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Removing carb vent lines solves starvation mystery

  • Thread starter Thread starter Anonymous
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Ever since I've owned my '80 GS 450, even when it was bone stock, it had a strange problem of running out of fuel with a full tank. If I tried to cruise over 60, after a few minutes, it would "run out of gas." It would bog, surge, and almost die; symptoms identical to running the tank dry. After slowing down to 30 or so for a minute, it would return to running normally. I, of course, suspected crap in the tank, clogged fuel line, crap in carbs, classic stuff. After over a year of fooling with every nook and cranny of the fuel system, and new carbs, and all dialed in, it ran great, but still freaking "ran out of gas." I thought I was losing my bloody mind! I'd heard something about a "vortex" in the carb vent lines, from a couple of different sources, but it sounded like silliness to me. Then Keith Krause on this forum explained it to me clearly enough that I finally got it. I pulled off my vent lines and... woohoo! It not only no longer runs out of gas, but it runs better than ever! Unbelievable! And I had this problem with a bone stock airbox! I bet there are lots of folks out there with this mysterious problem, thinking they need to rejet, etc. when all it is is a bloody mystery vortex thingy!

Anyhow, just wanted to make everyone aware of this issue, and that KEITH KRAUSE rocks! :D
 
Tell me if this sounds like the same issue ...

On my 1980 GS550L the bike runs great at any speed up to about 60mph. After being at that speed (or faster) for a short while it starts to cough, bog, and eventually die at the side of the road. I wait five minutes, and then it starts right up and will run reliably as long as I keep the speed under 45mph.

I covered electrical problems with none found (even updated to Dyna S ignition and Accel coils and wires, just to make sure). I checked for a clogged gas cap vent ... checks out fine. Looked for leaky vacuum lines ... none found.

I was just about to consider replacing the petcock when I saw your post.

Please post the link to Keith's description so I can double check whether this is my issue also...

Thanks!
Steve 8)
 
vortex thingy

vortex thingy

And here all this time I thought I was just reeeeeaally stupid.

This is something I will fix immediately on my 450. Once I get a new reg/rec, fix the leak in my gas tank ( do ALL GS gas tanks rust in the left hand rear corner??) and adjust the clutch, maybe the damned thing will run.

Course, I could still be stupid. But at least not about this problem.

Thanks for the tip.
 
Just wanted to say that it worked for me too! I still have some carb issues, but I went from topping out at 70 before pulling the vent line to... well... not sure how fast I've had it since but it's still pulling hard when the spedo pegs at 85 (and that's about my limit on the old girl anyways).

My performance improved a little bit when I pulled the carbs and cleaned them out, along with partially sealing up an air leak (hey, I'm on a budget here). My highway performance improved DRASTICALY when I pulled the vent tube!

Thanks guys!
 
Re: Removing carb vent lines solves starvation mystery

I have an '81 450 that wouldn't "die" after going over 60, but would really bog down. I ripped off the damn vent lines and WOO HOO I swear there's a glimpse of sunlight under my front tire now when I shift into second and third!!! :twisted: I LOVE IT

Need to get the 2-1 exhaust and pods on the bad boy now!!!

I have read a bunch of KK's posts and I agree with the consensus
Anyhow, just wanted to make everyone aware of this issue, and that KEITH KRAUSE rocks!
Click to expand...


Take care all....


David
 
What vent lines is everyone talking about? I have an 86 GS450L and there are no vent lines coming from my carb bowls. I am having a problem with gas starvation though even at idle (will die after 10 seconds) so I am interested to hear about these vent lines.
 
Hipster:
I swear there's a glimpse of sunlight under my front tire now when I shift into second and third
Click to expand...

I know egggzactly what you mean :twisted:



Wanago: The vent lines in question are two 1/4" rubber hoses hanging off the back of your carbs.

http://www.thegsresources.com/_forum/viewtopic.php?t=36158&postdays=0&postorder=asc&start=0

I think you have other problems, though. Sounds like clogged fuel line (does fuel run free when on prime with line disconnected?), bad diaphragm valve (runs fine when set on prime), dirty carbs, intake leak, etc. etc.
 
