RE: Help Diagnosing Poor Running, 1990 GS500E

[Iolo]

I starting to ride again after a longish break, and have noticed my GS is running poorly. I'm looking for some help to troubleshoot it. The GS exhibits the following symptoms:

- GS starts with full choke (idles at 4000 rpm). However, once warm, with choke off, the bike's idle is some what inconsistent. It will initially idle, off choke at 1500 prm or so. However, rolling on / off the throttle, the bike will idle high at 4000 rpm. Adjusting the idle screw does not change this behavior.

- GS when in neutral, or when the clutched, will easily rev up through 9,000 - 10,000 rpm with throttle application.  However, when in gear, the bike will not rev past 5,000 - 6,000 rpm.

In terms of troubleshooting, changing the petcock from normal to prime does not result in different behavior. Also, I've checked the plugs and verified that both of them are sparking, though one cylinder appears to be running rich, while the other one is lean.

Given all of this, I suspect the bike is getting enough fuel, as the behavior does not change when the vacuum line is taken out of the equation (i.e. running in prime). As a result, I'm thinking either: 1) Air filter is dirty / clogged not letting enough air, and / or 2) the carb needs some cleaning / attention due to sitting.

Am I on the right track here? Should I be looking at something specifically?
 

[Armandorf]

vaccumm leaks? spray wd40 or something flammable around carb boots and check if the rpms rise.

You have "hanging idle", search for it.

also if you suspect cylinders runs differently , unplug the spark plug on each side and see if there is an obvious difference.
try backing off the idle speed screw till its better.

If the cylinders are way off, you have:
carb problems: stuck jets, vaccum leaks, poor valve adjustment or unsynchronised carbs( a lot).

With no load the engine spins up through the sky, even with bad carburetion, with load you can see the truth.
try.

[Iolo]

With no load the engine spins up through the sky, even with bad carburetion, with load you can see the truth.
try.

Well I have seen what happens under load; when in gear the engine won't rev out over 5000-6000 rpm. It will easily sweep up to the rev limit when in neutral, or no load is applied. Assuming the petcock is not the problem, as it still exhibits behavior when set to PRI, I tend to think something is wrong with the carb, as I have not opened it up for a clean / new seals since I got the bike over a decade ago.

I'll check for vacuum leaks and backing off the idle screw first though.

[Iolo]

Okay. I got the carb apart and found the pilot jets were plugged.  I ended up replacing the pilot jets with new ones. I also ended up replacing the needle valves, the O-rings on the floats, the small O-rings under the carb slide caps, both idle mixture screw assemblies, and the O-rings / gaskets in the carb boots. 

After doing all of this, and reinstalling the carbs, I got the bike to idle at 1500 rpm when warm, as it should. Note the following blog post was very helpful -

https://joshkarnes.blogspot.com/2017/09/fixing-common-gs500-carburetor-issues.html


For others dealing with carb issues, the advice about replacing "everything" was spot on.



 

[mr72]

Glad my blog post is still useful.

Can't believe it's been 6 years since I wrote that.

I thought these GS500s were tricky, but my scooters have been kicking my butt. I'm just used to fuel injection nowadays.

[Iolo]

I'd like to think its just practice / familiarity. Digging into the carbs was intimidating at first. The fact that the prior owner apparently replaced certain components at some point did not help. However, once I figured out a Dynojet kit was installed previously, things started to make a little more sense. I have a much better understanding of it now that I've been through the carbs once.

Anyways, thanks again for the blog post. It was especially useful, especially since the carb rebuild kit I sourced did not come with instructions, and labeling of said parts, of any kind. Between that and the jetting table on GSTwins, I was able to get my GS back on the road.
 

[Chisel]

Good effort having a crack at the carbs, I've got to do the same myself over the winter.

Not sure how sure proof this is but when I know the bike will be parked up for a while I turn the fuel off and let her run until she cuts out. My theory is that the fuel won't be sitting in the carb getting guncked up.

Any tips for the carbs? The guy I sold the bike to messed around with the floats valves, since buying it bk and him messing with the carbs it hasn't run properly

[Bluesmudge]

The GS500 came with a vacuum petcock so the fuel off truck doesn't work unless you have a aftermarket peacock.    The best trick is to ride it at least once a month.

[HPP8140]

I agree...there may be some edge cases where not practical, but I don't winterize...best trick for the carbs is to ride it or start it...don't let it sit long

[Bluesmudge]

Any tips for the carbs? The guy I sold the bike to messed around with the floats valves, since buying it bk and him messing with the carbs it hasn't run properly

Unless you have money to throw at dyno runs or another way to measure the actual air/fuel ratio, the best way to get a bike running that has an unknown history is to get everything back to stock, since that's a known running set-up. That means clean the carb, make sure all the jets and slide needle are stock and replace all rubber and consumable parts and check/set the float height to stock. Replace the intake boots/o-rings, replace all o-rings and gaskets in the carb, check/replace the fuel lines and vacuum lines, remove all jets/float valve and clean them if they are the correct size and functioning or replace them with the stock parts. If you are feeling ambitious, completely dissasemble the carb and dunk just the metal carb body in a carb dip for a day or two to fully clean out all the carb passages. Use a stock airbox with a Suzuki air filter and a stock exhaust. If you have a bike that isn't running right and doesn't have stock parts, that's like the difference between solving and algebra problem and a calculus problem. How do you solve for multiple variable? Could be the float height, could be a vacuum leak, could be clogged carb passages, could be some additive combination of the three, etc. Best to set it all back to stock so you know what you are dealing with.  Oh...also set the valve clearances and vacuum sync the carbs.

[Iolo]

Any tips for the carbs? The guy I sold the bike to messed around with the floats valves, since buying it bk and him messing with the carbs it hasn't run properly

If you have not been in the carbs before, I'd make sure you take pictures of the disassembly so you can be sure where everything goes when you need to put it back together again. Other than that -

1. Make sure you have carb rebuild kit on hand with all the o-rings you need, carb bowl gaskets, needle valves / valve seats, and floats.  Note - pay particular attention to the year that your GS is; the carbs on 2000 or later GSs are different from the earlier ones (3 Jets vs. 2).  You may also want to replace the carb intake manifold boot gaskets as well.

2. In addition to a carb rebuild kit, if you've never been in the carbs before, you may want to also have new idle jets on hand as well. If they are gummed up, they can very difficult to clean / clear. Also, the pilot jets, if you never removed them before (and said pilot jet have not been touched since the bike left the factory), they may need to be drilled out.

3. Many of the bolts / screws holding the carb together have Japanese JIS standard heads.Having the right screw drivers are handy, as the fasteners can strip out pretty easily. Alternatively, you can pick up a cheap set of screw drivers, and grind them down to fit.

4. Be careful when removing the jets, as they are soft brass, and can also easily strip out.

5. Keep carb cleaner away from any plastic or rubber parts. Pay particular care with the diaphragms; if they tear or develop holes they will also need to be replaced.

As for winterizing, I find that filling the bike with fuel + fuel stabilizer, and running it for a minute or two, is all I've really needed to do before letting the bike sit for a bit.  I do agree though, that starting the bike once every few weeks during the winter though, is a good idea.