90% of the time it will be a stuffed exhaust clogging up the baffle with burnt oil. You have a few of options how to sort out the problem,
1/ try & source a good 2nd hand replacement.
2/ take the exhaust off & cut the silencer down the middle with an angle grinder, clean out all the crud & weld it back up again.
3/ the cheapest way if you're very careful & I don't recommend it unless you know what you're doing is to take the exhaust off, put a blow-torch down the spout of the silencer to burn all the cr*p. You may have to heat it for up to 10 minutes to get all the sh1te loose. You could use a rubber mallet to dislodge all the crud but only hit the exhaust at the weld seams as you will put dents all over the silencer & be back on this forum giving out stink about me. I also drilled a small hole just below the exhaust spout on the silencer box, it helped the exhaust gases escape easier but that's optional.
steo, July 2009