SOLVED! Carb Issues... maybe... Please help!

[Lluraeden]

Hi everyone! I'm having issues with my GS and can't figure out for the life of me what is wrong. Any and all input appreciated! Also I have been looking around these forums as well as others, and while I've found posts with similar symptoms the solutions given there have not worked for me yet.

So here goes, and sorry about the lengthy post. Bought the bike last spring all stock with 8k on the odo. I did 0 modifications, rode all summer into the fall before I put her away for the winter, clocking almost exactly 4000 miles with no issues. Over the winter I did: new stock replacement air filter (not K&N), inline fuel filter, and not that it's relevant but new brake pads, r6 shock and sonic springs in the forks.

When I put the bike back together this spring and started to ride, I noticed this issue: Above about 50mph, in any gear and at any rpm (excluding 1st and 2nd gears) when I roll onto the throttle at just about maintenance throttle and slightly above, the engine feels like it's running out of gas, sometimes popping/backfiring. This happens regardless of the speed at which I open the throttle. It is much more pronounced as speeds and gears increase, and is not noticeable below 40-50mph in 1st and 2nd. Sometimes it catches power but usually it just decelerates until I open the throttle further/WOT.

Full throttle has power, top speed is normal (I had to test it, you know, for science ), idle is set to ~1500 and it idles fine, it revs in neutral smoothly through the range, I start it on choke but it can warm up on it's own after less than a minute. Running with the petcock on prime has no effect. Removing the fuel filter had no effect.

Yesterday I tried to isolate the issue and gather data. On the highway at about 60mph it will only accelerate or engine brake, there is no way to smoothly maintain speed. I tried riding in this "dead range" for a few seconds, heard a few exhaust pops, pulled in the clutch let the engine drop to idle (took a bit longer than usual to drop below 3k to idle) then when I tried to rev it (still with the clutch in) it had no/minimal response and wouldn't rev until i held WOT for maybe 1.5 seconds. When it did finally rev, response was quick and normal even after letting it drop to idle and revving it up again. Not sure what this means, but it might be relevant.

One other thing I saw is that when revving in neutral with the airbox off to see into the carbs, gas gets sprayed back out into where the airbox should be. I don't know if this is normal, but it happens evenly on both carbs.

Obviously the first and second thing I tried was cleaning the carbs. They are clean, the slides appear to operate correctly from what I could see, all the o rings are intact etc. I added one washer on each side to lift the needles up thinking maybe it was just running lean in the mids, but no effect (and I shouldn't have to considering it ran smoothly last year). New spark plugs and oil/filter. I keep forgetting to check float height, but will update this when I know. I have not checked valve clearances or compression/leakdown, but I can't see why sitting for 4-5 months would affect those in any way or cause this issue. No noticeable vacuum leaks. I am out of ideas 


About me: I am not a professional bike mechanic, but I'm also not stupid. I grew up fixing my cars (they had a lot of issues...) and have helped rebuild an old CB750 as well as doing maintenance and low-key mods to my GS. I will take it to a shop as a last resort but would prefer not to.

About the bike: 1989 GS500E, 12000 miles, all stock- airbox and filter, carbs, jets (one washer on needles), exhaust, engine (to my knowledge). NOTE: I want to keep this stock and just get it back to how it was!

EDIT: It seems that the problem was the carb breather hose, which I had mistaken for a drain and routed down to the bottom of the bike. When I moved the hose to it's designed position between the airbox and the battery, the problem has (so far) completely disappeared! Big thanks for the help everyone, I'm glad it was simple and I would have never thought of that myself!

[Lluraeden]

Not sure why this post went up twice, sorry about that!

Also noob question, "what creates the electric arc that ignites fuel in the cylinder?" what answer does this form want me to give haha sorry I'm new here...

[dominickbuff]

sparkplug one word and thats actually the first thing i would check on your bike to

[Watcher]

Verify that this is all proper.



Since it's happening at a specific speed and not a specific RPM I have reason to believe either your carb breather (vent hose from upper T) is not routed out of the wind, or your intake boots are not sealing properly.

