Mercury Repair Questions? Ask a Mechanic for Answers ASAP
A faulty muffler will not cause any engine running issues, just exhaust noise concerns.
If its a misfire your getting then its likely in need of a tune up and possibly coil(s). If you have the use of a scan tool that can run a power balance test, you can then see which cylinder is to blame.
You want a excellent 5 star answer & I would like to be rated the same for providing it. Consider I don't know your skill level & I can't see, smell, hear, or touch the vehicle. It may take a few back and forth replies. Try your best to respond with good detail so I can do the same. I normally log in at 3pm and work late. A rating is required. Your rating me, what I type. I am not an employee of this website, nor have control over the fees or the time it took to get a response. If you rate negative or not at all you still paid, but I get nothing. I want you pleased with me and my efforts, so let me know if your experience here has come up short.
With the new details. I suggest a power balance test to see if any cylinders are misfiring.
Check for a restricted converter, by removing the upstream O2 sensors for a short road test.
Monitor the fuel pressure while running to verify it is staying above 25 psi while running.