Hi guys,

I'm stumped on this one.  I rejetted to 22.5/65/147.5 about 4 months ago, with a K&N Lunchbox and Yosh slip-on (stock headers).  Ran great.  All of a sudden, one day after a normal commute on a particuarly hot day, the bike started to hesitate with throttle application.  But the next day, even after it had cooled down, the bike had the same problems. 

Symptoms:

It idles just fine.  If you give it throttle slowly, it bogs.  Let go and it returns to idle.  Stay on throttle, and it dies.  If you roll on hard from idle, it dies.  If you give it some choke, it revs up to 5k, but still bogs.  It also knocks and pings while it's bogging, leading me to think it's running lean.  I opened the mix screws a bit, but that didn't help.

Suspicions:

1.  Bad gas
2.  Vacuum or air leak
3.  Dirty carbs
4.  Bad float height?
5.  ??

Troubleshooting steps

Same symptoms if I pull a plug wire, regardless of which side. 

I drained the tank and there was some weird crud in there.  Different viscosity than the gas, almost like oil.  So I filtered it out and put the gas back.  No change.

Sprayed carb cleaner all over carbs and on boots.  No change.

Sprayed carb cleaner into folds of air filter.  Some improvement, but only temporary.  Should I tear apart the carbs and clean them, on a regularly running bike with 12k on it?

The weird thing is that nothing changed on the bike between it running well and now.  Any thoughts?

Sounds like a classic case of a faulty vacuum petcock.Did try to run it on PRIme?Could be a vacuum leak at the carbs or intake boots.Try these if you believe it's a faulty petcock.

http://gstwins.com/gsboard/index.php?topic=19363.0

http://gstwins.com/gsboard/index.php?topic=36958.0

Yeah, no change on prime, reserve, or main. 

Tried spraying fluid on intake boots and carbs and petcock diaphragm, with no effect.

Quote from: sanjay on June 15, 2008, 07:29:58 PM
Yeah, no change on prime, reserve, or main. 

Tried spraying fluid on intake boots and carbs and petcock diaphragm, with no effect.

Could be that the valves are out of adjustment and the carbs aren't getting enough vacuum.Have you checked the carb float heights and balanced them?

http://video.google.com/videoplay?docid=-2015554469142545363&q=suzuki+gs500

http://cgi.stanford.edu/~sanjayd/gs500/Maintenance/FloatHeight

http://cgi.stanford.edu/~sanjayd/gs500/Maintenance/5CarbSync

Did a valve check at 8k and they were fine.  I haven't checked carb float heights, so I'll try that, too.

Valves should be checked ever 3000miles.Just wanna make sure we are using the same measurements.8k miles or 8 kilometers.either way I'd say it's time fur a valve check up.

I'm using miles here.  I thought a valve check was every 4k miles?  Also, there are no other valve-out-of-spec symptoms (noises and such).  I'm trying to figure out the most logical checks first, to avoid a full carb rebuild (or valve check).  Though I could just bite the bullet and do one.

Here's a video (.mp4, h264 format, so you'll need quicktime) where you can see how much throttle I'm applying. 

http://www.stanford.edu/~sanjayd/bike.mp4

Or, just audio.

http://www.stanford.edu/~sanjayd/bike.mp3

Sounds like it's not getting enough fuel or the plugs are going bad.I recently had a set of plugs go bad and they looked nearly new. 

Pics of plugs (left is left cyl, right is right cyl in both pics).  How do you read them if they're different front vs. back?

Looks like it's running rich.There nice and dry so there's not much if any oil burning thru the cylinders.Are you running 150 jets?If so try 142.5 or 145.I am running 142.5 and it's great.Also check this out for a spark plug guide.

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

I'm running 147.5's.  The thing is, the bike ran fine for 4 months with the same jets as now, with only maybe a few hundred miles on it.  So whatever happened, happened relatively suddenly.  That makes me doubt that it's crud in the carbs or a carb sync or a valve job....

Quote from: sanjay on June 15, 2008, 08:30:40 PM
I'm running 147.5's.  The thing is, the bike ran fine for 4 months with the same jets as now, with only maybe a few hundred miles on it.  So whatever happened, happened relatively suddenly.  That makes me doubt that it's crud in the carbs or a carb sync or a valve job....

I see.It doesn't take much for some crud to get knocked loose and end up in the carbs.Could still be bad plug(s).

Quote from: sanjay on June 15, 2008, 07:14:13 PM
All of a sudden, one day after a normal commute on a particuarly hot day, the bike started to hesitate with throttle application.  But the next day, even after it had cooled down, the bike had the same problems.

In my opinion, a sudden problem points to something stuck in the carbs (piece of rust from the tank etc) or something in the fuel (see below) - I doubt that valves etc would produce such dramatic problems so suddenly. What is the condition of the tank? (any internal rust?)

Quote
I drained the tank and there was some weird crud in there.  Different viscosity than the gas, almost like oil.  So I filtered it out and put the gas back.  No change.

Sounds like water in the gas tank. Was this weird crud sitting at the bottom? You drained the tank, but did you drain the carb float bowls too? Maybe you got some water in there? You sure you didn't top up with diesel? 

I would store the "filtered gas" in a container and get some fresh stuff to see if it helps.

WTF?!?!?!

Looks like water and some type of fuel additive settled out of the gas.

Indeed it did.  Took the carbs apart, gave them a thorough talking to, and cleaned all the fuel lines and petcock.  Runs like a dream!  Thanks for the help, guys! 

Still never figured out what that crap is.  Anyone know how to dispose of bad/dirty gasoline?

Thanks for the follow up!

If there's no gunk left in it, you can mix the bad gasoline with lawnmower fuel. Otherwise, they should take it wherever you can drop off oil motor oil. In Canada, I bring it to the local Canadian Tire.

It was nasty gunk, indeed.  Drained the tank and got as much out as I could.  The gunk is still in the fuel, so I'll try recycling at a lube shop.  Thanks again!

That is rust ... see it all the time. Even a clean looking tank manufactures rust in the crap pockets ... they dont call it crap pockets for nothing ...
Cool.
Buddha.