Author Topic: Need some help diagnosing a $700 bike  (Read 200 times)

Offline IUsedToWrenchCars

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Need some help diagnosing a $700 bike
« on: October 13, 2021, 08:39:15 PM »
Ok, so Iím a noob when it comes to bikes and this is my first one. Please be gentle. Also: sorry for the long post in advance.

I recently bought a used 2007 GS500F for $700 and Iíve been working to get it running. When I first bought the bike, it would turn over at full choke, rev up to about 4000 RPMs, then cut out. It would then turn over but would not catch. Hereís what Iíve done so far with plenty of help from other forum posts:

Oil and filter change (about 5 quarts came out of the crankcase. There was about 2 quarts of gasoline mixed in by my best guess. Replaced with 3 quarts 10W40 recommended)

Fuel tank drain and flush/clean

Tank petcock replaced

Spark plugs replaced (the left side one was pretty fouled with buildup bridging the ground strap to the electrode. Replaced with NGK)

Air filter replaced

Lower carbs rebuilt (the jets, float needles, relevant gaskets, etc. The float levels have also been corrected and set to about 14mm and the mixture screws were set back to stock 2.5 turns out)

Vacuum lines rerouted (this one was crazy. The vacuum lines were going a whole bunch of places they shouldnít have been)

And I recharged the battery for good measure.

After all this, the bike still refused to start with full choke. I tried it with no choke and it started and ran at about 4000-5000 RPMs fairly evenly without knocks and with only a pop every so often. However, when I apply choke, it begins heading towards 6000+ RPMs. I didnít make it past 1/3 choke after starting it because it was climbing towards 8 or 9k. I looked at the idle speed screw and it is all the way out. No more adjustment available on that. To cap it all off, thereís a good amount of white smoke coming from somewhere on the bottom of the bike. I havenít gotten the location pinned down yet.

Iím hoping someone has some ideas on how I can get this bike running properly. My understanding is that it should idle at 4000 on full choke and a dead cold start, then fall to around 1800 on half choke till warm, then 1500 after that. Any help would be appreciated.

Offline The Buddha

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Re: Need some help diagnosing a $700 bike
« Reply #1 on: October 13, 2021, 11:49:25 PM »
Throttle cable is too tight holding it open.
Or you're super super lean - which is very likely but still not 6K at no idle when cold.
Cool.
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Offline IUsedToWrenchCars

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Re: Need some help diagnosing a $700 bike
« Reply #2 on: October 14, 2021, 06:07:02 AM »
Throttle cable is too tight holding it open.

Iíll have to check that after work. Itís very possible although I made sure I was hearing the snap of the throttle closing like mentioned in another post when I set it.

Or you're super super lean - which is very likely but still not 6K at no idle when cold.

What could cause that? The jets are stock from what I can tell based on the numbers, and the air filter I put in is stock as well. Would I need to raise the floats? Or would that be something that should be fixed with the air mixture screws?

Offline The Buddha

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Re: Need some help diagnosing a $700 bike
« Reply #3 on: October 14, 2021, 06:51:38 AM »
Its not going to be 4K at cold start with the leanness from stock jetting. In fact it will not want to hold idle at all, it will stall when cold with jetting leanness. It will hove @4K when warmed up.

You're definitely looking at something pulling the throttle open.

Cool.
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Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #4 on: October 14, 2021, 07:17:56 AM »
You have to consider the physics of the problem. To rev to 4K at idle, it has to have fuel and air in the correct proportion sufficient to run the engine at that speed. Normally the air comes in by opening the throttle plate, which allows the slide to come up, bringing the needle out of the jet, which allows fuel to flow to match the air and bingo, it revs. If it revs without opening the throttle manually, then either:

a) as Buddha said, the throttle is opened, due to poor adjustment, wrong cable, or throttle stop screw ("idle speed screw") setting
b) giant air leak ("vacuum leak") AND ALSO somehow a ton of fuel coming through the pilot jet (or fast-idle jet/choke). With stock jetting, there is no way to get this much fuel, so a massive air leak would result in no-start condition or it won't run when taken off choke, not the other way around

