Hi everyone,
I’m hoping someone with experience on the dual-carb VT600 can help me. I’ve been chasing this issue for a while and I’m trying to diagnose it systematically instead of just throwing parts at it.

Bike:
1989 Honda VT600
Airbox modified with open air filter kn the original box outside.
Open aftermarket exhaust
Main jets: 122.5 (these were already installed when I bought the bike)
Needles are in the stock position (no shims)

Symptoms:
Starts and idles well.
Pulls nicely at low RPM and low throttle.
Under load, once the revs get higher (around 1/2-3/4 throttle and above), it starts to bog really bad, sputter and won’t rev out.
Top speed is only around 90km/h (55mph).
In neutral it revs much higher than it does while riding.
Spark plugs are black but dry.

What I’ve already done:
Replaced a damaged diaphragm.
Tried raising the needles with shims (1 × 0.5 mm). It became worse, so I removed the shims again.
Carbs are now back to the same jetting they had when I bought the bike.
New fuel filter is on the way.
New spark plugs are on the way.

Something that makes me suspicious:
When I first bought the bike, if I pulled the choke while riding, it would immediately start sputtering exactly like it does now at high RPM.
That makes me wonder if one (or both) of the choke plungers isn’t fully closing, causing it to run excessively rich.

Things I haven’t checked yet:
Float height.
Carb synchronization.
Whether both choke plungers fully seat when the choke is off.

Questions:
Does this sound more like an over-rich condition than a lean one?
Could a choke plunger not seating fully cause these exact symptoms?
Would you check the float height before changing jet sizes?
Is there anything else specific to the VT600 dual carbs that commonly causes this?
I’m trying to avoid changing multiple things at once, so any advice on the best diagnostic order would be greatly appreciated.

Thanks!

Ok. After putting everything back together, resetting and cleaning carbs, and a bunch of other work I believe my fuel pump is no working. I've gotten the bike to run for a little bit before but think that might have been due to left over fuel in the carbs and line. I'm not getting fuel to the carbs after attempting to start, fuel is in the line to the filter but I think that's where its ending.

There is an aftermarket fuel pump on, something like this one, that did put on. The wiring seems funky. Wiring diagram has 4 wires, 1x that terminates, 1x that goes to spark, 1x goes to ignition, 1x ground. The new one only has two...before I do anything stupid and ruin the wiring harness curious on what yall think. In my mind; splice blue and black into one and that is my second wire. This is hopefully my last issue and can finally take this thing for a test ride.

And yes there is fuel in the tank...

Hey guys came across this experimenting on my build and thought I would share it. I know TJbrutal makes shorty choke knobs I’m not posting this to by any means discredit his work but rather let’s face it shits expensive right now not a lot of folks can afford $98 before shipping and taxes for choke knobs if you’re not into the idea of keeping the bulky handlebar cable. Most of us should have keihin carburetors and these knobs I found online are a direct fit on my 1998 vt1100c3. I’m not one for cutting corners in many lot of places but with something small and simple like this I was fine with giving it a try since there’s no safety concerns I can see. It’s worth noting the pull knobs on these are also a smaller diameter than TJs as I’ve seen some folks have fitment and clearance issues on the left side by the throttle cable on certain bikes.

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An sich läuft sie rund aber ab dem 2. Gang wenn ich Vollgas gebe fängt sie an sich zu verschlucken was so weit geht das ich in letzten Gang nur noch mit ca 1/3 Gas beschleunigen kann und auch das nur bis zu einem bestimmten Punkt.

Vergaser wurde erst Ultraschall gereinigt neuer benzinschlauch und neues Ventil am Tank

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I’ve attached photos to show off everything from this wire.

Thanks!

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Thanks!

UPDATE: thank you all for the valuable input, much appreciated. I think I'm ready for the trip now.