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Installing Harley Fat Boy mufflers on XS11 motorcycles
by Curt MarshToday, I finally got around to doing the project.
Installing Fat Boy mufflers on OEM 4 into 2 system on an '80 G.
H-D mufflers are readily available because H-D riders take off OEM systems, as soon as they can afford it, to get the "right" sound. Near new mufflers can be picked up for $30 to $40 per set on eBay. At swap meets they are usually cheaper on the last day because H-D riders don't want them.
The total time is about an hour (less, if you already have the adapter). Tools: Hack saw, 8 mm Allen socket (best) or wrench, 1/2 box wrench, a 10 mm box wrench, hammer and 2 X 4 block.
- Remove the exhaust system and measure the diameter of the pipes.
- Cut the OEM muffler just in front of the cross over pipe. This eliminated the cross over pipe. The pipe that was left is just a hair under 1 3/4" O.D. and double walled.
- Install an adapter 1 3/4" I.D. by 1 3/4 O.D. by 4" long I.D. end goes over the muffler stub. I had to grind down the edge of the OEM muffler to start it. Then drove the adapter home with a board and hammer. This leaves the 1 3/4" O.D. to attach to the Fat Boy muffler.
- The Fat Boy muffler has 2 bolts holding the OEM hanger plate. Remove the H-D hanger plate and install the Yamaha hanger plate from the OEM mufflers by the rear bolt.
- Bolt on the headers, attach the muffler, bolt the hanger bracket into place.
- Mark the hanger bracket angle. Remove the muffler and drill the second hole and put in the second bolt.
- Install the muffler, again, and bolt the hanger bracket in place.
IMPORTANT: Install the muffler, so the writing is upside down. It changes the angle of the bracket welded to the muffler.
When you back it off in gear, it definitely says there something under this tank you don't want to mess with 
Idle may be just a little off. I'm going to wait a day or two and see if it needs adjusting.
I set aside half a day to do this project. If I had realized it was going to be this easy I would have done it a month ago