I'm running REM40E champ's in my O-320-E2A. They work fine, as do Unison's (now Tempest).
Are you running avgas? Try the ethanol-free mogas over at lebanon state airport-- not sure if it's 87 or 92 octane, but even 87 oughta be OK with a 290D, although a D2 might need 92.
If carbon fouling, try leaning more. If oil fouling, you've got bigger problems.
minimums are for busting