Hola, My 1990 miata Has had a high hydrocarbon problem. First time i went it failed miserably and it later removed the cat to find it had been gutted. I then installed a new mag-flow direct fit one and went again, and low and behold i still failed. This time however, was on the idle section i received a score of 304 when it needs to be at 220. I have replaced all plugs as the wires were in great shape changed the oil twice, and ran a bottle of fuel system cleaner through it with half a tank of gas.
A week ago the new cat began to have a VRUUUMMM noise upon deceleration. so does anyone have any suggestions?

 

Check and make sure base timing is not jacked up, should be 10-12. People up it to like 14 for power. Try running a can of seafoam through the vac lines( there are a few guides on line) this will clean the intake, tops of the pistons where as fuel system cleaner is more for the injectors ,filters, and pumps.

 

Timing too advanced, dirty injectors, carbon build up in combustion chamber, trace amounts of fuel in oil, engine and cat not warmed up enough, old plugs and wires, vacuum leaks, low compression, replacement thermostat is cooler than stock, etc.