I am trying to get my '97 running after sitting unused for almost 4 years.  I've cleaned the carbs and replaced the O-rings on the needle jet/pilot screws/float assembly, replaced the fuel lines, cleaned the frame petcock, cleaned and lined the tank using a Kreem kit, replaced the tank petcock (it was trashed), replaced the air filter, replaced the battery.

When I first fired it up, it idled well which I was happy about.  However, when I try to give it throttle it would stumble and cut out after 1/4 throttle pretty consistently.  It seems better when warmed up, but still there.  Then, while letting it warm up, it died while idling.  I put the frame petcock on PRI and it immediately idled better and had better throttle response.  It still stumbled after 1/4 throttle but would continue to rev.  Drivability is fair at best and it still kind of bogs at full throttle.  I tried spraying WD40 on the intake boots because they do have some thin cracking near the engine, but it did not effect anything.  I've had the carbs off again and checked the float height.

So here is my thinking:  when I cleaned the frame petcock I took the little back plate off and the little disk came out.  I was not sure which way to put it back in, so I guessed.  Also, when I was cleaning out the petcock, a lot of black gunk came out of the vacuum part of the petcock.  I also did not change the vacuum line, but was able to spray carb cleaner through it.  Is it possible that I installed the disk wrong or damaged something inside the frame petcock that is causing the vacuum problem?  If it even is a vacuum problem.

Also, the jetting is the same as when it was running before, and the only change that has been made is using a hiflo paper air filter instead of the uni that was in there before.  I am planning on checking that vacuum hose to make sure it is clear, and maybe cleaning that vacuum port on the carbs.  I didn't give it much attention when I cleaned the carbs since no fuel goes through and I didn't think it would be gummed up.

Any other suggestions?

Okay, so I checked the vacuum line from the carbs and it's definitely sucking.  Took petcock apart again made sure it was together correctly, which it seems to be.  If I suck air from the little vacuum inlet, I can hear/feel the diaphragm clicking back and forth a little.  All of my fuel pathways in the petcock seem clear (at least air will pass through them if I blow with the vacuum diaphragm thing off).

When I got it back together it did seem to be running a little better.  I was able to get it up to 80 mph indicated, though it still stumbles a bit at lower revs when I get above 1/4 throttle.  Also when I get WOT, it seems pretty flat.  Not as much "go" as I remember.  In top gear, I don't think WOT is doing much of anything near 70mph indicated.  I haven't tried a plug chop, but now I'm not sure if my slides are coming up correctly.  I think I'm okay on jetting.  It's 40/122.5, which ran pretty well before I let it sit.  I'm thinking of going up to 125 mains and see what kind of difference it makes.  Also, when I cleaned my carbs I sprayed cleaner on the slide diaphragms.  I've seen somewhere that carb cleaner can damage those diaphragms.

Any ideas?

Don't start messing with jetting just yet.  Remember that it ran okay before on the same jetting as it is, correct?

I'm very inclined to blame the air filter... I've read multiple cases on the forum that people start having conflicting mixture issues with that filter.  Maybe try your old air filter again if you still have it? Or order same old one/a new stock filter? 

- Bboy

My 02 stock air filter has a band of metal around the filter element an inch or so wide to reduce air flow.  I put 80k miles on a 97 GS with stock jets and was happy with it spring, summer, and fall but it really did need a jet adjustment for winter riding.

Would not be surprised if that hi-flow air filter wasn't the problem like BB suggests.  Why not just wrap a band of duck tape or something like that around it and give it a try, would enrich the mix a bit.

I always wonder when someone is messing around with the frame petcock to see if it's their problem why they don't just run a line from tank to carbs and take a ride to confirm whether that's the problem or not.

Quote from: BockinBboy on September 16, 2013, 06:26:48 AM
  Maybe try your old air filter again if you still have it? Or order same old one/a new stock filter? 

- Bboy

Quote from: gsJack on September 16, 2013, 07:24:33 AM
Why not just wrap a band of duck tape or something like that around it and give it a try, would enrich the mix a bit.

Yeah, there is a simple option too... 

- Bboy

I initially ran it with the old filter in there.  It was a Uni filter that I had oiled previously before letting the bike sit unused.  It was kind of falling apart at the mounting tabs so I trashed it and decided to go with a paper filter and the hiflo one was less expensive with positive reviews from the site where I purchased it.  It had the same issues with the Uni filter before I switched it up, but more pronounced.  The Hiflo seems to have helped.

I'll probably tape off some of the filter element and see if that changes anything.  I've got a bunch of main jets sitting around from previous tinkering, so it's not a huge deal for me to switch them out, but taping off the filter would be a good little test.

PS:  The hiflow filter design seems very similar to the stock filter that GSjack described.  I've always had the uni in before, so never saw the stock one.

Quote from: gsJack on September 16, 2013, 07:24:33 AM

I always wonder when someone is messing around with the frame petcock to see if it's their problem why they don't just run a line from tank to carbs and take a ride to confirm whether that's the problem or not.

I feel like that is basically the same thing as running it on prime.  If it runs well on prime than that would basically be the same as running a direct line since there in no vacuum working on prime.  The only thing you might see is, if it does not run well on prime but runs better with a direct line, then maybe the petcock is gummed up.  That wasn't really the case on mine because prime made it better, but on/reserve was problematic.  I thought maybe the vacuum system was the problem, but it seemed to check out.

Got back into the GS today and made some progress.  Took it out for a ride and warmed it up.  Did a carb sync which smoothed out the stumble when opening the throttle past 1/4 turn.  I also adjusted the mixture screws a bit.  So now it runs pretty well with "in town" riding.  It feels a little flat in the mid-range, but the only way to change that is with washers or switching out mains.  It still stumbles at full throttle.  It will pull through the stumble in lower gears and at higher RPMs, but in 5th and 6th it hesitates and feels super flat at full throttle.  I ran it up to 90 mph indicated today while testing.  If I'm cruising at 65 and need to pass, it doesn't really go in 6th, and feels slow to accelerate in 5th.

I haven't tried to tape off any of the air filter yet to see if anything changes.  That will be my next test when I get some more time.  Overall it is much more rideable now, but I'd be hesitant to take it on the interstate with traffic due to the WOT issues that persist.  I still kind of feel like 1 up on the mains would help.  I think my slip on flows a little more air than stock, and maybe a little richer than stock on the main might give me more responsiveness at full throttle.  I need to do a plug chop and see how they look too I suppose.