GS500 High Speed Fuel Starvation

[Nipedley]

Hi guys,

Been chasing this gremlin for a little while now - 1989 GS500e

What it does: I can accelerate up to 90mph no problem at all (and no doubt would go faster), but if I hold my speed over between 67 - 72 mph depending on how the bike is feeling that day, for more than 15 - 20 seconds, it will start to pull back (that running out of fuel feeling), start to bunny hop a bit, then slowly decelerate while the throttle makes no effect at all - just like running out of fuel. It then refuses to start until I 'PRIME' it for a while, and then we can take off. So, I presume the issue is lack of fuel

What I've done: Cleaned the carbs, twice. New float valve and seat, new fuel lines (not kinked), new fuel filter (and also ran without filter, no change). Cleaned fuel tap. Removed petcock from tank and cleaned (filter not like new but looks like it would flow fine).
I discovered as per a previous post I have large chunks of old fuel tank liner inside my fuel tank that I am unable to remove, but they shouldn't be fouling the petcock filter or flow I would seriously doubt.

So I am left with: Still dirty carbs (unlikely), faulty fuel tap? (vacuum operated), or the large chunks of old fuel tank liner are causing a problem

What do you think, or can you guys suggest anything else?

Ta very muchly

[J.M.]

Is it an older gs500? Run the bike on prime and see if the issue goes away.

If it does, fit the petcock from a newer bike - 2004 onwards.
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2004 R1 & 2018 XSR900

[Nipedley]

Sorry yeah it's a 1989. Prime seems to help slightly but the problem is still there, I can maybe get an extra 5mph cruising speed though before the problem exhibits itself.

I actually bought a newer fuel tap but unfortunately it wouldn't fit onto the 1989's mounting bracket as it was fouling on other parts

[J.M.]

Could you not attempt to make a plate, such that you can mount the newer fuel tap without fouling any parts?

The issue is supposedly that at high speeds, there isn't enough vacuum to draw the fuel that the bike needs for that speed. Running on prime (as prime is gravity fed only, no control via vacuum) usually resolves these issues and makes the bike run more nicely at high speeds.

The new design of petcock (2004 onwards) was supposedly better for this. I know my 2004 GS500 didn't suffer from this at high speed.

GSTwins has multiple things on it:
https://gstwins.com/gsboard/index.php?topic=19363.0
https://gstwins.com/gsboard/index.php?topic=36958.0
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2004 R1 & 2018 XSR900

[temeluchus]

Did you replace the vacuum hose going to the tap? is the vacuum hose clamped at both ends?
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Some shite cruiser. Now with guns and FREEDOM!

[Nipedley]

Yeah I also replaced the vacuum hose though the original one was airtight, it is clamped on the carb end and the tap end is on so tight it'll probably never come off again

I do suspect the fuel tap to be honest.. I opened up the old tap and cleaned it out, and it seemed to be okay, but I can't think what else it can be. Unless the tank petcock is clogged underneath the filter somehow.

I think my plan of action is: try a different tap, and take the tank petcock out again, remove the filter and blow that through with compressed air as well (I blew it through from one outlet to the other, but with the petcock itself closed)

[temeluchus]

Check the vents in the fuel cap are also clear.
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Some shite cruiser. Now with guns and FREEDOM!

[Coxyzxr]

[Nipedley]

Thank you guys I read on GStwins a bit more about the vacuum part of the tap, apparently there's supposed to be some sort of check valve in there I remember mine being a bent gooey mess, so that may well be the problem

Got a NOS tap on the way that will fit, going to blow through the tank petcock with compressed air and also going to clean out the tank cap vent's - if that doesn't do it, well... I'll probably be back for more suggestions

Thanks everyone!

[mysterious_rider]

i replaced the fuel lines once on my gs, made them a few extra inches long, would barely run properly. Make sure they are dead short! Or it will starve.
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KAWASAKIIIII