I would like to make a Brass muffler extention for my .91 4 stroke,how would i bend it at about a 45 degree angle without putting a kink in the tube?

Unless you have special bending equipment (what that would be I don't know) I doubt that you could actually bend it. You could just cut a "V" in one side and then solder it back together with the elbow acting as your bend. Not too cool, but if it is on the bottom of your plane and out of sight it might work.

Maybe visit your local muffler shop and get them to bend it on their machine that they use for bending tail pipes.

If you really want to do this, here is how.

Contact a metal supply business and get some Cerrobend. The stuff looks like lead, has about 96% of the density of lead, but melts at 156 degrees F. Plug one end of the tube and heat the Cerrobend with a soldering gun or iron. Let it drip into the tube as it melts, until the tube is full. Then use whatever type of bender you prefer.

Once the bend is made, put the tube in a pot of water and heat. The Cerrobend will run out, into the water. Be sure to recover the cooled Cerrobend. This stuff was not cheap, the last time I looked.

You can also plug the tube tube at one end, fill with sand and plug the other end. Bend and remove plugs and sand.

You will have to anneal the tube before you bend it and maybe have to again if it did not bend enough the first time.

Do you know anyone in the HVAC business. They use tubing benders for AC lines, that would work. The sand method works well too and its cheap.

Depending on the size of the tube, I have used copper plumbing fittings. Generally they have 22.5°, 45° and 90° in a couple of diameters. If the added nose weight is not a problem, then this has worked for me. Watch out for galvanic reactions though.

I've used rope (or cotton twine) in place of sand with some success. The idea is to fill the interior of the pipe with a relatively solid, but maleable material.

The "solid" bit prevents the pipe from kinking when you bend it, the "maleable" part allows the pipe to be bent.
Dennis-

If all else fails Performance specialties sells a (90) flex muffler. Not cheap but works well.

What size are you bending? There is also the "plug one end, fill it with water, freeze it rock hard, and bend it" method.

I have also filled it with plane old plumber's solder, heated it to melt it, let it harden, bend it, then heat it to drip the solder back out.

I've found that using a piece of cable in the tubing while bending keeps it from kinking. It also comes out of the tubing easily. My 2 cents.

This method is the traditional way to make bends for brass instruments: (trumpets etc):
Anneal the tube by heating to a healthy dark red glow
Let air cool
Plug one end and fill with molten lead
When cold bend the tube over a (wooden) form
If the inside of the bend developes wrinkles hammer them out wit a small suitable hammer (ball head)
Melt out the lead
Sand and polish the tube