Engine occasionally won't pass 1500 RPM

[Juan1]

I think this is most likely a carb issue.  Occasionally my bike won't give me any more than 1500 RPM.  I'll twist the throttle all the way, and still it stumbles and hovers at 1500 RPM.  If I release the throttle when this occurs, the engine dies.  Sometimes it happens when the engine is cold, and sometimes it happens when the engine is warm.  It only happens when the RPMs are low, like when I'm leaving a stop sign. 

The bike is new to me, but the carbs were serviced about a year ago.  The bike then sat in storage.  So what do you guys think, is it the carbs?  If so, should I do a full cleaning and balancing?

[The Buddha]

Ocassionally maybe carbs ... maybe fuel flow ... maybe air flow ...
Cool.
Buddha.

[beRto]

I think this is most likely a carb issue.  Occasionally my bike won't give me any more than 1500 RPM.  I'll twist the throttle all the way, and still it stumbles and hovers at 1500 RPM.  If I release the throttle when this occurs, the engine dies.  Sometimes it happens when the engine is cold, and sometimes it happens when the engine is warm.  It only happens when the RPMs are low, like when I'm leaving a stop sign. 

The bike is new to me, but the carbs were serviced about a year ago.  The bike then sat in storage.  So what do you guys think, is it the carbs?  If so, should I do a full cleaning and balancing?

Your problem sounds very similar to the one described in this thread:
http://gstwins.com/gsboard/index.php?topic=42192.0

"4 out of 5 times I would pull the throttle, the bike would do nothing.  The other time out of 5, the bike would rev up as normal.  When it would rev, it would feel very strong"

The main problem for pennstump was clogged pilot jets.

[Juan1]

Looks like this week's project will be cleaning the carbs and jets.  Gotta say, I'm getting sick of constantly working on this bike.     Then again, it only cost me $800 and the engine, when it isn't having this issue, is great!

[Juan1]

Last night I ripped open the carbs.  The tiny o-ring was missing (#26 on the fiche), and one of the previous owners put in adjustable needles.  Despite being careful, I managed to strip one of the pilot jets, so new ones are on the way.  We'll see if new jets all around (shimmed), new o-rings, a float height adjustment, and a carb cleaning fixes matters.

[The Buddha]

OK you have dynojet needles in it. OK then forget anything we ever said about running 125 or 150 or whatever ... all the numbers now dont mean jack to you. Pilots yes 40's.
Now I know I sent you somehting recently didn't I ... not jets mains from my kit will not do it for your needles.
Cool.
Buddha.

[Juan1]

Buddha,

Turn signals were purchased from you recently.

Can I go from dynojet -> mikuni?  Would I want to?  Are the mikuni 40 pilots still useable?  I stripped a pilot, so I was going to replace them with mikuni 40s.

[The Buddha]

Yea you can do the pilots, but not anything else.
I have recently taken a lot of DJ's out and swapped out needled with stock, but I am all out ... sorry. You cant use the 125 or 150 mains we all talk of. That's pretty much it.
Cool.
Buddha.

[Juan1]

There are 3 possible causes of the 1500 RPM problem.

1.  Plugs may have been fouled.  Both plugs were black.
2.  Mixture may have been set too rich at the mixture screws.  When I took the carbs off, it looked like they were out 5-6 turns.
3.  Throttle cable too tight, throttle slightly engaged when idling. 

Mistakes I made along the way/things to watch out for if you have the same problems:

- I tried to put the throttle cable back in the exact same position as I initially found it.  This was a dumb move since a P.O. had the throttle cable set too tight. The trick to doing the throttle cable correctly is to loosen the cable as much as possible at the hand control side, and then set the throttle slightly too loose at the carb side.  Put everything back together and you can fine tune the throttle cable tension at the hand control until its perfect.

-When setting the mixture, I didn't realize that the tight throttle cable was causing the engine to stay at a high RPM, so I kept enrichening the mixture thinking that I was experiencing surging from a lean mixture.  When I took the GS for a ride, the bike suffered the same 1500 RPM stumbling symptom that I originally had.  Once I discovered the throttle cable issue, I realized that my mixture was probably way rich.

-I knocked a fuel tube off when reconnecting the carb to the engine.  Do yourself a favor and buy clamps to put on all the fuel lines. 

My suggestions for anyone having this issue:

1.  Check your plugs, they'll let you know if you are running too rich.  If the plugs look fouled, change them and procede to step 2.
2.  Loosen the throttle cable at the hand control so that you have play in the throttle before you feel it engage.
3.  If step 2 lowered your idle speed, adjust the idle to 1300 RPM when the engine is warm.
4.  Tighten the mixture screws all the way, and then back them out 2.5 turns.  If that's too lean (you get occasional backfires on decel, throttle hangs, when warm the bike runs better with the choke on), turn the adjustment screws out one quarter turn.  See if it is still too lean.  If it is, try another quarter turn.   If you end up more than 4 turns out, the problem is bigger than your mixture screws.
5.  Tighten the throttle cable at the hand control so that the idle speed speed doesn't change when you turn the bars, and there is only a little bit of play before the throttle engages.

If that doesn't solve your problem, follow these steps:

-WD-40 all air lines, the airbox-carb boot and carb-engine boot, one at a time. An engine surge immediately after you spray reveals a leaking boot or air line.
-Check float height externally using the directions in the "how to" section.

Still haven't found the problem?  Sorry folks, looks like you'll have to remove the carbs and clean them.  While you're in there, you might want to spend the $30 on a carb rebuild kit and replace all o-rings and wear items.  Common carb issues are bent needles, bad aftermarket jet selection, clogged jets, missing o-rings and gunked up passages.

[beRto]

Glad you got it sorted out! 

And thanks for the detailed follow-up report