Bike cuts gas at top speed (165 km/h)

[danysan2002]

Hello there! My name's Daniel, I've just joined this forum because I read a lot of stuff about GS's around here and I have a small issue that I hope I'll be able to solve.

I've just bought a GS500E a few days ago, 1995, 31.000 km, new battery, new oil, everything runs smoothly, the bike pulls really nice through all the gears, it starts perfectly cold/hot, everything good. I've made a 350km trip this weekend and I've experienced a slight issue.

I've taken it on the highway too and I've tried to go for a top speed (it's not my first bike, I had a 125 for 2,5 years and rode many others in the 5 years that I have my licence, so no worries about pushing it just a few days after getting it ). After it gets around 160 km/h, it stops pulling and then it's like the gas cuts off. If I try to give it gas, the RPM drop and it doesn't pull. I have to leave it for 5-10 seconds and then it starts running and pulling normally. It happened 3-4 times, one time I was going downhill accelerating, again at around 160-165 km/h. Could it be something with the spark plugs? I haven't changed them, but the bike runs and pulls really good until 160, in every gear.

Also, the bike is imported from Germany and in the paper it says it has 25kw, but it pulls pretty hard so I don't think it's limited (there are a lot of bikes in Germany with 25kw in the papers, but not limited). It even lifted the front wheel when I tried to quickly pass a crossroad in the first gear, launched it at around 5-6k rpm from standstill (brought a smile on my face lol ). So I don't think it is limited, but who knows, maybe that's the problem and it's only limited on top speed?

It's not an important issue, but I'd like to solve it for my peace of mind, I like to have it in the best condition possible, without any issues. I've already contacted a service in my city and I'm waiting for a reply with an opinion, but maybe someone from around here experienced the same problem and has some advice.

Thank you for you time! 

[Watcher]

There are two primary causes to this that I would be willing to put money on.

Either the filter-side intake boots aren't sealing properly and at high speeds there's too much air intake.  Wind speed can force air in this way.

Or the carb breather hose is improperly routed, and wind into or across this line can cause issues with fuel delivery.  This line should be routed up and over the airbox and terminate between the airbox and battery.

[danysan2002]

Thank you for your reply!

If those boots weren't sealing properly, shouldn't the bike have other running problems, or just at top speed where there is enough air pressure to go past them?

[Watcher]

Pretty much just at that speed there would be an issue.

You could run the bike with no air filter at all.  It's not a good idea, and would require a rejet, but it can be done.

[pliskin]

I've had that happen on another bike before. The gas cap vent valve was stuck creating a vacuum in the tank thus starving the bike of gas. The first time I pulled over and gave it a minute then it restarted fine. 2nd time it died on me I popped the gas cap open relieving the pressure and it started right up. That's how I figured out it was the cap. I don't know if the GS has the same type of vent in the cap though.

[Watcher]

The GS has a vent tube from the cap that redirects down through the tank and exits underneath.  There's a hose there as well, it should also be routed behind the airbox like the carb vent.

[danysan2002]

I've just left it in a service, someone who had the same bike and the same problem told me it's most likely limited and the 25kw kit should be removed. I've told the guys there I wanted to do that, clean and adjust the carbs and also buy a carb repair kit since if you don't change those small bits (I don't know their english name), the bike will run poorly at low RPM''s and in general.

I'll wait to see what's happening with the servicing and I'll update with some progress.

[J_Walker]

Carb breather hose, you can attach a small air filter to it, like they use on pitbikes, or a piece of open cell filter foam ziptied to it, [what I did]