Quote from: haybaler on March 12, 2025, 09:03:06 AMIf it's carburetion, it can be very frustrating, they need to be really clean! I had two late model GS500 like yours and spent quite a bit of time working on both sets of carbs, though I was tempted to buy a replacement set. Finally got both sorted, so maybe some of my actions can help you sort yours.

1. the value of pictures, surprising how many times a member chimes in with, what's that thing in picture #1? mine doesn't have that, and problem gets solved. close ups of carb internals and snapshots of entire bike showing exhaust and the installed carbs with airbox and connections.

2. Consider replacing replaced parts, especially air filter. the one you bought was 149 on the website, but they list another for 249, wonder about quality difference. Covering intake is clever troubleshoot strategy, I usually wrap some plastic piece around half of the body of the air filter assembly. Also, suspect the fuel filter, some are junk and too restrictive, even though "new"!

3. 10 ml in bowl. That's not right, should be more like 45-55 cc in each bowl, plus what's in the fuel line. Take a close look at the physical operation of the float. I had one where the tang actually got stuck at the screw that holds the float pin. I had to grind down the head of the screw to solve the interference.

4. Was it difficult to replace the slides/diaphragms? This is a trick question! Anyone who says no problem is suspect; it should be difficult to re-install, difficult not impossible. Once installed you test by pushing them up and watch/listen for the "whoosh" sound as they come down.

5. Replace both pilot jets one size up; if you had 38, use 40's.

6. in #12 above the first picture shows the rubber piece/seal, but look where it goes, looks like a lot of corrosion. If it's corroded there, it's corroded throughout the carbs. It might need soaking in an ultrasonic cleaner to clean out passages, especially pilot jet passage. test by shooting carb cleaner into where pilot jet screws in and it should come out one of the holes in the venturi near the throttle plates. If not, blow compressed air same place and see if it comes through. It must be clear to run properly!

7. With throttle plates completely closed, turn throttle screw clockwise until plates clear about 1/2 half of the hole in the venturi. Set mixture screws around 3 turns out. Adjust idle as necessary after total warmup.

8. Here's a Youtube video that has some good ideas too: https://www.youtube.com/watch?v=AJh1hxNf7BA

Cheers!

**!SOLVED!**

Thanks for the tips Armandorf.
Looked like the fuel-air mixture were reaching the sparkplugs but not igniting properly, as soot were forming.

Also thank you for the suggestions, Haybaler. 

I had already tried all of that and more, including cleaning the fuel inlets and pipes connected to the idle circuit using a homemade small pipe cleaner. The last adjustment I made to the floats was setting them both to 12.68 mm. I did notice the guide, but it was firmly in place, though with slight play in each one, which I considered insignificant, as the needles did not exceed the guide height. 

Still, the issue persisted. 

I chose to run a 17.5 jet because the only source for the pilot jet was in the US, where I would have had to pay the part price plus 25% VAT and an outrageous 700% markup for shipping. 

So, I currently run a 17.5/60/130 setup with an aftermarket exhaust. 

The solution turned out to be simple but nearly impossible to guess. When I went to tighten the spark plugs, the right one snapped—despite only being hand-tightened and then given a half-turn with the plug tool. They were brand new when installed winter 23/24 and still produced a spark, showing the correct OHM readings on the coils. 

Nevertheless, I bought two new ones and replaced the other as well, and voilà! all problems, including the idle issues, disappeared. 

Although it's rare and unlikely, I suspect the faulty plug had a manufacturing defect, as the cylinder still fired but wouldn't handle load or run properly at idle with low power. 

Anyway, I'm finally a free man and should probably gear up for the next ride, as I've been itching to go! 

Cheers! 

Wow! Nice troubleshoot, and a good lesson for everyone about the quality of new parts. Most of the time new parts are fine, no problem, but once in a while a part gets manufactured wrong or injured at installation. That's why I don't throw replaced parts away until long after a repair is successful. Well done, cheers!

Thanks! Definitely a lesson learned always keep the old parts around for troubleshooting when things go sideways.

Here's a pic of the spark plug. Looks like gas were finally flowing but not igniting... judging by the fact that it's covered in what I'm guessing is fuel-air mixture (or maybe just tiny gremlins having a barbecue in there?).