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Stuff at work made in Bulgaria has 23% on it.
Hope Yambits is keeping their "immunity" or maybe still doing their paperwork right.

I have went through the left carb several times and have even swapped parts between the left and right carbs. I am not at all sure that the carb is my issue, but I have tried just about everything I can think of and some that you here have thought of too.

I'm able to relate to this.
Hope you find the smoking gun very soon!

If it's really the carbs, and it could be, I might have a fuel feed problem after all. Last time I had it out, it was lurching some when slowly dying on the open road.

My petcock flow passage (recommended by Mark) looks generous, but I also bought a genuine RZ unit with plans to put in a second outlet. Besides that, I would be able to turn it to reserve while riding under duress.

Wondering too about bowl filling inside the carbs through the needles and seats.

These ponderings would go better if Chuck Q were to jump in. Your concerns especially.

My '77 is harder to kick. A few factors including Chuck Q plugged the little cyl to exhaust holes.

Gotta be deliberate with it and watch the foot placement. It can kick back hard when it lights up before kicking through the stroke.

Much time spent here trying to make RD400 stock carbs work better. Never was happy until I did the Dave F mod.

It's a world of difference. I highly recommend.

See how it is done:

https://youtu.be/nRP-MMZlzJQ?si=WvohGGeEeg1CHQFt

Extended test ride today in 77 degrees F. Still more manageable with the gnarly needle, (it's not bent, just "chewed").
That still points me to new carbs.

However, riding 21 toward Rock Hill I noticed that going downhill and pulling in the clutch, the engine would stop. Obviously, I had felt the need to do that.
Turned around to go back and it stopped under power. Pulled over, had to turn off the key, then restarted.
I'm gonna change out the new Yambits coil to the original RZ one that I thankfully kept. It's ugly, but might just work better.

Gratuitous photos of a TZ750 engine make it much harder to focus on the lift.

I concur with Mark. The stock carbs are junk.

And I'm jumping right in on that.

Nobody wants RZ stock pipes. Experience has added stock carbs as well.

On the other hand, the RD400 is running crisp, responsive and quick today at 67 degrees.
Yay DaveF VM28's with fudged in UFOs and ChuckQ "update" porting with FPPs.

I'm going to drop back and punt the both of them. Wasted too much time this year trying to coax them into working better. Not getting that time back and I became an expert at removing stock carbs. Not really a coveted life skill.

I have those on my RD400. Had to fiddle with them because the fit wasn't spot on. They work very well. Pilot size goes WAY down and they make it crisp.

That left RZ carb though, it's not something I want to pour more effort into. Something is wrong with it that just isn't obvious.

Planning to look at the PWK options out there at this time. Can go the DIY route and save a few hundred or Ed Toomey complete kit for $700.

Still appreciate your carb loan offer and UFOs are def good advice.

I took it for a spin today. Since teazer mentioned my first post saying I had just changed the needle, it only made sense to change it back. Took the pristine new Toomey needle out of the left carb and put in the chewed-up old one. Turned the coil around, (not flipped, but turned 180 degrees to correct orientation - shouldn't have mattered), and had run a ground from a couple places on the engine to the battery negative and frame with a 4 connection ground wire "harness".

All that to say that the needle change is indicating that the left carb is the issue.
It really does pull through the lower rpm better now but reminds me of why I changed the needle in the first place back in May. Moves more boldly into traffic and is more usable at low throttle.

Will run it again later this week as we are supposed to approach 70 degrees but I feel there might be evidence to convict the left carb in a court of law. Will get it out more on the open road to check what I presume is/was an issue with the new coil.
The new coil wouldn't install the other way because the harness plug wouldn't reach, but since I lengthened the harness with direct wiring, it allowed the orientation that eliminated the ridiculous bends in the spark plug wires.

We'll see. I got my 30-year reward gift card from work, so that will help toward new carbs.
Still can't point to a detail that could be called a smoking gun after all of this. That old left carb that doesn't match with the right likes its old chewed up needle. Meanwhile it's spoiling the show.

Would be a dream come true to have it running great on the road for Spring.
PWK28s, one way or the other.

Not that I'm an expert, but that thing just delightfully rips above around 7k rpm. So much so that the clutch has started slipping - and that clutch slipping didn't impede acceleration either
Surely that can't happen with any type of reed problem.

So much stronger than I recall with the fresh '84 RZ that I used to own. It's at 64.5 mm bore.
Also, as I stated to mnein when he asked for a comparison, there is definitely more area under the HP/Torque curves with an RZ compared to the RD400, (both stock with pipes).

It smoothly ramps down on power, leaving time to find a place to either pull over or let off the throttle hoping that it will come back some before it completely dies. First time, I was sure it must be seizing. Crapped my pants.
It does not lurch like it's running out of fuel. Mark did a flow test on the tank/cap/petcock and the needles+seats are new.
I somewhat suspect the new Yambits RZ350 coil on this. There will be testing with that against the old coil - which was probably working just fine but became a victim of the parts cannon BEFORE this highway die-out situation came to light.

Also, I have switched from the new Toomey needle to the old, chewed up Toomey needle - mentioned in the very first post - and you pointed that out, teazer.
Figured out how to change the needle without removing the carbs. Fiddly but can be done.
Test ride is pending on that. Maybe Sunday with the 64 degree F high.

I'm not confident. Hopeful though.

interesting Suzuki has the front brake caliper on front side of fork tube. Common mod here is relocate caliper to back of fork tube. What are thought processes, pros and cons here?

Looks like they had to accommodate the possible "anti-dive" mechanism on the other side of the fork slider.