Bikes had been in pieces for over a year. i got a new 3.1gal peanut tank, new petcock (now...it's one of them $20 chrome valves that they use on big harley's and stuff that sucks way more fuel), and thats it. everything else just went right back on, and it starts up and runs like a raped ape unless you're in fifth and then it starves for gas and starts to cut out and die when im hauling balls. i can be jamming in 4th for a f*cking hour and nothing! soon as i switch to fifth it almost immediately starts dying. I tried cracking the gas cap thinking it was vapor lock or something...nope. and no i dont know sizes or diameters of anything. it shouldnt matter anyway.

So...whatcha guys thinking?

I have to switch to 4th gear to get it to stop. I had been jamming balls out in 5th gear for like 20 miles before it started happening and now it happens all the time in 5th. its starving for gas. but I don't know what that has to do with 5th gear because unless there's something I'm missing it shouldn't matter.
Actually...if I switch it up to 5th and it starts happening and I let it happen for a bit and switch to 4th still happens and I may have to even switched on the 3rd or let off the throttle for a while, like the bowl has to fill up... its starving for gas. filter on the petcocks is all I ever use. a friend told me to take that one off and put in an inline and it would restrict the fuel less but I don't see how this is connected to 5th gear. the last time I had a problem like this it was because the Jets in the carburetor were all switched around but I haven't touched those

im gonna open the carb up and check the jets too cuz it's possible i fiddled with it on my down time and accidentally switched the air jets or something. i seriously doubt rust or debris is the culprit but i'll clean the whole shabang out tomorrow. Does 5th gear at lower rpm's draw more fuel that higher rpm's in 4th?

Rpm drop going to 5th may drop vacuum too low to draw enough fuel...(not from the tank, but from the floatbowl)...
Do you have enough backpressure in your filter and pipes?...

Too open and all your torque is at the top of the rpm's... a Revzilla...
'Ceptin' our big thumper rev's out at 6500...

wait...serowbot...ur making sense but my phone is dead

I dunno hoot about the way the bike works, but all my cars had the highest intake manifold vacuum at idle. Vacuum drops as the carb opens. Maybe the bike is different,

I with serowbot on this one, a good debris tank check and petcock functionality test cant hurt either.

Confession time... I bought a Chinese petcock, looked good, the outlet was 8mm, it had a long neck, it was cheap.

before I could install it (already had a genuine yami on the daily) so there it sat til the dog found it and ate all the plastic off it... ie them little filter gizmos.  and the o-ring too, hoped it snapped when it came out.    
So in getting ready for the beast, I swapped engines in the 88 to the 96.  The 96 needed rings still from the oil tests. (don't change oils more than once if you can help it).  So a few weeks later I started getting this hitch in the getalong up my favorite WFO hill  
Yep them little petcock holes were stuffed full of rust chips.
So... check the FN petcock.

OK, so... low vacuum phen nom men na (doot doo de doo doot)
that's why you got a needle valve.
of course itsa crap shoot on the compromise.