RX7 with possible blown rotor

[Vince Vu]

Hey guys, just a quick question about the rotary engines. I was thinking about buying an '83 RX7 off someone, but they said it dies whenever it goes under 2,000 RPMS. Apparently the carb was rebuilt, and they don't think it's the vacuum or timing. They suggested that it is a "blown" rotor in the engine, and I was wondering what exactly meant as far as repair. I am a mechanic but haven't jumped too far into engine work, especially the rotary engines, so any help clarifying the issue is appreciated!

TL;DR: What has to be done to fix a blown engine rotor in an '83 RX7 1.2?

 

[magnumP5]

"Blown" means full rebuild, no questions asked. Not really a terribly difficult job on the Wankel engines as long as you keep all the seals organized.

That being said a rotor can't really "blow." Its seals can fail causing a loss of compression but rarely is the rotor damaged. You can check compression a number of ways, the best being with a Wankel-specific compression tester. On a budget you can remove the valve from a normal compressor tester and watch the needle bounce. Even cheaper you can just pull the trailing spark plug and listen for "pops." You want to hear three strong, distinct pops from each rotor. A faint pop means one or more side seals could be bad. No pops at all means a failed apex seal.

 

[Vince Vu]

"Blown" means full rebuild, no questions asked. Not really a terribly difficult job on the Wankel engines as long as you keep all the seals organized.

That being said a rotor can't really "blow." Its seals can fail causing a loss of compression but rarely is the rotor damaged. You can check compression a number of ways, the best being with a Wankel-specific compression tester. On a budget you can remove the valve from a normal compressor tester and watch the needle bounce. Even cheaper you can just pull the trailing spark plug and listen for "pops." You want to hear three strong, distinct pops from each rotor. A faint pop means one or more side seals could be bad. No pops at all means a failed apex seal.

So just need to rebuild with the current seals? Or should I just get new seals and rotors for it?

 

[magnumP5]

So just need to rebuild with the current seals? Or should I just get new seals and rotors for it?

Honestly, it depends on the condition of the current parts. Most people like to go with all new seals when they do a rebuild for peace of mind. If you're intent on going that direction I'd stick with the OEM Mazda parts. The rotors only need to be replaced if they're damaged or out-of-spec.

 

[Signal 2]

Minimum spec is set out in the Factory Service Manuals. Downloadable versions available from a number of sources on-line. http://wright-here.net/cars/rx7/manuals.html#FSM
Not unlike a piston engine, hard seals...apex and side, as well as other internal wear parts like oil control rings and obviously bearings can be reused. But it really doesn't make much sense if they have many miles. Regardless, I would almost always replace the hard seal springs. There are other major components that have to be 'in spec' too....rotor housings and side plates or "irons".
If you're considering the car, do a compression test using a traditional compression tester as mentioned above. The dying at idle could be a lot of things that are not related to the general health of the engine.