GS500 not running well

[curtis]

Okay so I'm trying to help my friend diagnose a problem with his bike. It starts and runs, but seems like the left cylinder isn't running properly. We spark tested both cylinders and seem to have spark. When the left plug wire is removed the bike runs similarly, but when the right wire is removed the engine dies completely. When the bike is running it will go up to about 4000rpm then starts to sputter and won't go higher without a lot more throttle. Also when I let off the throttle it doesn't drop down right away like it should. What steps can we take from here to diagnose the problem?

[Big Rich]

I'm guessing this is an older GS, that was neglected? I'd vote for a vacuum leak- around the intake boots (and / or the oring under them), or a leak from the vacuum line between the left carb & frame petcock.

If it is an older bike, there's actually a good bit of stuff that needs fixed / checked. Cleaning out the carbs, setting float height, check the valve clearances (very important!), etc, etc.

[nicksmo]

This would be the friend here.

Most definitely.  Picked it up for cheap off a guy who found it under a deck at an estate sale.  The bike is a '96 that has definitely been beat up a bit.

Ah good call!  I've come across a few ways to diagnose vacuum leaks (releasing flammable stuff around suspected areas and listening for engine surging).  Do you have a preferred method?   

[Big Rich]

It doesn't have to be flammable fyi. But a propane torch is what I've used, along with wd40 (wd40 will cause bogging).

Even a close visual inspection might work. The intake boots can appear in good shape, but aren't pliable anymore. And the orings underneath them should be round, not squished flat.

Honestly, a 21 year old neglected bike could use some new parts: any rubber (fuel lines, intake boots) and brake lines for starters.

[Watcher]

I never liked the WD-40 method, it makes the carbs all messy and tends to stick around if not cleaned right away.

A spray bottle of water serves the same purpose and may actually leave the carbs cleaner than before.


Whatever you use, make sure it's not brake-clean unless you want to destroy everything made of rubber.

[nicksmo]

An odd day with the bike.

We got the bike going and did sea foam (aerosol) clean of the top end of the bike.  We made sure to check each piston and see that it responded equally to the throttle.  We added some hose clamps to the carb-petcock line and did the WD40 test (the propane torch has gone AWOL) and found no vacuum leaks.  Overall we concluded the sea foam did help as the bike ran better after.

We then proceeded to adjust the floats to get a good idle of 1200ish with good throttle response and no weird bogging at the 4000rpm mark.  We confirmed again that each piston was working equally well by unplugging the plugs one at a time and seeing if the response was the same at idle and when revving.  At this point, we were feeling pretty optimistic about the bike as it was holding a strong idle and was responding well and consistently to the throttle.

Then, we put it into gear and took it down the block.  Within about half a block we got the weird bogging again where it felt like someone was yanking the throttle cable.  We tried then running it off prime which got us a bit further without the bogging, but it did eventually occur.  When we parked it, the idle was way up around 3000, and we could not seem to get it down again.  The choke did not seem to have much of an effect on the idle either.  We tried running it off each piston individually and we did find the left piston was weaker and required feathering to keep up, but the bike could be kept running off the one with just a slight amount of throttle, though it was backfiring every so often.

We are a bit lost as to where to go next.  We originally were thinking a fuel issue, but we have no idea why it behaved so differently after we put it in gear.  We are now playing around with the idea of a valve issue, with the backfiring being engine knocking but this is really throwing us for a whirl.

[Watcher]

I would check float height next.

The engine idles and revs fine but struggles under load, seems like less than ideal fuel delivery.  Jets clogged, floats too high, something like that.

[nicksmo]

 The thing that is really throwing us is the fact that it behaves differently now that we rode it around the block.  I'm going to do another vacuum check tomorrow and then I think proceed to valves unless someone has another suggestion.  None of us are that experienced with bikes but between the forums and the three of us we are quite sure the fuel supply is not the problem.  The carbs have been thoroughly cleaned and set to the upper end of factory specs (high altitude) giving an idle right around the 1200 mark.  The petcocks and all gas lines and fuel tank vents have been cleaned.  Again, it is possible we messed something up but if we go into the carbs again I have no idea what to look for as we have checked all the boxes we know of. 

[nicksmo]

To clarify, we checked and set the float heights to get the idle.