Need more help but i have made progress

[XS400D]

So previously i couldnt get the bike to idle at all, now for some crazy reason it will only idle reliably with the gas tank off which leads me to think the bowls are over filling. (my floats are set at 26mm but i wont rule out bad needles)

If i do let it idle with the tank connected the idle floats around 1500rpm but will dip to 1000 and rise to 2000. i have tried messing with the idle adjust screws and the throttle stop but i dont understand why my idle is so up and down.

also with the tank on if i try to apply throttle the bike dies but with the tank off i can redline.

any suggestions outside of new float needles (which i think i will order along with gaskets)

 

[XS400D]

so i shot a shitty video of it idling with my iphone. i let it idle for like 5 minutes and then took this video. i am thoroughly confused. it does this with idle mix screws at 3, 3.5 and 4 turns out.

 

[drewpy]

video is "malformed"

 

[XS400D]

try that drewps

 

[XS-Time]

I am always leary of the vaccume operated petcock on these bikes. Is it leaking gas into the vaccume port in the manifold?
Also when you took the tank off did you plug booth vaccume ports in the manifolds.

Just trying to think of what has changed between tank on and tank off.

 

[XS400D]

I am always leary of the vaccume operated petcock on these bikes. Is it leaking gas into the vaccume port in the manifold?
Also when you took the tank off did you plug booth vaccume ports in the manifolds.

Just trying to think of what has changed between tank on and tank off.

the vacuum makes me iffy too but it does the same thing in prime as well. i did not plug the vac port when i ran it without the tank. the biggest difference i could think of is that when i pull the tank some fuel drains from the carbs since the bike is on the sidestand

 

[XS400D]

i just looked at the manual and it seems that for my year the stock pilot air screws are supposed to be set to 1 1/4 +-1/2 out. does this seem right?

 

[drewpy]

are you on plastic or brass floats?

with the tank off, are you blocking the vac inlet barb on the right manifold?

 

[XS400D]

are you on plastic or brass floats?

with the tank off, are you blocking the vac inlet barb on the right manifold?

brass floats. with the tank off i did not block the vac barb but i did leave the hose on it.

 

[drewpy]

brass floats. with the tank off i did not block the vac barb but i did leave the hose on it.

still connected to the tap you mean?

 

[XS400D]

still connected to the tap you mean?

the tank is all the way off but the vacuum hose is on the vacuum barb.

 

[drewpy]

so your leaning out the mis then, as the hose will still vent to atmosphere. m,aybe that's your clue?

 

[XS400D]

i like to tinker but i am no mechanic so i am not really sure what the next step is. i think the thing that is most frustrating is that when i got the bike it ran great for a few months before i let it sit

 

[XS-Time]

If the rubber diaphram in the petcock is torn , fuel can seep past it and run directly into the intake manisold making the bike run rich. This can happen no matter what setting the petcock is set to.
When you take the tank off and don't plug off the hose or barb in the intake manifold the engine is pulling in extra air making it run leaner than when it is plugged off.

You need to plug off that barb (one on each manifold I think) to replicate the conditions that it is running under when the tank is on.

Then see if the engine is running any differently.