And not to dispute Buddha, but theory does not support the idea that it'll idle at 4K if it's lean. Lean or rich will both cause idle rpm to be lower than normal. The difference is if it's lean, or if it's rich, in either case, you can crank up the idle speed to try to get it to not stall, and especially in the case where it's lean, you may open the throttle plate enough with the idle speed/ stop screw to keep the slide up and pull fuel through the main jet.

But anyway, point of all of this is, it has to be a) above. logic says you are holding the throttle open somehow. Throttle stop screw, throttle cable adjustment, toothpick stuck in the venturi, bent or damaged/leaking throttle plate, some other crazy thing. Check the most likely first, adjust throttle cable until you have 5mm of free play in the throttle. Go from there.

Offline The Buddha

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Re: Need some help diagnosing a $700 bike
« Reply #5 on: October 14, 2021, 07:28:53 AM »


And not to dispute Buddha, but theory does not support the idea that it'll idle at 4K if it's lean. Lean or rich will both cause idle rpm to be lower than normal.

I didn't say that - I said lean condition cant cause 4K at startup - it will cause stalls at idle and barely run.
4K when warmed up is what I said.

Cool.
Buddha.
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Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #6 on: October 14, 2021, 07:41:12 AM »
That's exactly what I disputed :)

4k idle at any time won't happen if it's lean unless you are also intentionally adding lots of air through the throttle opening or otherwise.

4k idle is an idle speed/airflow problem, not a rich/lean problem. That's all I am saying :) Now, unintentionally screwing up the idle speed or airflow at idle in response to a rich/lean condition is entirely possible, and could result in a 4k idle when warmed up. Meaning, if it's way too lean then it won't start at all unless you crank the "idle speed" up so high that it will idle at 4k when warm. But that's because you are blocking the throttle open, it's not because it's lean. IOW, if you take a normal running bike and adjust the pilot screw to make it way too lean, and DON'T TOUCH THE IDLE SPEED SCREW, then it won't idle at 4k, ever. It won't probably idle at all. It will stall at stop signs if you don't nurse the throttle. It may not even start no matter what.

Offline Bluesmudge

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Re: Need some help diagnosing a $700 bike
« Reply #7 on: October 14, 2021, 09:18:41 AM »

Iíll have to check that after work. Itís very possible although I made sure I was hearing the snap of the throttle closing like mentioned in another post when I set it.

The Buddha is totally right here about it being the throttle cable. A totally cold GS500 with stock jetting should not start without choke, so either lots of extra air and fuel are getting to the engine (unlikely) or the idle ajust is set too high (possible) or the throttle is improperly adjusted (very likely based on your quote above).
The "snap" is not how you check that the throttle is correctly adjusted. A snap just tells you the throttle return spring is functioning. You have to set the correct free play (check the manual for specs).

When you first turn the throttle you should see the throttle cable pick up the slack where the cable attaches to the carb. If it starts out already tight then its holding the throttle open and effectively overriding the idle adjustment. Its slightly more complicated since your 2007 model has pull AND push throttle cables. If you want to simplify the diagnosis, remove the push return cable until you get the normal pull cable settings figured out. Older carbureted bikes got along fine with one throttle cable, the second one was added for safety.

Also note it could be as simple as the cable getting pinched when you put the tank back on. I had that happen to me recently and briefly freaked out that something horrible went wrong just as I was getting everything finished up. A quick tug of the throttle cable got it unstuck from the tank.

