Misfiring at 1800+ rpm

[romulux]

I just got done reassembling my 2001 GS (3-circuit stock carb).  I bought this bike with 10,400 miles on it about 2 weeks ago for $1500.  While it's in good shape, I don't think the two previous owners were meticulous about maintenance.

I've been going through my maintenance list fixing things up and getting it up to par.

I reshimmed all the valves.  They were very tight and I loosened them to spec or slightly looser.  I also cleaned the carbs, but they didn't look very gummed up.

Previously, I was experiencing stalling when pulling in the clutch coming up to a stop or any time I let the revs fall to idle while riding.  Starting the bike has always been fine, but I would have to use choke or rev it a bit, keeping it at 1500 to 2000 rpms or it would feel like it was going to stall.

While I have not had a chance to take it out and ride it, it started up and behaved better than I've ever seen it.  I used to have to play with the throttle when starting it cold to keep it running while warming up.  Now, it starts up and stays running without any throttle covering.  It slowly rises to 4000 rpm while it warms up and will idle fine down less than 1000 rpm (had the idle screw turned out too far).

The problem is that if I rev up a bit to 1800 rpm or greater, I get misfiring from both cylinders.  I believe it is from both cylinders because I hooked a manometer up to sync the carbs and the water was quite level except it would jump towards the cylinder opposite whichever cylinder misfired, and it was jumping towards both at random times.  Also, you could even see a flash sometimes during the misfire in the clear tubing hooked to the vacuum ports on the carbs.

The sound varies, I assume based on when during the engine cycle the misfire occurs.  Sometimes it is a mechanical "bump" which you can feel in the handlebars, other times it's a higher-pitched percussive sound firing back into the airbox area.

The spark plugs are not new, but they aren't terrible either.  I haven't checked them after running it.  The gas that was in the tank had some stabil and seafoam in it, both of which contain alcohol.  The Clymer manual says alcohol can cause misfiring.

I drained the tank and got some mid-octane fresh fuel, and maybe it runs better but I can't really tell.  It's still misfiring.  I don't know whether I ran it long enough to dilute the old gas sufficiently.

Throttle response feels good, I think.  There's no hesitation or sputtering if you crank it fast, and while the fall back to idle isn't instantaneous, it's not prolonged.  I blindly set the pilot screws by listening for changes in rpm.

Though throttle feels good, it seems like I can't make it hold between 2000 and 3600 rpm.  It's like if I try to open it a hair more than holding it at 1800, it bumps up much higher, close to 4000.  This is the only thing I've noticed that might be off with the throttle response.

Is there anything I should look into for the misfiring, and does my throttle sound like it's ok?  Thanks!

[ineedanap]

Sounds like you have a good handle on the troubleshooting.  Here's my 2 cents anyway.  

1.  You say it "idles down less than 1000 rpm".  You don't have it set there do you?  It should be set around 1300.  All the tuning in the world won't help if your base idle is too low.

2.  You said "I blindly set the pilot screws by listening for changes in rpm".  Setting the idle screws by listening for the changes in the rpm isn't quite foolproof.  Back in the day (4 years ago) when I raced motocross I would set my idle mixture screws by listening.  They would be close, but would always end up being on the lean side of things.  It would make for a not-so-responsive throttle when cracking it open out of turns.  I used to have to add an extra 1/4 turn out on the mixture screws to smooth out the idle to part throttle transition.  Don't be afraid to play with the screws a little bit.  Try 1/8 turn on each and ride it.  Try another 1/8th...

3.  If you're holding the revs at 1800 and the slightest bit more throttle kicks the revs up to 4000 you can almost guarantee you're really lean.  (unless you have a vacuum leak)  The only part of the carb that affects idle and near idle fuel delivery is the pilot and the mixture screws (pretty much).  If it's not one, it's the other.

