Hi, fellows.
I don't come here too often lately but I need your help.

My beloved '90 blue GS500 with more than 80.000 miles in the clock started to fuel starve.
Changing to PRI made some difference but didn't totally resolve the issue so:
- I totally refurbished the carbs (New fload needle, and all o-rings were replaced and also jets were replaced, all original Mikuni original Suzuki parts). Checked float height and everything was ok.
- I replaced the fuel petcock by a '05 model.
- I made the valve job and all was under specs.

Still starving.

- Changed to another set of carbs I had.

Still starving.

Last week after a ride it started to idle very poor.

- With cold engine it barely could idle.
- With warm engine, it was idling at 3000RPM, somehow like it was running lean.

Engine intake boots were replaced in September when I serviced it. I noticed then that there was a severe lack of compression in the left cylinder.

May this cause problems with the vacuum activated petcock?

Changing to PRI makes difference, but still starves at 5000RPM and 6th gear.

What is this instable idling? And popping? Did  burn exhaust valves? I've noticed that spark plugs rubber are cooked due to high temp. I bet that the bike ran lean and things are getting worse.

Any considerations? Thoughts?

Is this the start of the end?

I'm afraid of installing a new head and burn it again. Can't guess what's hapening here. Replaced carbs, petcock...

Do not give up! Do a compression check. How much oil does it burn? A top-end rebuild is not all that expensive if you do it yourself. Keep スズキ on the road.

as long as it never started making a knocking sound, [sounds like marbles in the engine case] you can still save it, probably valves not adjusted? could be rings I suppose? maybe the 05 petcock valve/switch is turned around the wrong direction? Did you check this? just pull off the face plate, it should slide right out.

need more details what lead you to replacing all this stuff to begin with.

Did you check the rest of the fuel path from start to finish?  ie rust in tank, tank filter, lines?

Sorry for the delay.

Back with some news. In fact, good news, since the bike is back to the road, but the issue is not totally solved.


I installed the original carbs ('90), that are severely worn out but they worked until now, so...
I installed new rubber seals, with new rubber seals everywhere, and I installed the original float needle, propperly cleaned with a professional ammonic carb cleaner. Even I installed a new rubber ring under the slingshot valve guide (not available from Suzuki)



Changes:

- This time I replaced the 125 main jet with a 122.5 main jet. Stock was 120. Original Mikuni.

- This time I set the mixture screw to 2 turns out instead of 3.

- I replaced the fuel vacuum tube, installed the 2005 petcock, and installed new fuel lines 7.5mm internal Diameter, polyvinyl instead of silicon "fuel hose".

- Replaced both spark plugs caps with original NGK with 5K resistor.

- Checked the compression issue: could be both rings or valve seats. To know where's the problem, an easy test is to pour some oil on the piston, place the spark plug, and recheck compression. Oil will prevent the air from leaking through the rings, but will not prevent it from leaking through the valve seats (take care not to overfill with to much oil). Result: great compression after oil drop. Hence my valves were sealing well. The compression leak was due to ring wearing.... but.... while doing some compression tests, I heard a "clack" and then, I could notice a somehow increased compression on left cylinder. 
I asked a local mechanic and he said that I may have sticky rings on left piston. I've just read that this also happened to someone in this forum. So this makes sense.

After reinstalling all, no more fuel starvation (well, in fact I had, but it was that the carb breather tube, that got caught between the airbox and some other part, and the bike ran very very poor with issues with wind, bumps and some very weirg behaving). Just take care when reinstalling.

The bike works, I got over 100MPH, so there is some power, mileage is good (over 200Miles/tank) but there's still something that does not work well. It idles at 1500RPM, but this is not stable, starts to missfire, starts to slow down until it stalls.

Tried to open to set the air/fuel mixture to 3, but the idle turns to very unstable: after revving it it does not slowdown to 1500RPM but 3000. If I adjust idle screw, I can't get it to idle to 1500RPM, either idles at 3000 or stalls. So, I close the air/Fuel mixture screw to 2.

Beyond this, it works reasonably well. 80500Miles on the counter. Oil consumpion is about 2qt/1000Miles, more than a 2-strokes.

Cold is not the best friend of a worn out engine and battery is not in the best shape.

Could this be part of the problem? http://gstwins.com/gsboard/index.php?topic=72142.0

that part there was just a vaccum cap which may or may not work but truth is a top end rebuild is very easy and not expensive, i would say put old carbs back on.

and you have done a compression test and you have said its down on compression so in that case if you are going to do the rings or new pistons you might as well fit new valves, and valve seals honestly for 80k the engine has done amazingly well, you have replaced alot allready. so its worth fitting new rings and valves and seals as you know the engine and it has been looked over by yourself,

also check the exhaust for any leaks

That idle problem is because you have the "idle speed" (throttle plate stop) set too high. Don't try and make it idle at 1500. Set it to 3 turns out then close the throttle plate stop ("idle speed knob") until it idles at about 1200 rpm when FULLY WARMED UP. See my blog post about this for more details.

Do this right and you'll fix the hanging idle (3K). it's a common problem due to setting the idle speed too high.

If you have bad compression due to rings then it probably will still run ok but just burn a lot of oil and be low on power. If you had valve sealing problems then you'd get running issues with the engine hot (won't idle, dies below 3K rpm, etc.). I agree a top end rebuild is reasonably easy. If you are really confident about the valve condition then a ball hone and some rings plus a weekend's worth of wrenching will get the rings sorted and that's cheap. You can always pull the head again later and do the valves.

I'll read your comments.
I just found this gold:
https://joshkarnes.blogspot.com.es/2017/09/fixing-common-gs500-carburetor-issues.html?m=1

Quote from: user11235813 on January 10, 2018, 03:55:52 PM
Could this be part of the problem? http://gstwins.com/gsboard/index.php?topic=72142.0

Checked, they are tight. I even sprayed wd40 over caps, boots, and couldn't find a leak.

Quote from: mr72 on January 12, 2018, 11:51:59 AM
That idle problem is because you have the "idle speed" (throttle plate stop) set too high. Don't try and make it idle at 1500. Set it to 3 turns out then close the throttle plate stop ("idle speed knob") until it idles at about 1200 rpm when FULLY WARMED UP. See my blog post about this for more details.

Where is your blog post? I just found some golden post (see a couple posts up). Going to give it a try.

About the top end rebuild, I wasn't able to find replacement pistons. Oversized are no longer available in Spain. Original ones are extremely expensive. A ring set is over $120 per piston in Spain. Head honing is about $120 plus new valve guides and seals. Camshaft chain is reaching end of life. I expect oil pump reaching end of life too. Gasket kits are not cheap too. I can hear some rod click (piston bearing and/or crackshaft-rod pad wearing). I'm not sure if it is a good deal to try to keep this engine running. Maybe I can find a spare engine with less mileage.

Quote from: marc on January 14, 2018, 01:59:20 AM

Quote from: mr72 on January 12, 2018, 11:51:59 AM
That idle problem is because you have the "idle speed" (throttle plate stop) set too high. Don't try and make it idle at 1500. Set it to 3 turns out then close the throttle plate stop ("idle speed knob") until it idles at about 1200 rpm when FULLY WARMED UP. See my blog post about this for more details.

Where is your blog post? I just found some golden post (see a couple posts up). Going to give it a try.

That was the blog post I was talking about.

BTW you likely won't need pistons. Can't you get rings from an online source? If you are doing the head you'll need to machine the valves (or replace) and replace the valve seats too. Not worth the effort for just guides and seals. A little oil leaking in is not affecting compression.