Trying to fix top end stumble

Mikuni bs34's with the non adjustable needles
1980xs400sohc
I have owned the bike for almost six months now and ever since the first ride there has been a BIG flat spot on the power curve at the top end. It is most noticable when the engine is under heavy load (60-70mph) at around 5500rpm it acts like it isnt getting enough gas and stumbles. This occurs consistently. The first time i took the carbs apart they were pretty clean but the floats were jacked up so i replaced the floats, float needles, and needle seats. That fixed the gas spitting out the carbs but not the stumble. The PO had 156 mains and the idle mix screws WAY out!!! So i re jetted to 142.5, 3.5 out from snug on the mix screws, and while my fuel economy increased, the stumble was still there. So today i slipped a washer under each slide needle to raise it about .5mm and jetted down to 140. stock exhaust, pods. we will see. Could the petcock going bad cause this?

It could be a clogged petcock keeping the gas from running just enough to stall at WOT. That would be rare,but possible. You could always drain the tank and check the filter,and put an inline filter in the gas line. lha

Have you had the carbs synced?

How's your timing?

Sounds like the PO tried to fix the top end problem with bigger jets? This
does kind of sound like a ignition issue. What are some symptoms of incorrect timing?

One sign is a flat spot. Sometimes if an engine is down a hair on compression as happens with wear and not a problem, the engine can use a little more advance than recommended when the bike is new. However, I wouldn't mess with the advance until I was sure the plugs, wires, coils and all stock ignition parts were working well.

HoughMade said:

How's your timing?

Click to expand...

"How's your timing?" - Hougmade

my 79 runs better with a little advance

wow, ok, shimming the needle seemed to help a little bit, and its running fine on 140 main jets, by fine i mean the stumble hasnt gotten worse. I wouldnt know timing from a hole in my head. Im hoping it's a fuel delivery issue, will def check the petcock next time im empty.
i have not had the carbs syncd, did a bench sync last time I tore into em.

Mmmmmmm HHHHHMMMMMMMMMM!!!
FIXED!!!

freaking fantastic, I can now redline it without it acting like its running out of gas. Had to go up to a 147.5 on the mains. but it runs great now. way way way more responsive. thanks all for the advice anyways. Taking it in a 600 mile round trip next weekend.

I guess your timing was good then!