Strange Highway RMP activity

[mennobike]

I've got a stock 97 GS500E. Great bike, love it to bits, but it started having some issues earlier this year. First it was flooding (float needle seat o-ring was shot), then it was having issues with a lag in revving and a low, unstable, untunable idle (forgot to put back in the O-rings i found under the main jet needle c-clamp), and now, a different (although probably related) problem.

As I was on my celebratory ride for finally having fixed the bike on my local interstate (going right at 75) the RPMs started wavering. I've experienced this to a small extent in the past, but always assumed it was a tach issue since I never felt a real power change or surging. This time, however, it felt like it was pulling hard, and would lurch and surge. This continued for maybe a mile, until it stopped surging and simply died. LUCKILY i was already planning on exiting there, so I coasted off, and swore loudly. Perhaps out of fuel, I thought. So I put the petcock on PRI, and let her rip.

Started immediately. Put it over to reserve, and got to the gas station with no issues. Looked in the tank, probably 4/5 full. Didn't add any, left it on reserve, pulled back on the interstate. Same situation, seems to start wavering after about a mile, and kills the engine near the second, going 75.

I took a different road on the way home and went 65, no issues besides a barely noticeable wavering in the power and the tach. Didn't kill at 65 though, and the issue was less pronounced.

On a side note, while adding the o-ring under the c-clamp did improve the strength of the idle when the bike was warm, it's still too low, and I can't convince it to come higher with the idle adjustment without making the RPM drop REALLY slow after a rev.

So, what'd I break? i'm assuming this is something to do with the carb, but I have no idea where to start.

Really appreciate any insight you can give!!

[Atesz792]

The highway issues could be caused by a partially blocked (dirty) gas cap vent, I'd think.

[bombsquad83]

Does leaving it on prime fix the problem?  If so, then it's your petcock. 

I'm not sure what you are talking about with the o-ring under the c-clamp.  What do you mean by c-clamp?  Are you talking about the e-clip on the needle?  So basically you are shimming the needle up?

If the petcock on prime doesn't solve it, then you will want to check your float height.  Your symptoms kinda sound like your floats might be set too high and not allowing enough fuel to flow.  Did you modify the float height when you replace the needle valve seat o-ring?

[mennobike]

Sorry for the late response, been busy! (headed overseas for a year in a week) yeah, I'm talking about shimming the needle up   (read somewhere on this forum that 1 o-ring up was the ideal setting for stock). I have adjusted float height, leaving it on prime did NOT fix the problem. When I finally get back to the bike I'll adjust the float heights again and see if that fixes it. So you're thinking the floats are too high (fuel level too high in the bowls, that is)? Which causes fuel to flow badly?

Thanks so much for the help!

[bombsquad83]

Actually if the floats are set too high (with the carbs upsidedown as they should be when float height is measured directly), then the fuel level will be too low because the float will cut off the flow too early.  Best way to check float height is to check the actual fuel level in the bowl with a clear tube to the carb bowl drain.  Put the petcock on PRIME, and check the fuel level in the tube.  It should rise to be even with the float bowl gasket.  Any lower and your floats are set too high, and any higher and they are not sealing or set too low.

If setting to prime didn't solve the problem, then you should check your gas cap vent. 

http://gstwins.com/gsboard/index.php?topic=41061.msg461590

[mennobike]

If anything, my floats are too low (when the carb's upside down that is). I'll check the vent.

Thanks again!

[RedMark]

Are you sure it wasn't just wind gusts?

Had this happen a couple of times on the highways, at different speeds ranging from 80KMH-140KMH.

Wouldn't this be re-creatable by placing the bike in neutral and holding the throttle at 6000RPM?