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.

[kieranduggan12]

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.

Thats a good idea, maybe it's a clogged vent in the gas tank cap starving the carbs for fuel. I also have a fuel filter in the line, I'll try removing that, too

[kieranduggan12]

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.

I had them vacuum synched before I took them off to try dipping them again, but haven't since then. I noticed the plenum drain was open when I was putting the carbs back on the bike. When that didn't fix the hanging idle I stepped away because I got frustrated

[Grimly]

I had them vacuum synched before I took them off to try dipping them again, but haven't since then. I noticed the plenum drain was open when I was putting the carbs back on the bike. When that didn't fix the hanging idle I stepped away because I got frustrated

If you took them off after synching, you need to address that again. It would be worth your while to get a set of gauges or a Morgan Carbtune and learn how to use them (it's easy enough).

[gustovh]

4 turns out on the pilots is a bunch. When nothing is working..stop have a beer contemplate life…then start from the beginning, compression test, check plugs, you have addressed the carbs so set your pilots at 2 turns out from lightly seated. Start bike adjust idle and let warm up, adjust idle if needed. Turn pilot in slowly one at a time til the bike stumbles then back off 1/8 turn or until even idle. You may have to reduce the idle during this process. This sets you up for lean best idle…

When you adjusted the valves, did you turn the engine over by hand a few times and recheck the clearances?

V

[salty_monk]

Check you didn't snap the ends off the pilot needles in the hole. Doesn't matter how much you "adjust" them after that.

Does it run better with the choke out?

[kieranduggan12]

Ok, so yesterday I worked up the courage to try running the bike again. I warmed it up with the gas cap off and the fuel filter removed and vacuum synched it, with the same result; idle hangs hard at 2500 rpm. So I guess its not a fuel delivery or synch issue. I also found an air leak on 2 and 3 carbs throttle shaft seals again, which is what prompted me to change all of them out in the first place. Not really sure how I could have installed those wrong on the inner carbs and correctly on the outer, but something isn't right. It makes sense though, 2 and 3 seem to be running more lean than 1 and 4. Turning out their idle mixture screws increases the rpm until they're backed all the way out at about 4 turns, while 1 and 4 seem happy at about 3 1/4

[kieranduggan12]

Check you didn't snap the ends off the pilot needles in the hole. Doesn't matter how much you "adjust" them after that.

Does it run better with the choke out?

I wish that was the case! I made sure they were clear the last time I had the carbs off the bike. I haven't really run it much with the choke out, mostly at idle until the bike is warm. I don't remember the rpms hanging with it out though

[kieranduggan12]

4 turns out on the pilots is a bunch. When nothing is working..stop have a beer contemplate life…then start from the beginning, compression test, check plugs, you have addressed the carbs so set your pilots at 2 turns out from lightly seated. Start bike adjust idle and let warm up, adjust idle if needed. Turn pilot in slowly one at a time til the bike stumbles then back off 1/8 turn or until even idle. You may have to reduce the idle during this process. This sets you up for lean best idle…

When you adjusted the valves, did you turn the engine over by hand a few times and recheck the clearances?

V

I'm not sure if I rechecked all the valves after turning it over, its been about 6 months since I did it. Probably worth another look

[salty_monk]

I wish that was the case! I made sure they were clear the last time I had the carbs off the bike. I haven't really run it much with the choke out, mostly at idle until the bike is warm. I don't remember the rpms hanging with it out though

The reason I asked about the choke is that if it runs better that would back up the air leak, lean running diagnosis.