Cutting out at 70-80mph - Any ideas??

[coreyadcook]

Hi guys,

I'm new to the forum so sorry if this has been a previous topic.

I have an old GS500E from 1991. I bought it off an old Spanish man who used it as a commuter bike in Barcelona. It has been a great little runner and I've not needed to do anything to it but replace the battery. she's starting to get close to the end of this lucky period I fear as lots of oil spots appearing, but I can handle that.

However lately it has started to cut out when doing 70-80mph on the motorway, with a feel as if I'm running out of fuel. This stops immediately when I switch the fuel chicken to prime. This is the only time it does it. When I'm driving round town at lower speeds the bike has no issues running on normal fuel setting.

I have cleaned out the float chamber of the carbs, there was a bit of rust, as the bike does sit around unridden for long periods of time, but it wasn't as bad as i thought it would be. But still the issue comes back when I am out on the motorway.

Any ideas?? is maybe the timing out on the carbs?

My mechanic skills are basic entry level stuff!

thanks for any advice.

Corey

[DrtRydr23]

If there is rust in the carb then there is almost certainly rust in the tank.  For it to get to your carbs it has got to go through the filter that is in the tank.  I'd guess your tank filter is partially clogged with rust and restricting your fuel flow at higher loads.  It could also be debris in your main fuel line, since that is where you run it most, but the reserve line is not (or at least less) clogged.

Short term fix would be to pull the main fuel line between the tank and frame mounted petcock and try to clear it out.  You'll need to eventually get into the tank and see if it's rusty and remedy that situation though,or it will likely recur.  Or you could just run it on reserve until that circuit starts acting up too (I guess that is the actual short term fix).

[coreyadcook]

thanks. It does the same thing on the reserve setting as well. So it might be both the main and reserve lines are clogged. I've already taken the fuel filter like thing out of the tank and checked that to see if there was rust build up around that, but there wasn't. So I'm hoping that the rust in the tank isn't too bad. although I can see small patches when I look into the tank.

thanks again. I'll try replacing both fuel lines to start.

[Meuryn]

My '02 GS had a similar problem, it felt like it was stuttering / running out of fuel at 60-80mph.

It turned out I had routed the carb vent hose incorrectly, so that it was exposed to the wind - I guess when I got to motorway speeds enough air was forced into the carbs through the exposed hose to create a positive pressure, affecting the fuelling. I routed the hose into a more sheltered location (alongside the battery I think), and 'insulated' the end with some wire wool, again to prevent motorway speed winds from affecting it. After doing that, the problem was no more.

Not sure whether this is your problem exactly, mine showed only a small amount of improvement switched to prime or with the choke on, but it's easy to do and worth a shot!

[Badot]

If it cuts out on 'on' and reserve, but not prime -- that generally means that there's an issue with the vacuum operation in your frame mounted petcock.

Check your vacuum line from the petcock to your carb, and if that's in good condition and unobstructed it's probably some form of leak/stickiness within the petcock.

You can test/convert the frame mounted petcock to a manual operated petcock by removing the cover plate and rotating the valve core, lever, and cover plate half a turn. Note, this reverses your 'on' and 'reserve' lines from where it's indicated (or you can just swap the hoses) and the 'pri' is now your 'off' position.

[DrtRydr23]

If it cuts out on 'on' and reserve, but not prime -- that generally means that there's an issue with the vacuum operation in your frame mounted petcock.

Check your vacuum line from the petcock to your carb, and if that's in good condition and unobstructed it's probably some form of leak/stickiness within the petcock.

You can test/convert the frame mounted petcock to a manual operated petcock by removing the cover plate and rotating the valve core, lever, and cover plate half a turn. Note, this reverses your 'on' and 'reserve' lines from where it's indicated (or you can just swap the hoses) and the 'pri' is now your 'off' position.

Yeah, the frame petcock is the next likely culprit then.  You could have debris in the vacuum fed area of the petcock.  The prime setting runs off the reserve line I'm pretty sure so the line is probably okay.  Try disassembling the frame petcock and cleaning it up. There is a little spring and vacuum diaphragm in there.  Pay attention to how they come out, and be careful with the diaphragm.  Also check the vacuum line as Badot has suggested.

[coreyadcook]

thanks guy, gonna try all of these! I'll let you know

[coreyadcook]

ok, so I didn't get time to do anything other than re route the carb vent hose before I had to go on a journey of about an hour on the motorway. The good news is that the re route of the carb hose seemed to fix the issue. I had no cut outs at 70-80mph, apart from one slight splutter, but it soon sorted itself out.

I am sure the fuel hoses all need changing and the petcock changing or adjusting. but good to know a simple fix for the short term.

thanks for all the advice