86 Honda Civic Sudden High Idle, pre-ignition,, red hot

In any vehicle I've ever seen, when the cat gets red hot it is due to excessive unburned fuel getting past the combustion chamber and into the exhaust manifold. Possible causes are an incorrect fuel mixture, incorrect timing, corroded spark plugs, a faulty oxygen sensor, sticking float, faulty fuel injector or a malfunctioning check valve.

On the rare occasion it could be the catalyst material has fractured. When the car is on and you rev up the engine can you hear anything rattling in the cat? It is possible that it has become clogged. Are you getting anything out of the muffler?

Hi mvannessjr, i all so think it has something to do with unburnt fuel, however i think there is another problem causing this. i think the problem stems from what ever it is making the car idle higher. the fast idle happened very suddenly, which lead me to believe it had someing to do with fuel, however i have checked the carby out and it looks fine.what concerns me is the problem happened so fast. i have taken the cat off and started the car ,but idle was still high, the plugs look in great shape i think the mixtures ok because had no signs of any problems before, the float looks free. yes i am getting exhaust out the rear, however i don't know how to chech the oxygen sensor and check valve or where it is located. i know quit a bit about cars and have never come across a sudden high idle before, always been slow idle which is usually sucking air. i have noticed that the upper part of the exhaust mannafold is a whitish colour which could be caused by unburt fuel like you suggested but carn't locate where or why this is happening. i would love to hear your suggestions on this annouing problem. THANKS :?