GStwin.com GS500 Message Forum

im riding my bike  and pick up speed(70+) then all of a sudden the bike loses power and shuts off on me in the middle of traffic!!!!!!........its definitely not getting gas at this point because to restart it i have to put it into prime and it will drive just fine again.....is it bad to drive in the prime position???....the carbs are clean and the floats are good to go maybe an 1/8 under gasket...new air filter and fuel filter and all fuel lines replaced and in working condition...im thinking petcock not good vacuum cutting off fuel flow for some reason when in on position... :dunno_black:.....


one more thing...the idle is really bugged out lately, revving really high at stops and stuff like thaT,constantly adjusting screw to keep it under control

Prime is fine- just turn it to on when you park it.

Is your choke mechanism stuck at all?

i think the choke is fine ill have to check though good point.....i read here if you ride on prime it could overflow into the airbox and get into oil even while your riding.... recently left the bike in prime while it wasnt running  and it poured gas like crazy had to do an oil change and all...same concept????


side note:
also noticed when i ride in prime and park it...(turning petcock to on when bike is off)... little bit of somethin is coming from the drain line on the air box...thinkin its gas spraying on the air filter from the carbs and mixing with oil on it and dripping out..i checked my float heights though and they were good....they shouldnt be leakin or anything the pins and o rings were good to go....cant figure this out

I think a good troubleshooting checklist would be to:

1. Check fuel lines for sharp bends/kinks/squeeze spots.
2. Check inline fuel filter for proper flow if you have one.
3. Check in-tank petcock mesh filter.
4. Check frame mounted petcock for proper flow.
5. Assuming everything else checked out, check your carbs. At this point, you can be 99% sure that the problem is a dirty carb. Possibly, partially clogged main jet(s)

its crazy because the bike runs fine until i open it up on the parkway then it doesnt wanna get gas in the on position...as soon as i change it to prime the bike runs fine all the way through the various gears and speeds

Do a search on here for WOT petcock. You will find it's a common problem.

Technically, running on prime shouldn't get gas in your oil (since it would flood the combustion chamber first and bog down the engine). And technically, the floats should stop any gas from entering the carbs once the bowls are full- but that's not always how it works.

Quote from: Big Rich on April 14, 2011, 09:49:48 PM
Technically, running on prime shouldn't get gas in your oil (since it would flood the combustion chamber first and bog down the engine). And technically, the floats should stop any gas from entering the carbs once the bowls are full- but that's not always how it works.

:thumb:

I usually leave mine on prime all the time because I forget to turn it. I never have flooding or similar issues. If your carbs are in good health, you shouldn't get any such issues.

The vacuum petcock (frame petcock) just doesn't flow enough fuel to feed the engine at sustained high RPM. You will find this is a common problem above 70 or 75mph.  You are not alone, my friend.  Searching the forums on this site will turn up lots of people with the same complaint.  A few have modified the petcock, others just replace it with something different. 

It's possible that you have a partly clogged/blocked fuel line or filter (if one has been added to your bike). But if it's running fine everywhere except above 70mph then I don't think you have any particular problem except the "usual" fuel starvation issue.
:cheers:

Dont overlook the vacuum pipe that leads from the LH carb to the back of the valve. Make sure it has a snug fit at each end and that its not kinked. I have seen a few on older models that have over time become weakened and prone to collapsing in on themselves reducing the vacuum seen at the valve.

Wonky idle = Check your valve clearances, then check your carb synch.

Mine runs 80 MPH for 15 minutes on my work commute with stock fuel system essentially set in prime position without acting up.