4.  You say "and while the fall back to idle isn't instantaneous, it's not prolonged".  This can be a great way to tell if your mixture screws are set correctly.  It should pretty quickly return to the 1300 rpm.  It shouldn't take more than a second or two.  Any longer and it's lean.  Also if it's lean, you might see it return quickly to 2k or so, and then slowly go back down to the 1300 or whatever you're set at.  On the other hand, if you chop the throttle and if the idle quickly drops to a rpm less than 1300 then increases back to 1300...you're rich.  

Sounds like you probably knew all this already, though.

[ineedanap]

Oh and welcome to GStwin.  Congratulations on your first post!!!

[romulux]

Sounds like you have a good handle on the troubleshooting.  Here's my 2 cents anyway.  

Just trying to be descriptive, and I've been through at least some similar things on my first, older bike.

1.  You say it "idles down less than 1000 rpm".  You don't have it set there do you?  It should be set around 1300.  All the tuning in the world won't help if your base idle is too low.

Nope, just didn't have it set correctly the first time running it after reassembling carbs + top end.

3.  If you're holding the revs at 1800 and the slightest bit more throttle kicks the revs up to 4000 you can almost guarantee you're really lean.  (unless you have a vacuum leak)  The only part of the carb that affects idle and near idle fuel delivery is the pilot and the mixture screws (pretty much).  If it's not one, it's the other.

Now this is helpful.  If it's lean at this throttle/rpm point, would that be because of the stock main jet that everyone here on gstwins replaces with a richer one?  You say mixture screws, what does that mean?  Externally, the carb has an idle screw that just bumps against the throttle assembly, and it has a pilot screw (air screw?) underneath the carbs in front of the float bowls.  I'll try to set the pilots on the rich side.

4.  You say "and while the fall back to idle isn't instantaneous, it's not prolonged".  This can be a great way to tell if your mixture screws are set correctly.  It should pretty quickly return to the 1300 rpm.  It shouldn't take more than a second or two.  Any longer and it's lean.  Also if it's lean, you might see it return quickly to 2k or so, and then slowly go back down to the 1300 or whatever you're set at.  On the other hand, if you chop the throttle and if the idle quickly drops to a rpm less than 1300 then increases back to 1300...you're rich.

I'll play with this some more and get a better grasp of its behavior.

Thanks for the post.

[ineedanap]

Now this is helpful.  If it's lean at this throttle/rpm point, would that be because of the stock main jet that everyone here on gstwins replaces with a richer one?  You say mixture screws, what does that mean?  Externally, the carb has an idle screw that just bumps against the throttle assembly, and it has a pilot screw (air screw?) underneath the carbs in front of the float bowls.  I'll try to set the pilots on the rich side.

It isn't the main jet, but it could be the pilot jet.  Most people swap out the pilot jet on the older style carbs.  I'm not sure what the "hot" pilot setup is for the newer ones.  Check the wiki for what other people have had success with.  

Here's a neat chart that will help you with jetting.  All you have to do is find the throttle position where you're having a problem.  It'll tell you what parts of the carb are affecting it.  It will save you some trial and error.



The terms "air screw slow jet" and "slow air jet" don't really apply to us.  For the GS500 that whole shaded area would be our pilot jet.  You can ignore the throttle valve cutaway completely, since we're not going to be modifying that.

You are correct by calling the little screws by the float bowls the pilot screws.  Sometimes the pilot screws will be referred to with different names.

pilot screw on a four stroke= sometimes called idle mixture screw or fuel mixture screw.  
pilot screw on a two stroke = sometimes called idle mixture screw or air mixture screw.

Hope it helps...and doesn't make it worse.  

[romulux]

Thanks for the reply.  I rode to work today for the first time (in 30 degree weather, no less) on this new (to me) bike.  I've ridden it around other places since I bought it 2 weeks ago, but not since I reassembled it last night.

It's freakin' awesome not having it stall when I pull up to stops.  My old bike was a 1978 KZ200 that I never got running good enough, and it would stall too.  I'm loving it.

