GStwin.com GS500 Message Forum

my bike is a 1990 and it was having a problem vapor locking... did it every 20 miles to the tee. fixed it, found out it was my tank bag killing the air intake on the top.

well now i havent had any trouble for almost 1k miles and today it starts bucking me again. doesnt do it till about 70mph when im on the highway.

i tried to troubleshoot solo and took the tank off, found a hose underneath that was a bit crimped.. seemed like a breather one. i replaced it because it was the wrong size anyways.  anyways, i put it back together and now it acts as if it has no gas and dies in the "On" position and the "reserve" position, but if i put it in the "prime" position it runs as if its fine...

any ideas.. tips?

P.s. any1 know where to get pirelli sport demons cheap? my challenger tires are AWFUL! :x

thanks for input in advance! :thumb:

on a side note the on and off valve underneath the gas tank... i think i might have turned it a quarter turn the wrong way, meaning it would be 180 degrees from the on position.. is it possible it would act that way with the valve in the wrong position or would it get no gas at all?

any pics of what the valve should look like when its in the on position... didnt see any arrows :dunno:

possibly the diaphragm of your vacuum operated fuel petcock is damaged.  the 'res' and 'on' positions are both vacuum operated, so when the engine is not running fuel is not flowing.  but the prime position bypasses the diaphragm so you can fill up the carbs after draining, cleaning, etc. without running the battery down.  oh, and there are a few fuel petcocks on ebay.  please don't bid on the one for the tank, i'm currently bidding on it.  good luck.

Quote from: ppriderany pics of what the valve should look like when its in the on position... didnt see any arrows :dunno:

Here you go.  (Hope it's not too dark on your monitor!)



The ON position is with the slot VERTICAL.  Turning it to the horizontal position (in EITHER direction) turns it OFF.  Any position between vertical and horizontal means it's partially closed.

BTW, you can't turn it further than 90 degrees from vertical in either direction.  That's as far as it will go.

I'm pretty sure that the hose coming off the bottom of the tank is just the overflow, so that shouldn't cause any problems.

If the tank petcock was turned slightly, its possible that it could be cutting off some fuel flow. I don't think that it would start acting up all of a sudden, though.

What did you do to solve your problem with the tank bag? One thing that comes to mind is that the wind at 70mph could be pushing down on the bag enough to cover the vents. If thats the case, then maybe making some sort of brace or spacer to make sure that the vent wouldn't be blocked would be in order.

Do you have fuel starvation issues at other times than when you are running at 70mph? Are you sure that its still vapor lock? Do you have problems with fuel starvation at hwy speeds when you aren't using your tank bag?

Are you sure that  you have the vacuum hose hooked up right? When the frame petcock is turned to "ON" the vacuum from the engine allows fuel to flow. "PRI" turns the fuel flow on all the time, regardless of vacuum.

Quote from: pprider...........any pics of what the valve should look like when its in the on position... didnt see any arrows :dunno:

Seventh post down in this topic.

:cheers:

so what is needed to fix the vapor lock?  i have the same issue with fuel starvation at any given speed however mine could work for days on ON then all the sudden act up.  then i switch to PRI and all is well.  this diaphragm that was mentioned, is there any way to get to it to see if its damaged?  also how do i check the vent?

Quote from: ppriderany1 know where to get pirelli sport demons cheap?

I just got my brand-new 2005 Chaparral catalog in the mail today.  I haven't sat down to run the numbers from umpteen different vendors yet, but since I'm looking at the book now anyway...

Pirelli Sport Demon       Bias Sport Touring Tire

FRONT                PART #           PRICE
110/70H-17        329-1342900        $101.99
120/70H-17        329-1342500        $106.99


REAR                 PART #           PRICE
130/70H-17        329-1343100        $108.99
130/80H-17        329-1343200        $116.99
140/70H-17        329-1403900        $115.99
140/80H-17        329-1342600        $116.99
150/70H-17        329-1342700        $126.99

ok i understand that the diaphragm is a possibility but we didnt touch the upper area of the carbs, why so suddenly would it have severe problems.

"I'm pretty sure that the hose coming off the bottom of the tank is just the overflow" can any1 confirm this? i thought it was a line for sucking air...

the tank petcock is definitely vertical so i guess that isnt the problem...

as for the tank bag solution that was easy i took it off lol, it wasnt mine anyways it was a friends who was bikeless who now does have a bike so i dont use one anymore anyways...

as for the reasoning that it was the cause... i made space pulled it up and it never locked again once it was removed same thing, so yes it was strapped down so hard it blocked the air...

before i removed the tank i only had problems at 70mph+ now i cant be in any gear at any speed in the on posicion or the reserve position...


GOAT which hose are you speaking of when you say the "vacuum hose"?

o and kerry add www.ronayers.com to your price list...

SPORT DEMON 120/70V17 FRONT         $78.87

SPORT DEMON 140/70H17 REAR           $87.95

Quote from: ppriderok i understand that the diaphragm is a possibility but we didnt touch the upper area of the carbs, why so suddenly would it have severe problems.

They were talking about the diaphragm in the ON / RES / PRI fuel selector switch.  With a 15-year-old bike, who knows?  Maybe it has gotten brittle or something....

Quote from: pprider"I'm pretty sure that the hose coming off the bottom of the tank is just the overflow" can any1 confirm this? i thought it was a line for sucking air...

See this old post.  Be sure to follow the link and examine the diagram.

Quote from: ppriderGOAT which hose are you speaking of when you say the "vacuum hose"?

There is a thin hose which goes between the inboard side of the left carburetor and the most-inboard spout on the ON / RES / PRI fuel switch.  The vacuum pulse transmitted through that hose is what operates the diaphragm in the switch and allows fuel to pass through to the carbs (in the ON and RES positions) .

Neither the diaphragm nor the vacuum hose are involved when you switch to PRIme.  That's why goat and weazelnoze pointed you in those directions.

Quote from: ppridero and kerry add www.ronayers.com to your price list...

SPORT DEMON 120/70V17 FRONT         $78.87

SPORT DEMON 140/70H17 REAR           $87.95

Hey, thanks!  :thumb:

well i definitely know part of the problem now... the hose coming off the tank was crimped and i thought that was a air intake hose... so i switched it with the one on the upper  t-connection... puttin the smaller hose on that T connection from  the diagram i can tell now that the bigger hose definetly needs to go back to the upper T section