Terry, Too small.  A good place to start is .270/.272 diameter.  My experience is that with .260 range is it is just starving for (air) volume.  You were smart to start small so that you can open it up in stages.  You will be happier as you get closer to the .272, and maybe a bit larger if that's not enough power.  Your "mileage" will drop as the diameter increases.  Lots of info under the LA46 heading at the top of the forum engine section.  Good luck.

If you don't use a pressure fed tank then opening up the venturi will alter the engines ability to draw fuel for the worse! So don't go too big!
Many people use the "stock" FP20 venturi and NVA on the LA46. In fact perhaps the majority do. I have never understood why this is done as one loses a considerable amount of power this way. I always use The FP 40 "stock" venturi and NVA. It gets the job done far better! (The stock FP 40 venturi has had some variations in bore , but any of them are better than the restrictive FP 20 venturi)

Andrew.

I asked that question in another thread, if anybody had used an audio tach to see what the rpm increase was from static to inflight. Brett gave an answer, but I don't recall for sure what the rpm change was...maybe 1,100? If you did a search for "audio tach", you'd probably find it. 

I've mucked about with 'venturii' (restrictors) a fair amount, since I can make my own out of Delrin easily enough. On a K&B .40 (#4055), I used as much as a .312" bore with a .156" spraybar through. It was still pretty gutless, but certainly didn't need pressure feed. 

With the .46LA, I used as much as a .290" with a .156" spraybar through, but the plane had only 4.5 oz tank, and it wasn't enough with 10-22 fuel, even with only an 11.5 x 4 APC...and even at Tucson's altitude on 10-22 fuel. I put in a .272" 'venturi' and got the run time up enough to work ok. If I'd had enough ground clearance to use the 12.25 x 3.75, I would have needed even more fuel. There was no need for a .312" venturi, but it would have drawn fuel well enough, I have no doubt about that.

Regarding the smaller venturi sizes, there have been some reports of engines running wonky with such a setup, for the simple reason that there's no longer enough oil being run through the engine, so it got hot. I have not tried such a setup myself, but for example, Norm Whittle reported that his Stalker .76 ran much better and more reliably with a .330" venturi (replacing the .312" venturi). More fuel often makes a 2 cycle engine run much happier. Not always, but often enough to suggest keeping that in mind. That can be done by increasing the nitro content, but it can also be done by increasing the castor oil content...or increasing the venturi bore.

Terry...I'd suggest putting the baffle back into the stock OS muffler. You might experiment with drilling out the muffler outlet, maybe even drilling out the hole in the center of the conical baffle a little. They do a nice job of keeping the noise down and help control the engine rpm, just like the OS .20 "BBTU" setup. I used one from a .46LA on my Magnum XLS .36, and it was pretty sweet. Not much noisier than an electric. Used muffler pressure and liked it. Also used muffler pressure on the .46LA with a Randy Aero CNC tube muffler and it was also nice, but beware of leaks.   

Yes, your Cardinal will probably be nose heavy with the OS muffler, but a little bit of tailweight will fix that, and the Cardinal wing can easily carry a lot of weight. Narrow the flaps down about 1/2" to 5/8" and work with the control ratio to get a little more elevator than flap travel and it'll be good.    Steve   

PS: Read all the "pinned topics" at the top of this forum!