I had the exact same problem. I have a 140 DZ running magnum DZ fuel also. I use the old style NMP Central header. I noticed the problem after i lost significant RPM. The header inside was contricted to an 1/8" opening from carbon deposits.I also called Varrano at YS, and he stated he hasn't really heard of this prblem with DZs. Now that makes at least two of I know of. All I've done is just clean out the header. I have a feeling this is going to happen to more of us.