Hi,
I'm in a need of ideas and or advice to help solve this problem.
Here is the situation:
My NC30 track only bike , suddenly started to develop a back fire and loss of power when passing the 5000 RPM.
It will continue to erratically backfire and never reach more than 10.000 RPM. (either with the bike on the stand or riding)

I have read every topic regarding fuel, ignition and electrical but I still can not solve the problem.

So far I did:
-Rebuild and synchronized the carburetors. (with new boots)
-Tested with two different (good) batteries. I have good voltage reading at 5000 RPM
-Tested the coils, (primary and secondary) and Pulse generator (They are not perfect, but in range) I ordered a new set of coils, but not got them yet.
-I have good ground.
-I have spark on all the plugs. (they are new)
-I have tested with two CDI's (ignitech)
-I'm running total loss system , with no alternator (stator and rotor) and the reg/rec

I have not yet done a Valve clearance. Could this be the problem?

During all these tests, something particular happened. If I used the Ignitech CDI, (stock map) the bike will turn on and run perfectly until I rev it pass 5000 RPM. If I use the original CDI (Last time I used was working perfect), the bike will not start at all. The ignition will just go and go, but not start.
I tried another Ignitech CDI and the same problem, so I'm ruling out that could be that, but It puzzle me that the original CDI will not get the bike running.

Any help or advice will be much appreciated.

Thank you

You have an adapter that allows to swap between an Ignitech Honda TCI and a Honda OEM. How many rpm was the engine doing when it lost power. What exactly was the battery voltage at 5000 rpm. Have you tried disconnecting the tacho. You have free flowing fuel from the tank. Vacuum fuel tap has been bypassed. tank vent not blocked.

I would check if you have a broken or weak valve spring. you can see it on a vacuum gauge (not steady but very erratic or nerviness) It could also be the pulse gen. (can be checked on a scope)

UPDATE:
Thanks for the replies. While I was cleaning the carburetors, I noticed that the jets were all 115 and all the emulsion tubes had only 4 holes, when they should be 4 and 5 holes. I went ahead and changed the jets to 120 / 122 and the emulsion tubes to the correct size. Now the bike will run strong
up to 12.000 RPM and then it will cut off/ backfire. If I slow done the bike will come back to work perfectly under the 12K. Could this be fuel related. Should I go with smaller jets?
Thanks!

vaccum hose / tap could be buggard i think.

the high vaccum is causing the line to leak / collapse and shutting off fuel till you back off throttle, seen it many times with same symptoms.

cheap and easy enough to inspect / replace