I did a quick search and may have missed this, but I am looking for what is a good ID size for the venturi on the TD049 that will give good performace on suction from a uniflow tank. The math I use for the larger engines would require an opening smaller then what is on the reed valve motors, so I do not trust the math for this size.

I am trying to get some of my 1/2A planes running better.

Thanks
Robert

 

Why suction?  The side nipple pressure works really well.  Take a 2x56 "T" nut and turn the flange down to make a nipple out of it.  Screw it onto the side nipple on the carb housing with a bit of epoxy or Goop adhesive to assure a seal.  Drill the pressure tap per Cox's instructions and plumb to the tank.  I get incredibly good, reliable runs with this setup.  You can even drill out the venturi a bit if you need even more power and can handle the fuel load required in your plane.

Why suction?  The side nipple pressure works really well.  Take a 2x56 "T" nut and turn the flange down to make a nipple out of it.  Screw it onto the side nipple on the carb housing with a bit of epoxy or Goop adhesive to assure a seal.  Drill the pressure tap per Cox's instructions and plumb to the tank.  I get incredibly good, reliable runs with this setup.  You can even drill out the venturi a bit if you need even more power and can handle the fuel load required in your plane.

I have a Madallion .049 that I run on pressure.  I have trouble keeping the fuel line on the pressure tap. Is the above a fix? do you have pictures? 

As a side comment I have found pressure hard to hand start but an electric starter works fine.

To John Rist:  see above for the best way to keep the hose on the nipple.  There are a couple of other ways as well.  One way is to super-glue the hose on.  Another is to gently scribe anullar lines around the nipple with a #11 blade.  This adds friction to prevent slippage.  The 2x56 nut filed down to become a real hose nipple is far and away the best.

Here is a photo of the nipple modification I use on the Tee Dee and Medallion.  You screw a 2x56 "T" nut onto a bolt and chuck it into your power drill.  Use a file to cut the flange down until it is a nice hose nipple shape.  It will then screw onto the plastic nipple on the carb housing of either engine, and if you use a bit of Goop, epoxy, or JB weld, will seal perfectly.  Drill out the tap per factory instructions, and you have a very reliable pressure tank drive system.   

I like the ultra fine control that the Kirn or Klaus needle system provide, but it is quite workable with the stock needle.  I seem to need about 1 ounce of fuel to do the classic pattern on 42.5' lines with a bit of margin left over.  I do tend to set it screaming all the way, keeping attention on not going over-lean on the verticals and overheads.  If you want something closer to a 4-2-4 break, you will need a bigger tank.  Currently, I like the APC 6x2 prop to give me lap times in the high 4-second region.