Interdasting Vacuum Problem (maybe?)

[danofthejungle]

So I lowsided after a taxi cut me off, which resulted in a bent handlebar.  I got a new Bikemaster handlebar and installed it.  Took it for a spin, but noticed that the handlebar was a bit more vibey than before the lowside.  I initially passed it off as a lighter handlebar, but thought, 'what the heck, let's try cleaning the carbs, and maybe there might be a bit of a vacuum leak causing extra vibrations..."

So I clean the carbs (they were a bit clogged), popped the air/fuel mixture screw cherry, adjusted the air/fuel mixture screw to 3 turns out, put the bike back together (without the tank), synched the carbs at 1500 RPM, and the bike seemed to idle OK. 
Put the tank back on the bike, start the bike up, then get popping noises at idle, slow return to 1200 RPM after twisting the throttle, and revving up to 3000-4000 when applying a bit of throttle.  WTF?

Take the tank off, take the carbs off, try to reseat everything, try starting again, then the idle seems fine. 
Put the tank on again, then the same issues as before.  Hanging idle, revving up to 4000 RPM.  Check for any vacuum leaks with WD40, but nothing seems to change.
Something seems to be messed ONLY when I put the tank on.   

With the tank off again, I tried pushing on cables to simulate the tank sitting on them, but nothing makes it screwy like the actual tank sitting on the bike.  I checked for any kinked hoses, and pinched hoses, but nothing.

Tonight, I tried going through the same issues that were in the FAQ, but nothing seemed to go.  Played with the mixture screw and checked the spark plugs afterwards, but it seemed too rich (black sooty residue).  Reset to 2.5 turns, and the idle seemed ok, plugs seemed ok.
Put the tank back on again, start the bike up, and even without throttle, the idle starts zooming up.  Thought back to Step 0 of the FAQ for Hanging Idle and figured what the heck, might as well try.  I brought the idle back from 4000RPM to 1200RPM by only the idle screw.  All of a sudden, no popping noises, RPM comes back down quickly.  I have only taken it up and down the driveway, but it seems to go normally without any issues (lower RPM, up to 2500RPM).  I'll give it a spin tomorrow to see if everything else is running normally.

TL;DR - get hanging idle issues only when the fuel tank is on.  Idle seems to be different when the tank is on vs. off.  Anyone else experience this at all?

[Slack]

Do you have a stock air box? One of the reasons I use a K&N Lunch box is because on more then one occasion I pinched this tube or that and caused issues. The lunch box leaves more room for everything.
Second thought: How are you supplying fuel when the tank is off the bike. The vertical drop is important, too much will cause extra pressure, equivalent to raising the float levels.

[danofthejungle]

Stock airbox.  Didn't see any pinched hoses (only one that is a bit pinched is the airbox drain hose which is capped, and it is pinched due to the routing.  how the heck is it supposed to come out the right hand side when the opening in the box is on the left hand side?)

Fuel is via a large syringe connected directly to the carb fuel intake hose.  The vacuum hose connected to the petcock is disconnected during testing without the tank.  Does this cause issues?

[bmf]

Did you block the vacuum hose to stop air leak to the carb?

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[Janx101]

I'd just prop the back end of tank up a bit with a stick and set everything so it's running right... then lower tank down.... sure it's gonna be kinda cramped but as handy as the syringe setup is... its not the same as the tank with all hoses attached. .. (shrug)

[danofthejungle]

vacuum hose was not plugged.   I synced the carbs and when I blasted through the videos, I didn't note any mention on blocking off that hose when taking off the fuel petcock, so I didn't think it was an issue.

I couldn't leave the tank on as I was syncing the carbs (and needed access to the sync screw)

[bmf]

So you then tuned the carbs  to suite a non normal situation - when it returns to normal (no vacuum) everything will be out. Plug the tube,  retune and then everything should be good.

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