Motorcycle Repair: CB900C, air fuel ratio, stock pipes

Question
Actually Two questions...I have been trying to bring this old 1981 CB900C back to life and have struggled with the Carb Jetting. With Stock Pipes I was able to get it to run reasonably well until 8000RPM's where it would die. I then change to a Mac 4 into 1 exhaust system. Then it would break up at 4500RPM's. I rejetted in steps from the stock 105 mains to what I have now..115 mains. Its better but still only runs well until 6000 rpm.

Question one...should I keep increasing the main jets a little at a time or am I chasing another problem?

Question two ever since I put on that "Mac" system, there is what sounds like a mechanical rap in the top end. When I look for it with a stethoscope, I hear no tapping at all on the hard parts of the motor. It sounds smooth both in the top end and the bottom end. WHERE is that "rapping" coming from? Is it possible the exhaust system itself has an anomaly where it creates a rapping sound? This "Rapping" is only there after the bike has been running for a few minutes..and then goes away for the most part over time. Even when its at its loudest, I can't hear anything from the motor with a stethoscope. This bike is Driving me nuts!

I want it to have all its revs and to run smooth. It runs REALLY strong from 3500rpm until 5000 rpm where it begins its fading act. (Plugs are light tan)

Answer
Walter, the main jets only control the fuel above the 3/4 throttle.  Between 1/8 and 3/4 throttle the needle jets in the slides along with the main jets the control fuel metering.  You'll need to raise the needles too.  If you go to the hardware store and get some small washers about .025 thick you can use them to raise the needles.  I wouldn't be afraid to go as high as 125 mains if that's what it takes.  My 1977 750 F had 125 mains but it depends on how open the air box is, the restriction in the exhaust as well as cam timing to determine how much fuel is required to get the right air fuel ratio. However, to make sure you are working on the right problem use the choke to richen the mixture by supplying a little extra fuel and make sure that the problem is really fuel related.

Regards
Rich