Motorcycle Repair: 1979 honda cm400t, cm400, length of pipe

Question
I just bought a cm400T for about 700 bucks, and I have two and a half big problems, all related.

#1 I took it to a shop to get the carbs cleaned - I figured the first time I would have someone else do it.  The guy who did the work said he wasn't sure the carbs are OEM, and was unable to get it properly jetted (and admitted that) - see below. I got it home, and the next day the carbs started leaking.  Everywhere.  Like a sieve.  Nothing stops it.  Not shutting off the fuel at the shutoff, not cranking the throttle, on or off.  I suspect that the floats may need to be replaced, and that the valve is stuck somehow, but I wonder because of the other issues I'm having if my mechanic might have been clueless.

#1.5 Somewhere along the line an owner replaced the exhaust with a 2 to 1.  This eliminated the collector and one of the mufflers, as well as a length of pipe (I'm awaiting a scrapyard replacement from a 1982 cm400, but I think the collector has an extra cat in it - the only part that has a different model number from a 1979).  This wouldn't be a problem, except:

#2 I have NOTHING above 5800 rpms.  The engine nearly dies.  it sputters, loses power, etc.  The bike can't do 55 mph in any gear as a result.  This is a baaaaad thing.

Are these problems related?  If I replace the muffler, will that fix the 5800 rpm issue?  I suspect fixing the carb jetting will help too - can I manage that (If a pro couldn't, I'm intimidated)?

Because of all this, I'm getting about 24mpg. This sucks. Is it worth fixing this bike?  Can I fit a 34mm carb to this, and if so, what jets would I use (the 1982 has 34mm keihins).

Thanks for whatever help you can give.

Answer
Yea, the problems are all related.  First, there's no reason a good mechanic can't clean a set of carbs without creating the problems you describe so I'm guessing something else is going on.  I suspect you have a problem with rust in your fuel tank which creates debris that prevents the petcock from shutting the fuel off, makes the carbs leak and can plug the jets and passages in the carb.  

I'm not familiar with the exhaust you describe so I can't say much about that but any aftermarket exhaust that claims a performance enhancement is probably less restrictive and will tend to allow more air through the engine and make the air to fuel mixture lean out.  If you are going back to the stock exhaust, I'd recommend you go to the stock carb jets and settings and you should be good to go.  

You need to take the petcock apart and see why it won't shut off.  I may give you a clue about your other problems.

Regards
Rich