Watch these videos for help:
https://youtu.be/zy2dj9VvEdI
https://www.youtube.com/watch?v=XcLU_Y22oko
« Last Edit: October 14, 2021, 09:32:47 AM by Bluesmudge »

Offline IUsedToWrenchCars

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Re: Need some help diagnosing a $700 bike
« Reply #8 on: October 14, 2021, 07:28:31 PM »
You guys are awesome! Iím gonna try this tomorrow after work. Hopefully this solves the issue and I can move on to mixture adjustments and carb balancing. Fingers crossed!

Offline The Buddha

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Re: Need some help diagnosing a $700 bike
« Reply #9 on: October 15, 2021, 03:26:03 AM »
4k idle at any time won't happen if it's lean unless you are also intentionally adding lots of air through the throttle opening or otherwise.

No. This isn't correct. It can, and has happened to many many people I'll bet.
If it is lean and you set it to cold idle at 1300, 20 mins of riding later it will be near 4k.

The thing you're missing is, the engine can suck enough air from a pair of butterflies set to idle to get 4k. Easy, and it does so when there's not enough gas to suck in with that air. Viscous resistance of air is very very very low compared to gasoline, which this bike sucks defying gravity too. If I got a carb rack that someone said its 1500 when cold and 4K when warm, without touching anything else, put 1 larger pilot in it and see what its hot and cold idles are.

Barely hold idle @1300 when cold, idle hover @4k when hot = classic lean.

OP condition however is - 4K and climbing when cold - Not the same as above. That cant happen without open butterflies to well above idle.

Cool.
Buddha.
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Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #10 on: October 15, 2021, 05:42:10 AM »
That's exactly what I said. It doesn't idle at 4k because it lean, it idles at 4k because the hot idle speed is set to 4k. If you set the idle correctly when hot, it may not idle at all cold, because it lean.

Offline The Buddha

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Re: Need some help diagnosing a $700 bike
« Reply #11 on: October 15, 2021, 06:02:37 AM »
That's true, but someone should do this experiment. Set hot and cold idle to 1500 - aka properly jetted bike.
Then swap in 1 size smaller pilots - without touching anything. See what the hot idle is then.

My bet is - its 3k+.

AKA hot idle went up without touching idle screw.

So Its not that hot idle is set high in the lean case we are talking about because hot idle is fine with correct jetting.
Cool.
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Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #12 on: October 15, 2021, 07:43:10 AM »
That'd be a lot of work to just prove a point.

There's NO WAY less fuel (leaner idle) will result in higher RPM. It defies the laws of physics and chemistry. Less fuel = less energy = less power to overcome pumping losses and sustain unladen engine speed.

Offline Bluesmudge

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Re: Need some help diagnosing a $700 bike
« Reply #13 on: October 15, 2021, 08:56:30 AM »
None of what you two are arguing over is relevant. We are talking about 4k idle from cold. Its the throttle cable or idle screw.

You only have one idle speed adjust, so you set it for hot. I disagree with The Buddha that hot and cold idle should be the same on a properly jetted bike. The cold idle should be unknown because the bike will die without choke. If you don't need choke when the bike is cold then you're pilot jet or mixture screw is probably way too rich and you are just wasting fuel the entire time once the bike is warmed up. I guess that would make sense if you never ride more than 5 miles but otherwise bad. The choke is there for a reason, the bike is supposed to need it.
« Last Edit: October 15, 2021, 09:07:04 AM by Bluesmudge »

Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #14 on: October 15, 2021, 10:16:11 AM »
Totally agree Bluesmudge.

I'm just hoping to avoid adding to a wealth of misinformation about carb tuning for old motorcycles that abounds on the internet.

You're right, only warm-engine idle speed matters. Idling a cold bike is the job of the choke. A properly jetted GS500 won't start cold without choke and won't idle without choke until it's been running a few minutes.


