Dead spot has me stumped? Help please!!!

My 79 KZ1000 has a dead spot at 5K rpm at WOT. The engine loses power and "stumbles" or "flutters" until it reaches 6K then pulls strong to redline. Only at 5K-6K does this happen. I can accelerate half throttle from 2K to redline and it will not do it at all. If I twist it WOT at 2K in any gear it will pull smooth to 5K, flutter till 6K then pull like gangbusters to redline. This is driving me crazy! My motor is freshly rebuilt with 10:1 1075 kit, K410 cams degreed to 108*, home ported head, stock valves, 4 into 1 header. The valves are properly adjusted, the spark is good and advance looks good with a timing light. It has BS34 carbs with factory airbox and K&N filter. Carbs were rebuilt and jetted professionally and I've been through them twice again looking for this problem. I installed an O2 sensor and it showed 12.5:1 through the entire rpm range. I took the carb needles and turned grooves in them on my lathe and tried adjusting them up and down a couple of notches. The O2 sensor showed the mixture changes but the stutter was still there, although the stutter was at a lower RPM when the needles were lowered (leaner). It feels like the bike is lean for a few seconds at 5K WOT as it transitions from midrange to main jets? Sorry this is so long, I am just trying to give as much info as possile so maybe someone has been here before can help me.

Define "home ported" When I was an inexperienced young lad I ported a head and I was never able to tune out a major flat spot. You need to be very carefull when opening up the intake ports. Mix that with some aggresive cams and you can make an engine which can never be tuned right. I'm not saying that is your problem but it's worth a look at.

Please consider this as just a wild blind shot in the dark.

Is it possible the diaphram has a pinhole leak or the slide not completely free (for whatever reason) to move. Thinking stumble might be due to delay in movement of throttle slide because of diaphram weakness or some slight interference with normal slide movement, until vacuum belatedly overcomes the problem between 4k and 5k.

Can tell I don't know zilch about BS34's.

I checked the diaphrams carefully, twice, in addition to the fresh professional rebuild. It might be my porting? I carefully blended the transition from the carb holder to the head. I opened the runner up per Larry C. calculations, going from 34mm to 29 mm at the SSR. I cc'd the runners to make sure they were equal volume. I used various sized valves to measure the port size as the porting progressed. I'm 52 and although no pro I have amature ported a few heads in my days. I was wondering about some sort of reversion or harmonics with the system as a whole. I was just asking, maybe someone had fixed this problem before and hoping I was overlooking something simple?

Still open for ideas! TIA

Have you checked the ignition advancer?

my kz900 acted up around 4-5k and it turned out to be the ignition. i messed with the carbs til i was blue in the face and nothing worked. i put a dyna ignition system on it and it went away. this may not be the case for you but it might help. good luck with your problem.

roystaylor wrote:

Have you checked the ignition advancer?

It is clean and free and I can watch it advance with the timing light. It was my first and second guess?

Starving for gas? Do you have an inline filter on your lines that might be slightly clogged and not getting enough gas down to the bowls when you get on it possibly?

I cleaaned out the lines and replaced the filter. That is what stumps me, once past 6K it runs great till I shift at 11K. Less than 5K rpm runs great, over 6K runs great. Between 5K-6K stumbles at WOT, but not at half throttle????

What about the possibility of the carb boots not sealing and matching up with intake runners? I didn't see this mentioned as an area to look at.

If there is any ridge or mismatching on the two mating surfaces this might cause the stumbling as the ridges cause an interruption in the air gas mixture being drawn into the intake runners.

I had this problem and did three things to clear it up. Went one groove leaner on the needles and also did a little better job on sealing and matching the boots to the head. I am not sure which cleared the problem but combined they seemed to take care of the stutter and flat spot around 6 grand.

Just a couple of ideas to look at.

Cheers-Colin Firth-Ontario Canada