I'm not experienced enough to fine tune the carbs, I think.  It seems like it's running well, and the only time I'm in the 1800 to 3000 rpm range is when I'm passing through it accelerating.  For now, I guess it's good enough.  I will check my plugs and try to determine whether I'm running too lean or rich.  I have a very experienced friend I can take it to for more detailed tuning.

I am still concerned about the misfiring.  So far, your advice, while very enlightening and much appreciated, has been limited to carb tuning.  Any idea about the misfiring, or am I missing the point because your suggestions about carb tuning would effect the misfiring as well?  I couldn't have adjusted the timing when changing the shims, except obviously the small amount changed by the valve clearances themselves.

Thanks again.

[ineedanap]

Any idea about the misfiring, or am I missing the point because your suggestions about carb tuning would effect the misfiring as well?  

I promise what you're feeling is a lean misfire.  If it were ignition problems or valve problems you would have different symptoms.  Dial in your carbs and you'll fix the misfire.  

[romulux]

Revisiting this after riding and tweaking it for a while.

I have both carbs set to 4.5 turns out on the pilot screw.  This is the fewest turns out I could go and not get any misfiring.  This seems like a lot to me if the stock setup is 2 turns out.  Is this excessive?

Will the behavior be different with cold weather versus warm weather?  It's between 25 and 50 degrees out here.

Also, I've never known how to gauge whether the engine is "too" hot versus "normal" hot.  Is there some indication that it's too warm besides checking plugs?

I will be checking my float levels tomorrow (should have done so already).

[The Buddha]

1800-3K misfire could be lean on the pilots.
4.5 is fine on mix screws, up to 6 is OK, past that it has a possibility of falling out.
You may be fine, or you may want to do +1 all around.
Cool.
Buddha.

[ineedanap]

I have both carbs set to 4.5 turns out on the pilot screw.  This is the fewest turns out I could go and not get any misfiring.  This seems like a lot to me if the stock setup is 2 turns out.  Is this excessive?

Most people would recommend going up one size on the pilot jet if you have more than 3.5 turns out on the mixture screw.

Will the behavior be different with cold weather versus warm weather?  It's between 25 and 50 degrees out here.

Maybe.  A 25 degree difference might account for small fueling changes since cold air is slightly more dense.  More importantly, though, you might find it really hard to get the GS up to operating temp when it's 25 degrees out.  Even if your carbs are dialed in right, you might find you're using part choke for the first 15 minutes of your ride.  

Also, I've never known how to gauge whether the engine is "too" hot versus "normal" hot.  Is there some indication that it's too warm besides checking plugs?

I don't even know if it's possible to overheat one of these motors.  I've done 20 minute track sessions in 100 degree weather and been stuck in freeway backups in August without any issues whatsoever.   I guess if it's on fire, run away.  

Glad to hear you're making progress on the misfire.  

[romulux]

Cool, that's reassuring.  I was concerned because today it was wanting to stall when I turned the choke down and I ended up running partial choke on the way home, yet it seemed nice and hot to me (the bike, not the air).

I'm keeping everything stock until I have a few thousand miles of familiarity with the bike, then I'll know better whether larger jets will suit me.

Thanks again.

[The Buddha]

Romulux - I would suggest +1 all aorund, not just pilots.
Yea I'll make sure I have it and get it ready for you. Paypal ID follows in the PM.
Cool.
Buddha.

[Pigeonroost]

I just asked a new-B question about these newer three stage carbs.  Should have read this thread first.  Buddha; by +1 all around, I assume you mean to do something to each stage, such as open the idle mix, shim the main jet and .... do what to the other?  Romulux seems pretty close to his goal, but I'm wondering if he may have a small vacuum leak.

prs

[Pigeonroost]

The Buddha be quick -- he answered my pondering on the other post before I could get done with this one and get back to the other.  "Cool".........

prs

[romulux]

Yeah, I think mine is real close, but my air screw is out as far as I want it to go and it still has mild misfiring.  I'm getting a jet kit from Buddha who has been awesome answering questions I had.

The bike comes with #17.5 pilot jet which I am replacing with a #20, and I'm moving the main jet up to #130.

