The automatic 2004 Mazda RX-8 I looked at today had a rough idle. The guy said it was because of a rotary. Tr

could be. a rotary is part of the distributor system

Yes, it is true that rotary engines have rough idle. It is due to the fact that they have low compression. If you are concerned, you can take it to a mechanic to check it out. One thing to remember is to NEVER put synthetic oil in it. It probably needs a tune up.

A rotary should purr.
l think it has a problem that is unique to rotary engines.
l would get it checked out by some one who knows rotary engines. Any thing less is like taking a 747 to a piper cub mechanic.

l doubt that.

Rotary engines run much smoother than piston engines.

l would not buy a Rotary engined car like the Mazda RX-8 if you do not know about them.

You probably do not know anything about rotary engines because only the RX-8 offers that engine.
They require different types of oil and maintanance and must be driven in a different way.

http://youtube.com/watch?v=9jVDr44IP1A
This is a properly working RX7 at idle.

http://youtube.com/watch?v=dg-vWC-7CRI
This is the same car with a bad idle.

Do you hear the puttering? A properly running 13B rotary engine will have a noticeable 'mechanical' ticking sound, like a regular engine with messed up valves. Ask him if it burns oil. If he says no, walk away.

It is true that all RX8 is have Wankel Rotary engines, which when running properly should run very smoothly. This engine does not so there is something wrong. Although it could be a couple things causing this, it is likely the issue below.

The '04 models, especially the automatic tranny version, have had motor problems. This is not a design issue but simply the fact that auto tranny drivers tend not to rev the motor very high. This coupled with low viscosity oil (5W20) lead to low internal lubrication issues which damaged the apex and/or side seals on the rotors. A rough idle is indicitave of this problem.

A full engine replacement recall was issued for any affected cars, & the oil metering pump was reprogrammed to inject more oil to prevent this in the future.

If this car still has full warranty then it is likely the engine can be replaced. If the owner was an idiot & did not take the car in for the recall check on the engine & is now out of warranty run away. Get the VIN# & check it at a Mazda dealership to get ur options.