Hello!
I have a 1993 GS500E, low miles but pretty fresh out of repairs.
It runs good, with one exception - between 3900ish to about 4400ish RPM, is a "hesitation zone" - it does not exist if you rev from 3000 to 5000, it goes right through it no problems. But due to shifting, I find myself hitting this zone each time I come to a stop/start.
If it helps, I will upload some video of the throttle actions and the results.
Any ideas?
As I mentioned - it has no other strange behavior that my novice hands and ears can find, just this spot.
How strange. If your bike had a direct cable to slide carburetor, I'd say to check the needle position--sounds lean. However, with vacuum carbs, I do not know. I'm sure that someone on the forum can help.
Most likely hesitation is because it's not getting fuel, i.e. it's lean.
So, some questions, because this is odd behavior AFAIK.
You say it doesn't do the hesitation if you rev through from 3K to 5K. Is that only unloaded?
I am assuming you mean in 1st gear if you go from a standing start it doesn't hesitate, but only does the hesitation in the event that you pull the clutch, then shift (or coast down) to a gear so that it'll wind up starting off in this 3900-4400 rpm range, then under load it will hesitate. Does that sound about right?
Just trying to be clear on my understanding of the issue.
If it consistently does this at a certain rev range and varying throttle position under load then I'd have to suspect a vacuum leak or something related. My explanation is this: when underway you accelerate and then close the throttle like to shift gears, then the slides drop (because vacuum goes way down under closed throttle) and the main jet is closed by the needle. Then when you shift and open the throttle again, the slides don't come up as quickly as they should, thus leaving the needles in the jets and causing it to run lean. The reason the slides are not coming up is probably a vacuum leak, insufficient vacuum to pull the slides up by the diaphragms.
The reason you don't see this when accelerating through this rev range may be that you open the throttle bigger at a lower rpm when accelerating, say at 2500 rpm, which makes a LOT more vacuum and it's more than enough to pull the slide up hard. You are using more fuel because you are accelerating through the range, and the slide is up to deliver the fuel. But when you are midway through and make a gear change and wind up in this rev range there's not as much vacuum as there is when accelerating from a lower rpm (with more throttle, likely), and since there's a vacuum leak, there's just not enough vacuum to get the slides moving quickly enough.
I'd go through and check for vacuum leaks (DO NOT spray carb cleaner on the carbs!), but most likely culprit is going to be the intake boots or intake boot o-rings. Could be missing "little o-rings" under the carb cover vacuum ports or maybe missing/bad cap on the vacuum port somewhere or even a vacuum leak at the petcock vacuum line at one end or the other.
I could be completely wrong but this is where I'd start looking for this issue.
Now should come the parade telling you it's likely valve adjustment, ignition, or you need to rejet. which all could be true but I'd still start with vacuum leaks.