The biggest thing that made a difference in how well my bike runs was the valve adjustment I did after I bought it used.  I have nagging doubts about them being perfect, so I'm going to adjust them again in the spring when the snow melts.

[black and silver twin]

Yeah, I think mine is real close, but my air screw is out as far as I want it to go and it still has mild misfiring.  I'm getting a jet kit from Buddha who has been awesome answering questions I had.

The bike comes with #17.5 pilot jet which I am replacing with a #20, and I'm moving the main jet up to #130.

The biggest thing that made a difference in how well my bike runs was the valve adjustment I did after I bought it used.  I have nagging doubts about them being perfect, so I'm going to adjust them again in the spring when the snow melts.

Stock main jet on f model 3 circuit carbs is 130. If the bike is stock try 20 pilot, 60(stock) mid, 132.5 main +1 washer and 3 turns on mix screw. if you get cruise-surge (40-75mph) try upping the mid to a 62.5 as well. if you get high-speed-cruise-surge (80+mph) or poor high rpm response/power (7000+rpm) try using 135 mains.

[romulux]

Stock main jet on f model 3 circuit carbs is 130. If the bike is stock try 20 pilot, 60(stock) mid, 132.5 main +1 washer and 3 turns on mix screw. if you get cruise-surge (40-75mph) try upping the mid to a 62.5 as well. if you get high-speed-cruise-surge (80+mph) or poor high rpm response/power (7000+rpm) try using 135 mains.

I'm being pedantic, but according to the service manual, '01 year bikes came with 2 different model carbs (Mikuni BST34SS and BSR34SS) with any of 4 possible main jet sizes: #115, #127.5, #130, #135.  They all have a #17.5 stock pilot jet.  They break it up based on numbers like E-03 depending on region, and also on another indicator that I don't understand.  A 2001 GS500 from the E.U. could be a GS500-H, GS500-U, or GS500-HU, all of which have different parts and original equipment settings.  If you have an E.U. GS500 from 2001, you could have a #115, #130, or #135 main jet.

I believe mine is a US bike, all of which came with #127.5.

The pilot is really all I'm concerned about, and I'll throw some washers on with the #130 I'm getting.

The Suzuki service manual is so much better than the Clymer.  Clymer has missing and inaccurate information.  Won't be buying a Clymer in the future when I change bikes someday.

[black and silver twin]

Stock main jet on f model 3 circuit carbs is 130. If the bike is stock try 20 pilot, 60(stock) mid, 132.5 main +1 washer and 3 turns on mix screw. if you get cruise-surge (40-75mph) try upping the mid to a 62.5 as well. if you get high-speed-cruise-surge (80+mph) or poor high rpm response/power (7000+rpm) try using 135 mains.

I'm being pedantic, but according to the service manual, '01 year bikes came with 2 different model carbs (Mikuni BST34SS and BSR34SS) with any of 4 possible main jet sizes: #115, #127.5, #130, #135.  They all have a #17.5 stock pilot jet.  They break it up based on numbers like E-03 depending on region, and also on another indicator that I don't understand.  A 2001 GS500 from the E.U. could be a GS500-H, GS500-U, or GS500-HU, all of which have different parts and original equipment settings.  If you have an E.U. GS500 from 2001, you could have a #115, #130, or #135 main jet.

I believe mine is a US bike, all of which came with #127.5.

The pilot is really all I'm concerned about, and I'll throw some washers on with the #130 I'm getting.

The Suzuki service manual is so much better than the Clymer.  Clymer has missing and inaccurate information.  Won't be buying a Clymer in the future when I change bikes someday.

huh did not know that. anyway if a 130 is too small its too small, regardless of what came stock. I guess what I should say is a 130 main jet is too small for a 3 circut carb gs500. unless the function of the carbs changed (doubt it, but your manual might say). and not all US bikes came with a 127.5 mine had 130s stock and its a US bike. if you put 130s in you will essentially have what my bike came with stock, which was too lean (13:1 afr verified with wideband O2 sensor on dyno)