on the tank, and normally is not submerged in fuel. Any leaks will give a very inconsistent run. A throttle rarely works with a pressure system.
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Old 10-20-2017, 11:26 PM
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Yes, there is no need for fuel pressure on the stock medallions. The RC ones also have a rotating restriction in the venturi that emulates a crab. This too helps with the fuel draw at lower throttle settings. In this the one you have?
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Old 10-21-2017, 11:25 AM
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ochsnm00
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Looks like what I have. It looks as if this nipple (on the red body) is placed to pick up crankcase pressure. If it's not used should it be plugged? Is it there for tank pressure?
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Old 10-21-2017, 12:41 PM
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I finally got it to run reliably. I plugged the red nipple and raised the tank up.I couldn't get it to run with the vent line hooked to the pressure nipple.
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Old 10-21-2017, 12:56 PM
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The pressure nipple is likely not drilled through, which is good. And no, it will not run if the vent is attached to the nipple because it will not let air in the vent. As I said before, I usually saw it off flush, as I will never use it, and it gets in the way on some motor mounts. You can leave it on though, maybe for resale purposes. If I want pressure feed, I use backplate pressure or a bladder. That would be with no throttle. Tee Dees are more suitable for pressure as they are more of a performance motor.
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Old 10-21-2017, 04:43 PM
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Originally Posted by aspeed View Post
The pressure nipple is likely not drilled through, which is good. And no, it will not run if the vent is attached to the nipple because it will not let air in the vent. As I said before, I usually saw it off flush, as I will never use it, and it gets in the way on some motor mounts. You can leave it on though, maybe for resale purposes. If I want pressure feed, I use backplate pressure or a bladder. That would be with no throttle. Tee Dees are more suitable for pressure as they are more of a performance motor.
If I had just thought of blowing through I would have known it was plugged. Anyway we are one step closer to flying it for this gentleman. It seems to me that back plate pressure would be pulsing but the nipple would be open only to crankcase pressure because of how it is situated to the crankshaft opening..
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Old 10-21-2017, 07:12 PM
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Backplate pressure does suck back a little bit but overall gives pressure. The front one gives slightly higher pressure. Sometimes the backplate pressure fills with fuel and floods the motor. I prefer not to use crankcase pressure at all.
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Old 10-22-2017, 01:16 AM
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For future reference:
You can just poke a needle in that hole to measure depth, odds are there is no way the old man drilled it out would have to dissemble
to finish the process he has not even ran the thing.

This is a Medallion engine with superior fuel draw, needs no pressure to run well. If the carb body collar is tight and the back plate
the engine should have no trouble drawing fuel assuming the cylinder is tight too and the glow plug and your sanity to be gentle
with it and insure you have good glow, 20% castor, 25% nitro should function very well with the throttle sleeve.
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Old 10-26-2017, 12:05 AM
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Default Cox Medallion pressurisation

Originally Posted by ochsnm00 View Post
I finally got it to run reliably. I plugged the red nipple and raised the tank up.I couldn't get it to run with the vent line hooked to the pressure nipple.
As supplied ex factory-both the TD and Medallion ranges come with the pressure nipple moulded into the plastic carb housing-but of course this fits over the crankcase nose extrusion-and this is NOT drilled ex factory. The usual way of doing this-if it is needed- is to strip the motor, drill thru using the plastic nipple on the housing as a drill guide, deburr, and reassemble. So if yours has not been drilled then there is no need to block off the nipple.-there is no path for crankcase pressure to reach the pressure line. Very few users get satisfactory results using the crankcase nipple pressure set up-it produces 'timed' pressure as opposed to 'untimed' from a backplate nipple-with a higher peak pressure-and not the least-because the nipple is so small and smooth and it is tricky to keep the pressure line on it. I've seen a few TDs over the years with a pressure tap drilled through one of the engine lugs.

Assuming it has the muffler throttle-as distinct from the pure exhaust restrictor throttle -then it is potentially feasible to install a pressure tap in the muffler and use this for tank pressure just like a larger R/C motor-you would need to allow sufficient extra length in the pressure line to accommodate the throttle movement-but this would only be about 1/2" or so extra length. One way valves are available from some sources to prevent backflow flooding the motor-Kustom Kraftsmanship used to do a very nice compact small diameter one-but he's long out of business. Some of the specialist C/L racing suppliers probably still supply an equivalent item-as the CL racers still use them.

ChrisM
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Old 10-26-2017, 07:33 AM
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Originally Posted by ffkiwi View Post
As supplied ex factory-both the TD and Medallion ranges come with the pressure nipple moulded into the plastic carb housing-but of course this fits over the crankcase nose extrusion-and this is NOT drilled ex factory. The usual way of doing this-if it is needed- is to strip the motor, drill thru using the plastic nipple on the housing as a drill guide, deburr, and reassemble. So if yours has not been drilled then there is no need to block off the nipple.-there is no path for crankcase pressure to reach the pressure line. Very few users get satisfactory results using the crankcase nipple pressure set up-it produces 'timed' pressure as opposed to 'untimed' from a backplate nipple-with a higher peak pressure-and not the least-because the nipple is so small and smooth and it is tricky to keep the pressure line on it. I've seen a few TDs over the years with a pressure tap drilled through one of the engine lugs.

Assuming it has the muffler throttle-as distinct from the pure exhaust restrictor throttle -then it is potentially feasible to install a pressure tap in the muffler and use this for tank pressure just like a larger R/C motor-you would need to allow sufficient extra length in the pressure line to accommodate the throttle movement-but this would only be about 1/2" or so extra length. One way valves are available from some sources to prevent backflow flooding the motor-Kustom Kraftsmanship used to do a very nice compact small diameter one-but he's long out of business. Some of the specialist C/L racing suppliers probably still supply an equivalent item-as the CL racers still use them.

ChrisM
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I was able to download 2 pages of the instructions but unfortunately the third page with pressurization was missing. All there was was "On this engine a pressurizing method.......". Thanks to you guys I now have an understanding of the port. We will just run it without pressure.
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Old 11-30-2017, 08:19 PM
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Crankcase pressure serves multiple purposes depending on the intentions of the user. 1. is for simple improvement to fuel draw. In the Medalion one would need to drill the precise sized hole in the crankcase directly in line with the nipple on the plastic venturi mount. The rotating crank shaft acts as a valve to control pressure leaving the nipple.
In the TD the function is essentially the same for low performance sport flying. Higher pressure is for higher performance competition flying.
2. Crankcase pressure bled from the rear crankcase cover using a nipple with a check valve ( or check valve in the pressure line) to increase pressure for both fuel draw and improved atomization from the spray bar. Some modelers use needle valves and spray bars with more precise metering with the higher pressure. These aftermarket parts are rare these days and difficult to find if at all.
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