A 155 main sounds way too small even for 2800 ft. What are you using for premix?? Fuel?? Have you checked your reeds? If your are getting stuff out the tailpipe is it black or is it real runny? I blew a head gasket and it had quite a bit of brown runny stuff coming out the tailpipe. Head gaskets are cheap. Do a test like Motorrad said.
You had nothing better to do this week anyway right
Always do the "BIG 4"..
these tests point you to the problem every time... no guessing, wasting parts, or time etc... dont even need to buy gaskets to do them.... and if anything can save you alot of money from melting down a good motor.
Step back, take some time, and do some tests... if anyting, you may discover other problems that are hurting power besides the major one at hand...
The big 4Compression check
Fuel Level check (external)
Ignition System check (Ohm the coil, and stator) only takes about 15min
Leakdown test
Also a VIDEO works great... we can tell alot by how it sounds..
and if you could pull the pipe, and snap some pictures of the kips drums... that would be helpfull...