Had a problem with a rough idle mostly when driving and the car slows and the revs drop too much and there's is a rumble but it never stalls. The revs flick back up which I've heard is normal. I cleaned my idle valve with carb cleaner but still looked dirty inside at the back so in case it just wasnt working correctly, on Sunday I changed my idle valve to a brand new one. I let the car run for 5 mins then took it out for a drive for 15-20 min's. Still not right. When I turn the engine on the autochoke doesn't keep the revs high initially before they go to normal after 1 minute and when the revs drop when slowing the rumble is still there. I've checked all hoses but could it be due to corrosion in part of the exhaust?