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I was wondering if I can run strait pipes on a 81 Yamaha 650. What problems could I run into if I go this route. I noticed new mufflers are very expensive for this bike, plus I like the loudness.
Thanks
BR

You may have to rejet and balance your carburetors.

I tried to run a couple of short side-draft mufflers on my 650, but they were just too loud, even with packing in them. You might run into problems with excessive noise as well.

You may have to concoct a story to tell the numerous police officers who may stop you to listen to and test your pipes with a decibel sound meter. Also, regardless of rejetting, your performance may actually suffer.

Now I see Harleys with strait pipes all the time, what is the difference in the loudness then.

Probably a difference in frequency and pitch. You should get all the loud you want. Don't forget to protect your ears..

An open pipe four cylinder bike tends to be louder than your average V-Twin cruiser. Maybe it's because the fours run at a higher RPM, not sure. My 1980 Yamaha XS 1100 broke one of the collector pipes off right behind the headers once. It was my only transportation at the time, so I had to ride it until I could get it fixed. It was actually fairly quiet that way until I gave it gas. But on the gas, at around 7 or 8K RPM, it was ear-splitting.

Guys on Harleys with straight pipes would look around to see where the noise was coming from.

Even with rejetting you will lose some power. The cruisers that run straight pipes do the same thing. More noise does not neccessarily mean more power. Not mention the areas that are trying to make any aftermarket pipes illegal because of too many people running straight piipes. It will also give the police another excuse to pull you over.

Point taken. I guess I'll look into a different exhaust sys. I want it loud but not so loud that the cop will bug me all day long.
Anyone know of what kind exhaust I should look into?

I would suggest a Supertrapp system. They have a series of plates that you can add or remove to change the sound, and/or tune for more power. That sounds like something you might like, since you can play around with it a bit until you get the sound you want.

If your bike is an XJ series, there are only two companies still making exhaust systems for it, and nobody makes a slip-on specifically for it, so fabrication of a bent pipe in front of the muffler may be required. Once by that part, you can indeed use a Supertrapp universal slip-on. Mac and Jardine make complete systems, including the header. They are nicely described in the Dennis Kirk catalog, online it is www.bing.com and Bike Bandit also has the same products at www.bikebandit.com.
As your bike is air-cooled, rejetting should also be considered if you get a new system.