[Jim Moore]

That's a weird one. Have you tried all three positions of the petcock? I'd also replace the fuel filter, just because it's cheap and easy. After that I'd make sure my carb tops were fitted correctly and the diaphragms weren't torn.

[Lluraeden]

Thanks everyone! For some reason when I posted this it went up twice, so for the sake of simplicity it's probably best to just use one thread: http://gstwins.com/gsboard/index.php?topic=71573.0

sparkplug one word and thats actually the first thing i would check on your bike to

I replaced the spark plugs, issue remained.

Verify that this is all proper.



Since it's happening at a specific speed and not a specific RPM I have reason to believe either your carb breather (vent hose from upper T) is not routed out of the wind, or your intake boots are not sealing properly.

I will definitely try this as soon as I get out of work, I would be ecstatic if it was this simple!

That's a weird one. Have you tried all three positions of the petcock? I'd also replace the fuel filter, just because it's cheap and easy. After that I'd make sure my carb tops were fitted correctly and the diaphragms weren't torn.

Removing the fuel filter had no effect, and neither did running the petcock on PRI. other people have suggested the diaphragms as well so although they appeared fine last time I checked, I will look again when I have a chance.

[mr72]

other people have suggested the diaphragms as well so although they appeared fine last time I checked, I will look again when I have a chance.

They are easily ruined with carb cleaner. People often spray carb cleaner on the carbs without considering this, and then it seeps in and destroys the diaphragms. So just because they were good when you checked last, they may be hosed now if you've ever sprayed carb cleaner on them.

Also very tiny pinholes can cause them to fail, and they can be misaligned when installed under the caps causing them to leak as well. They basically have to be perfect and installed perfectly to work correctly.

Your problem definitely sounds like it might be the diaphragms, but it would be odd for both to fail. My money is on a vacuum leak.

[rscottlow]

other people have suggested the diaphragms as well so although they appeared fine last time I checked, I will look again when I have a chance.

They are easily ruined with carb cleaner. People often spray carb cleaner on the carbs without considering this, and then it seeps in and destroys the diaphragms. So just because they were good when you checked last, they may be hosed now if you've ever sprayed carb cleaner on them.

Also very tiny pinholes can cause them to fail, and they can be misaligned when installed under the caps causing them to leak as well. They basically have to be perfect and installed perfectly to work correctly.

Your problem definitely sounds like it might be the diaphragms, but it would be odd for both to fail. My money is on a vacuum leak.

I had one that was just slightly misaligned, and it was pinched under the cover. Even after putting it in correctly, the crease in it kept the bike running poorly as the OP describes. I replaced it, and voila!

Even if that's not the issue, it sounds like you'll find and correct the problem given all of these suggestions. Good luck! 

[user11235813]

Not sure why this post went up twice, sorry about that!

Also noob question, "what creates the electric arc that ignites fuel in the cylinder?" what answer does this form want me to give haha sorry I'm new here...

There is no answer, it's a prank and drove me nuts, but I was promised it goes away and it does after a predetermine no. of posts

[Lluraeden]

Problem could be as simple as the carb bowl vent hose isn't probably routed.  It comes off the top T between the carbs and should be run straight back over the air box and and hang down between the air box and battery where it's open end is not affected by air pressure or vacuum caused at the bikes higher speeds.

http://img.photobucket.com/albums/v443/jcp8832/top%20view_zpsdgdjolxi.jpg

This appears to be the problem! If you're ever in the Hartford CT area I'll buy you a beer for figuring this out  I had thought that the breather hose was an overflow drain, so had routed it out the bottom of the bike not behind the airbox as it should be. I moved the hose, and the problem has completely gone away from what I can tell  Thanks so much everyone, I was going crazy not being able to ride I'll definitely be back here when the next thing comes up!

[ajensen]

I had a strange, perhaps related, problem. My GS500f started running really badly--it felt like a 125cc. Later in the ride, all of a sudden it went back to normal. The next ride it did the same thing only it never went back to normal. I checked the petcock and fuel lines. Then I cleaned the carbs. Then I took off the extra vacuum stuff. Then I unplugged all the electrical connections that may have had anything to do with the ignition system, sprayed them with WD40, and plugged them back together. I do not know what fixed the problem, but Suzi runs better than ever now.