gpz750 going, going, not going?

So heres one for the gurus to wrack their brains over. Carbs are clean, idle is good, vacuum is 17(whatever unit) on my mercury gauge across all 4 @1500rpm. When I go for a ride everything is great until I go past half throttle, after that, the only way I can decribe it is like turning the ignition off. It just slows down, eventually around 3000rpm it gets small surges. If I go back to pre-half throttle, no issue. I have changed reg/rec, ignitor, vacuum caps, plugs look new, tried setting enrichment with no change, carbs are seated and sealed, petcock is working fine.

There is a small exhaust leak between the header and the port (which I will address eventually but not now) but im sure theres no way that would cause this.

There was some shitty fuel in the tank, which I drained but couldnt get the last half cup(?). Filled the tank almost completely full. just wondering is maybe that "gas" was 80% water or something, could that half cup do this? I would think it would run like shit all over?

Is the bike stock?

vacuum petcock might be closing up as the manifold pressure gets closer to atmospheric pressure when the throttle is open. it thinks it is not running and closes. had this problem on my gpz550. fixed it by turning the vacuum petcock into a manual one.

Nessism: Yes as far as I know, minus the F1 exhaust.

Stardon: I tried running it in prime position with zero change. I do have another (bad) tank with a known good petcock I could try. I guess I'll make a 60th trip out and try it.

Does the bike have the stock air box and air filter installed? Ed

Tried the other tank, I think it may have gained a bit more throttle usage but could also be the bike temp or gas quality. In ether case the problem is still there.

Ed: No I'm running large pods, with custom side covers. I ran them on my 550 as well without these issues. I'm not sure I could even get the stock air box on to try.

As the sun is now setting, I cleaned up for the night, I'll be back at it in the AM to the neighbors dislike I'm sure.

Pods and a header necessitate rejetting the carbs. The bike isn't going to run properly without it, and if it does run, it will run HOT.

I believe the "header" is stock? its 2 sets of 2 that have a cross over pipe connecting them behind the oil filter, the cans for sure are not. I'll see if I can find some jets, we just gave away most of ours when we sold our race bike. Also the jets that are in there are plastic, stock?.

Plastic? Pics?
Steve

I'll continue this ether Monday or Tuesday night. I'll try to take pictures of the jets then.

Sounds like a main jet issue, the plastic jets are definitely not original. Your linked 4-2 pipes are standard?

Maybe a PO had it with a different pipe or something before you got it? See what mains and needle profile and clip position you are running for a start.

I bought a few sets of jets from my local KZ dealer years back and he only had 3 brass and one nylon or white plastic jet in that size ..... he told me it was the same and would be fine but I ended up picking up another brass one and never using it.......

Just wanted to add I don't think they are causing the problem cause they are plastic ..... more probably cause its not jetted correctly or another problem.