Hanging idle troubleshoot help

[kieranduggan12]

Hi everyone,
I purchased a non-running 1982 GS550L a little over a year ago and have slowly been working on making it road worthy. The last major hurdle I have is this bad hanging idle, that I assume is from a vacuum leak; I'm just having trouble finding it. I thought I had a breakthrough today when I realized I didn't have the plenum drain between the airbox and the carbs plugged, but it runs the same whether it's plugged or not. I'm really losing patience with this thing. Hoping its something simple I'm overlooking as a newbie.
The bike starts right up on choke, and once it's warm with the choke off I can set the idle down to 1200 rpm, but when I blip the throttle, it won't come back down below 2500 rpm unless I back off on the idle adjustment screw. If I crank the throttle passed around 6000 rpm, the rpms will drop back down until the bike cuts off.

Relevant maintenance:

• Re-shimmed valves into specs (all but a few were very tight)
• New OEM carb to head boots and O-rings
• New carburetor O-rings (Including the ones under the idle mixture screws)
• Float heights set from bowl gasket surface without gasket
• New OEM throttle shaft seals
• airbox sealed with weather stripping
• carbs have been completely apart/unganged and dipped overnight at least 3 times
• New dyna S ignition and coils/wires (cleared up a bad misfire on 2 and 3 cylinders)

Running conditions:

• Fully charged AGM battery
• stock airbox and exhaust
• idle mixture screws fully out at 4 turns
• Petcock on prime with carb vacuum tube plugged
• 11.95 volts at the coils

I've been spraying the carbs/vacuum tube and obvious places like the carb boots, with carb cleaner with no change in rpm. The boots from the air plenum to the carbs aren't cracked, but they are a little hard (not as bad as the ones I replaced at the head). I'm hoping someone with more experience than me can offer some advice. Thanks guys!

[Rob S.]

Once it's running, why do you leave it on prime?

Is prime the only way you can get it to start?

[KiwiAlfa156]

Hanging idle is symptomatic of a lean idle mix. Air leaks make a mixture lean as it adds additional air. If the carb spray isn't changing the idle and you're sure it's not an air bleed, then it can only be an idle circuit issue. Lean = not enough fuel for the volume of air, so it may be fuel blockage in the pilot circuit. Clogged pilot jets? Clogged by-pass offices? Can you squirt carb cleaner through the pilot system to visually confirm the pathways are clear?

The pilot jets in the Mikuni BS (I'm assuming here as you say 82). Have a small hole at the pointy end which is the main fuel path way into the pilot circuit you should be able to see daylight through them. The holes on the sides of the jets are air bleeds.

Here's the official Suzuki diagram of pilot circuit fuel and air flow.

[KiwiAlfa156]

Just saw the 6000 rpm bit. Temporarily replace the fuel line with clear tube. You'll be able to see if fuel supply from the tank is the issue.

[Nessism]

Did you vacuum sync the carbs?

The 550's are more problematic than most regarding a hanging idle. Sorry, I know this doesn't help fix the problem, but just sharing.