So as many of you know my '92 GS500 is pretty well dialed in terms of carb setup, etc. The bike runs super great. I have no complaints. BTW I have 40/127.5 jets, OEM airbox, paper filter, chopped Yoshi muffler (loudish).

Today I had to troll around a giant shopping center parking lot looking for a certain business, this was all 1st gear barely above idle you know? After doing this for like 2 minutes the bike decided it wanted to stumble just a bit going from above-idle to more throttle/accelerating. It just stumbled/hesitated for maybe 1 second, burbly kind of exhaust note, almost like a misfire? This is not a consistent hesitation coming off idle or just above idle when riding normally, like accelerating from a stop. Just when accelerating from a 10mph roll. Bike was fully warmed up.

Ideas? Normal? Ignore it (my strong tendency)? Indication that the engine is about to grenade?

Oh the only other running issue I recognize is that the bike really feels like it's running out of air above about 8.5K rpm. It sure doesn't seem to really love hitting 11K. But I almost never ride it above 8K so it usually doesn't bother me. Wonder if the 127.5 jets are actually too big? But it seems it couldn't be rich at >9K causing it to not want to run, but fine from 4K-8K. IDK if this is to be expected or not. Like I say I very rarely need WOT or >9K rpms, but the times when I get the inkling for the old Italian tune-up the bike seems less willing than expected. Maybe I need bigger main jet? Needles shimmed? ?? Maybe this is related to the stumble when rolling up from parking speed?

My first thought is to ignore your parking lot problem, although I might try to re-create it riding around the neighborhood. If I could get it to happen again I might start trying to figure out the cause.

I'd be more interested in the high rpm issue. That bike should happily bang itself off the rev limiter all day long. (I don't understand why you avoid high rpm and WOT. In my mind that's what motorcycles are for. But it's your bike.) I have a hard time believing you're too rich. I'd try a bigger main jet. A shim is only going to affect the 4000-8000 range. At high rpm the needle is fully out of the jet.

Quote from: Jim Moore on August 22, 2017, 03:25:31 AM
My first thought is to ignore your parking lot problem, although I might try to re-create it riding around the neighborhood. If I could get it to happen again I might start trying to figure out the cause.

Actually I noticed it today in 2nd gear between 2000 and 2500 rpm with just a hair of throttle. Like maintain-speed throttle. It stumbles. Give it just a touch more gas and it takes off normally.

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I'd be more interested in the high rpm issue. That bike should happily bang itself off the rev limiter all day long.

Well, there's no rev limiter ... besides the valve springs and connecting rods...

Quote(I don't understand why you avoid high rpm and WOT. In my mind that's what motorcycles are for. But it's your bike.)

It's not avoiding it, it's just that it's not really needed or warranted in the places I ride. Plus, like I said, it kind of runs out of steam above about 9K so why rev it any more? There's no more power there. It accelerates quicker if I shift.

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I have a hard time believing you're too rich. I'd try a bigger main jet.

Yeah sounds like a plan. I'm trying to remember whether there are 125s or 127.5s in it now (I have both).

Both my 97 and 02 GSs were stock all the way, never touched the carbs, never thought they needed it. Copied this from an old post of mine on my thoughts about shift points for spirited riding:

Don't know about others but my stock GS500 power band was 7000-9000 rpm.  Max stock hp is at 8500 and nothing is gained going more than 10% over max hp since the power drops off rapidly after 8500.  Shifting the GS up one at 9k if your still accelerating results in a drop to approx 7k rpm putting you right on top of the torque curve where you want to be.

Doesn't mean much on the city streets but running in the mountain twisties with bigger bikes it matters.  I remember running 7-9k rpm for hours in the mountains back 15 years ago when I was a kid of 70.

GS Jack--85 years old? I am a youngster of 70 who has been riding for 55 years. I hope to make it a few years longer. I have owned dozens of motorcycles, but my GS500 is my first with an electric starter. I do not count my 1962 Zundapp Super Saber 250 because smoke would pour out of the wiring loom whenever I tried to use the starter dynamo.

"is pretty well dialed in terms of carb setup," --well, perhaps not eh?!
It should be smooth on and off throttle at low speeds, pilot jet and just above sounds suss.

Put the standard muffler back on, it'll probably be the culprit.

Nothing to do with main jet at low speeds. Plug heat range?
There's not much more power available above 8500 anyway so I personally wouldn't be too concerned about that. Complete wast of time thrashing it above that, get a Gixxer if that's what you require. But worth doing a full-throttle plug chop anyway.

Quote from: Joolstacho on August 22, 2017, 05:46:57 PM
"is pretty well dialed in terms of carb setup," --well, perhaps not eh?!

Right on. Mostly pointing out that I'm not just starting from scratch. I've put serious time and effort into the carbs and I'd say even with this rare stumble they are 98.7% dialed.

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Put the standard muffler back on, it'll probably be the culprit.

Can't. Don't have it, never did. But that's an interesting point.

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Nothing to do with main jet at low speeds. Plug heat range?

Another interesting point. Plugs could use changing anyway. Worth an experiment.

Quote from: gsJack on August 22, 2017, 04:38:41 PM
...stock GS500 power band was 7000-9000 rpm.  Max stock hp is at 8500 and nothing is gained going more than 10% over max hp since the power drops off rapidly after 8500.  Shifting the GS up one at 9k if your still accelerating results in a drop to approx 7k rpm putting you

Thanks, this absolutely confirms my perception, power drops off after 9K and it doesn't seem to make much sense to rev past it and the bike clearly doesn't like it. It's plenty quick shifting at 9K. Won't make any sense to shift at 10.5K and wind up above the power band in the next gear, plus it certainly adds a ton of wear and tear for no good reason.

To me, 7K rpm is where the GS500 "comes alive" and it goes back to dead after 9K Now I routinely shift long before 7K anyway because most of my riding is around a neighborhood with 30 & 40 mph speed limits, hard/fast/loud acceleration doesn't make any sense. When I rarely wind up on a >45mph highway I just accelerate up to the speed limit (55-65mph in most cases) at the pace of traffic, which is to use 1/8 throttle and shift at 5K rpm, and then cruise along at 4K rpm in 6th gear. It's not like I'm riding on a race track where there's no traffic, no speed limit, and no traffic controls.