It really depends on how long you have had a drop in RPM ? the plug in the hole is sometimes foul and it just needs a good sorting from time to time ! you will find that once you have taken it out ..given it a good scrub..then you watch that baby ignite !
P.S. Any idea if the the Tranny was a 115bhp or 130bhp? the 130's go like a gud un !!
EL