Do you have a picture of the carbs where you can point out the lines? I have rebuilt my carbs completely and was staring at them last night after reading these posts about the vent lines... didn't see any. Maybe you have a different carb then mine. I have the stock carb for an 86 GS450L which is a CV 34mm carb I believe. If they are hiding somewhere I want to find them! :)
 
wanagohueco said:
Do you have a picture of the carbs where you can point out the lines? I have rebuilt my carbs completely and was staring at them last night after reading these posts about the vent lines... didn't see any. Maybe you have a different carb then mine. I have the stock carb for an 86 GS450L which is a CV 34mm carb I believe. If they are hiding somewhere I want to find them! :)
Click to expand...

On the earlier 450 models we had BS34 carbs. I am sure you would see the vent lines if you had them. They were about 10" long fuel lines coming off each of my carbs. I am not familiar with the CV carbs so I can't help there... sorry about that. I am sure there is someone else on the GSR that can help you. Good luck
 
Wow! Pulled them off of my '81 GS650G and what a difference. It pulls way harder at highway speeds and accelerates in low gear much better. Had serious winds riding to works and no bogging at all. Before I couldn't accelerate into a strong head/crosswind without it bogging and or dieseling (pinging) like mad. I was thinking of selling my bike because not being able to 'get on it' at 75MPH can be dangerous on bay area freeways, but now I think I'll keep her until she just can't make the commute.

I'm still getting a little pinging, but not bad at all compared to before. I think I need to set the floats just a tad higher. After completely rebuilding my carbs, I can't for the life of me, figure our how anyone measures the float height. I've just done it empirically.

On the 450, there are two of these tubes... one for each carb, if I remember correctly from rebuilding the carbs on my gf's 450T.

I think the tubes in question are #68 on
http://houseofmotorcycles.bikebandit.com/partsbandit/assets/schematics/Suzuki/SU0082_014.gif
 
Hey, Espumoso. Glad to hear! Ain't it great? I was so bummed about my bike, exactly like you say... can't buck a headwind? Wha? So awesome!

Regarding float level, you probably already know this, but using that to set mix is marginally effective, and probably not best practice. You might try running a drill bit that's just the same size or a hair larger through your mains. I used a micrometer to determine that my bit was the same size as my mains, and ran it through, and it removed a lot of sediment I couldn't see. If you are pinging, a little enlargement on the mains sure isn't going to hurt.

from my limited experience, and for what it's worth
 
bazango,

It's like I have a new bike. I was all grins riding to work today!

Regarding the pinging. After my carb rebuild, I set the floats *way* too low, but since I can't figure out exactly how to get them to spec, I think I'll pull them one more time and move them up a tiny bit more. If I'm still pinging at load, I'll consider rejetting, but after reading some of the stories about jetting, it's not something I want to get into unless I have to. When I rebuilt the carbs, I did the whole dip and clean, including all the holes in all the jets with a wire, so I hope they are still clear.
 
Don't you just measure from the gasket surface (with gasket off) to the bottom of the float (the top, at the moment, as you are working on it upside down)?
 
OK... so I found the carb bowl vent lines. The previous owner had put a small hose connecting them together. I had assumed they were some sort of vaccuum line (not vent lines) and that it was assembled correctly... ha. I disconnected them and put seperate hoses on that just vented to outside... the bike runs great now!
 
I have what I think are two vent tubes on my 1980 GS850L. They are between carbs 1&2 and 3&4, and they hang over and behind the airbox.

Is this what the discussion is about? And if so, and I remove mine, do I need to cap the tubes they were attached to?

Thanks.
 
Connected together! Hah! What a riot. Not a lot of venting going on there, eh?

A T, they need to be open to allow the bowls to remain at atmospheric pressure. I don't think just leaving them open is a problem, unless you let your bike sit for several years, and bugs get in there. Of course, that's a totally sucky idea anyway. I guess if you were concerned, you could put short, stubby little lines on there and see how that works, after verifying that removing them makes a difference.
 
Yeah pretty ridiculous right. I had just assumed it was correct since that was how it was when I bought it. Then again... the gas tank, seat and battery werent bolted down correctly either so I probably should have known.
 
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