Very annoying fuel/vacuum issue

[rerb]

Hey, I've been tasked with repairing my friend's GS 500, (90's something). The bike has a very hard time starting sometimes, whether it's warm or not. The bike will occasionally start easy and run fine, idle at 1.1k, and rev through the rpm perfectly. Then a few minutes later it will idle at 4k and sputter and lose power below 5k. Then starting becomes hard. (Full choke when warm, it will thump along at 600 rpm and slowly build up. Any throttle input will kill it. Won't start any other way)

When it is running "nice" the idle screw is as far in as it can, and that only lets it idle at around 1.2k or so. Any less and it won't start at all.

Things I'm pretty sure are good:

Fuel delivery, carb bowls always have fuel in them.

Diaphragm looks mint

The Jets are very clean


Things I'm uncertain of:

At max idle adjust, the butterfly valves are barely, barely open. (Is this normal?)

The needle in the diaphragm slide is missing hardware. (#4 - #6 on the diagram of the carb)

The right carb's vacuum line isn't capped. (However, it creates no pulse like the one to the petcock, and holding my thumb over it didn't help.)

Any ideas? I'd really like to finish it soon before snow comes.

Thanks

[Sporty]

Hey, I've been tasked with repairing my friend's GS 500, (90's something). The bike has a very hard time starting sometimes, whether it's warm or not. The bike will occasionally start easy and run fine, idle at 1.1k, and rev through the rpm perfectly. Then a few minutes later it will idle at 4k and sputter and lose power below 5k. Then starting becomes hard. (Full choke when warm, it will thump along at 600 rpm and slowly build up. Any throttle input will kill it. Won't start any other way)

When it is running "nice" the idle screw is as far in as it can, and that only lets it idle at around 1.2k or so. Any less and it won't start at all.

Things I'm pretty sure are good:

Fuel delivery, carb bowls always have fuel in them.

Diaphragm looks mint

The Jets are very clean


Things I'm uncertain of:

At max idle adjust, the butterfly valves are barely, barely open. (Is this normal?)

The needle in the diaphragm slide is missing hardware. (#4 - #6 on the diagram of the carb)

The right carb's vacuum line isn't capped. (However, it creates no pulse like the one to the petcock, and holding my thumb over it didn't help.)

Any ideas? I'd really like to finish it soon before snow comes.

Thanks

I think you need all the parts in carb. It cant be missing slide and needle parts.

The other vacuum port is probably not drilled through.

It's quick and effective diagnostics  to run the bike with a spark plug wire disconnected. Try each side to find if both cylinders are running.

The jets can be clean, but other passages and air bleeds can be clogged.

If it were me, I would buy kits from this seller on eBay,

Completely  disassemble and clean the carbs. New needle and seats, set the float levels.

the seller also has kits with floats and separate floats if needed.

https://www.ebay.com/itm/2X-Carburetor-carb-Repair-Rebuild-Kit-1989-2000-GS500-GS-500-GS500E-US-/323338869308?vxp=mtr&hash=item4b487f9a3c



Replace the intake boot orings.  You can just match up metric orings (or std) at the hardware store




The idle screw is turned all the way in here

[rerb]

Wow, thank you very much that was a very detailed response. I'll go buy a rebuild kit, thank you~