My 420 (149,000 miles)was idleing rough and the timing was making noise so I replaced the timing chain, chain guides, water pump, belts and hoses and installed new platinum spark plugs. The distributor cap and rotor looked fine so I did not replace them. After the last bolt was tightened I started the engine and noticed it was still idleing rough at ~600 rpm. If you kick it up to ~1000 rpm or higher it smooths right out and I must say purrs with that new chain. Any how, even after driving it for a while the Idle is still rough, cold or hot, does'nt seem to make a difference. Acceleration is great, cruising is great, just rough idle. Any suggestions? are the platinum plugs ok for that model year? What is the recomended gap for the plugs? Any help would be appriciated.