Offline IUsedToWrenchCars

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Re: Need some help diagnosing a $700 bike
« Reply #15 on: October 15, 2021, 05:01:11 PM »
Alright, I adjusted the throttle cable so it has the requisite free play and the issue persists. I have a shirt and pretty useless video but Iíll throw it up anyways. In the video, I attempted to show what would happen when at full choke and the damn thing nearly headed to 9k before I could turn off (if thatís the right terminology) the choke so thatís the momentary panic you see right at the beginning. Iím not sure what the best way to share videos on the site is so pardon my YouTube link: https://youtube.com/shorts/IMKh54oQagM?feature=share

Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #16 on: October 16, 2021, 04:07:58 AM »
Ok, I reread your first post, it's clear something went wrong with your carb work.

Starting on choke and revving to 4k as it did before you took it apart is correct behavior, or close enough. IDK what you en by "then cut out" but if you want it would stall with the choke off, that points to clogged pilot jet, very common.

Now you clearly have two problems. First, too much air at idle. I think the throttles have to be not fully closing. Or maybe your vacuum reroute has resulted in a big vacuum leak bringing in air. But somehow enough air is getting in when the throttles are supposed to be closed to let it run at 4k rpm.

Second problem is it's way too rich at idle. That's why it won't start on choke, and why there is enough fuel to run 4k rpm at idle. Most likely cause of this, and probably related to the air problem, is one or both of the main needles are hung up, so the slides are stuck open. It's easy to assemble the carbs and get the needles misaligned so they don't go all the way down. That would cause the main jet to be open and let in a ton of fuel at idle.

Also, if you didn't replace the pilot needle/screw o rings when you had the carbs apart, then they are very likely leaking fuel, making it ultra rich at idle.

If I were you I'd be pulling the carbs back off and redoing the carb assembly. Double check the float levels. Change the pilot needle o rings. Verify the top of carb assembly and slides are moving freely up and down. And check anything that could cause the throttles to not close completely. Then when putting it all back, make sure the vac hose routing and connections are exactly right, no leaks, etc.

There are a hundred things that can go slightly wrong when assembling and installing the carbs without you noticing, so you just have to be super meticulous and very critical of your work. As many times as I've done it, I just now assume I made mistakes when anything isn't perfect. I'm going to be pulling my own GS apart today to fix an impossible to diagnose issue that's probably my own error.

Offline IUsedToWrenchCars

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Re: Need some help diagnosing a $700 bike
« Reply #17 on: October 16, 2021, 09:08:02 AM »
IDK what you en by "then cut out" but if you want it would stall with the choke off, that points to clogged pilot jet, very common.

I mean that when I would start it at full choke, it would catch and start running, rev up towards 4 or 5k RPMs, and then the engine would die and it wouldnít restart for a bit. It would just crank over with the starter. That was however before I set the floats to their current setting at about 14.3 mm and redid the vacuum routing (by which I mean put the vacuum hoses back in their correct attachments and replaced cracked hoses)

Now you clearly have two problems. First, too much air at idle. I think the throttles have to be not fully closing. Or maybe your vacuum reroute has resulted in a big vacuum leak bringing in air. But somehow enough air is getting in when the throttles are supposed to be closed to let it run at 4k rpm.

Yes. I did some more research on the subject and Iím going to closely inspect the intake boots today to make sure theyíre not leaking. I doubt itís the vacuum routing as the hoses are now in their stock configurations.

Second problem is it's way too rich at idle. That's why it won't start on choke, and why there is enough fuel to run 4k rpm at idle. Most likely cause of this, and probably related to the air problem, is one or both of the main needles are hung up, so the slides are stuck open. It's easy to assemble the carbs and get the needles misaligned so they don't go all the way down. That would cause the main jet to be open and let in a ton of fuel at idle.

Iím not sure what you mean by the main needles, but the slides were both closed the last few times I had the carbs apart and they moved freely on their pins up and then returned quickly back down when released. They also both made a woosh noise like bellows when operated manually when the carbs were out. Additionally, in that video I made, you can kinda hear, but I was able to start it on full choke and it didnít die. Instead it went shooting towards about 9k rpm before I went to no choke cause it scared the crap out of me. Iím not sure what changes were made to allow it to run at full choke again, but it doesnít die out immediately after starting on choke anymore it seems.

