If the whole tone changes then the combustion isnt correct

could be spark plugs,sensors god knows what

try this ecu scan trick first to see if it throws any codes http://www.ozhonda.com/forum/showthr...-ECU-for-j00-!

check your ignition leads are on properly,pull the spark plugs and check or even better put a fresh set to be safe than sorry

check oil level is up to top marker line