potential fueling / carb issue

[510rob]

Greetings GS community - this is my 1st post

I bought a used & stock '93 GS500 with around 35k kms on it.  It's a really fun bike.  I immediately took it on a long trip, and it performed flawlessly.  I recently took it on another trip and it started acting up under load at one point...

It starts fine, runs fine, idles fine, the choke works as expected, all functions seem relatively normal, but sometimes when riding (depending on temp & humidity), the motor feels a bit sluggish in and around the 4-5krpm range, where/when it feels like there's a big mushy spot in the throttle response through the midrange.

I mentioned that it was "acting up" at one point on my last road trip.  After riding in traffic (being blocked by a motorhome is more like it) for a while, it started to surge (felt to me like lean surge) and the mushy spot in the throttle got to be about 3 miles wide.  It would hesitate, sputter, and act like a bratty kid unless I was completely off the throttle, or on full throttle; under full throttle, it ran the way I would expect it to run at full throttle.  I pulled into a gas station and filled up to full.  It idled fine, and ran fine in and out of the gas station, but then started to act bratty again once I was back on the highway for a while.  I came to an intersection with a light - it idled fine at the light, and after I left the light, I buzzed it up to 11krpm without it skipping a beat.

Any ideas???  Is this normal behavior for a GS500 from the 2-jet era?  Has anyone else experienced any similar midrange sluggishness?

[Bluesmudge]

Could be a clogged jet or a petcock problem. The older petcocks don't have a prime setting so it is tougher to test but a fairly common problem with GS's leading to fuel starvation at freeway speeds. I'm sure someone else will be of more help.

Welcome to the forum!

[Robzor]

The GS frame petcock has that pesky diaphragm in the ON position, the design purpose of which is to only allow fuel to pass thru the petcock to the carbs when it receives vacuum from the carb rack and not at all when the bike is off.  Trouble is, that diaphragm deteriorates over time, and leads to any number of intermittent fuel starvation issues.  This could be the root of the issue.

If you haven't yet, it would be easy and a good idea to remove the carb rack and give it a good inspection and cleaning.  It's easily done and you'll feel better knowing the bits are all there and clean.  It should only take an hour or two at most to remove the carb rack, clean the bowls, bore and jets, even if it's your first time doing it.

[The Buddha]

Old GS also has prime position. However the inlet and outlets are smaller.
Any way clean and rejet is the answer to the OP. Check air filter for flow and exhaust for blockage.
Cool.
Buddha.