Also, if you didn't replace the pilot needle/screw o rings when you had the carbs apart, then they are very likely leaking fuel, making it ultra rich at idle.


I replaced all the other o rings but I might have forgotten those. Are we talking about the air mixture screws?

If I were you I'd be pulling the carbs back off and redoing the carb assembly. Double check the float levels. Change the pilot needle o rings. Verify the top of carb assembly and slides are moving freely up and down. And check anything that could cause the throttles to not close completely. Then when putting it all back, make sure the vac hose routing and connections are exactly right, no leaks, etc.

There are a hundred things that can go slightly wrong when assembling and installing the carbs without you noticing, so you just have to be super meticulous and very critical of your work. As many times as I've done it, I just now assume I made mistakes when anything isn't perfect. I'm going to be pulling my own GS apart today to fix an impossible to diagnose issue that's probably my own error.

Well good luck to you. Hopefully weíll both have well running bikes by the evening  :woohoo:

Online mr72

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Re: Need some help diagnosing a $700 bike
« Reply #18 on: October 16, 2021, 01:20:48 PM »


 closely inspect the intake boots today to make sure theyíre not leaking. I doubt itís the vacuum routing as the hoses are now in their stock configurations.

the o rings between the intake boots and the cylinder head are almost definitely leaking. You can't inspect that without completely disassembling it, and then you might as well put new ones in.

But that's not causing your problem.

Quote
Iím not sure what you mean by the main needles, but the slides were both closed the last few times I had the carbs apart and they moved freely on their pins up and then returned quickly back down when released.

the "pins" you are talking about are the main needles.



Quote

I replaced all the other o rings but I might have forgotten those. Are we talking about the air mixture screws?

well, it pays to be precise if we want to understand what's happening. So, no, there are no "air mixture screws". If you actually removed the part you would see clearly that it's not just a screw, but in fact a needle which you adjust but turning as you would a screw. But it meters fuel, not air. Which is why when the o ring is shot, it leaks fuel and makes your pilot mixture adjustment meaningless. To think of it as an "air mixture screw" encourages misunderstanding what it actually does.


Quote
Well good luck to you. Hopefully weíll both have well running bikes by the evening  :woohoo:

thanks. Unfortunately I didn't fix my problem, but I am closer to knowing what it is. I just got tired of working on it. I get spoiled with my fuel injected motorcycle.

Seriously if you haven't, go read the blog post in my signature. Might help a lot.
« Last Edit: October 16, 2021, 02:25:18 PM by mr72 »

Offline IUsedToWrenchCars

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Re: Need some help diagnosing a $700 bike
« Reply #19 on: October 19, 2021, 04:54:51 PM »
So I got the idle taken care of. It seems it was a combination of old intake boot o-rings and my own incompetence. When I set the idle screw to the minimum adjustment previously, I hadnít taken care of the free play on the throttle cable correctly. So when I went to adjust the slack in the cable to the correct point, I adjusted it to what I thought was the minimum adjustment for the idle screw and of course, it was too high. I realized my mistake with the carbs off and it now idles at the correctish speed until it gets really warm.  :icon_razz:

My first time actually riding it, it died out after about 20 minutes of riding around. Before it died though, it started missing every so often. I took it out again today for a longer ride to get familiar with riding on the street, and again it started missing again. At first it was just when Iíd decelerate. Closer to that 20 minute mark though, it was any time I had constant throttle or I was decelerating. Then it started doing it while I was accelerating too. The exhaust was popping, the bike felt like it was missing every so often or like I was being pulled back by a rubber band, and a kinda grey smoke was coming out of the tailpipe in pretty good quantity.

So thatís where I am with that. The answer to my posted problem though, was ďuse your eyes next timeĒ. Thanks to everyone who suggested that check!

 

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