Removing carb vent lines solves starvation mystery

[phaeton]

Is this considered a performance mod? Seems like it

Does anyone know if this applies to an 84 GS550E?

 

[Anonymous]

I would think it would apply to any bike with the Mikuni BS carbs, no?

 

[Anonymous]

can you remove this from any of the gs bike (850gl)? I was going to do that when I rebuilt the carb but thought of road stuff getting in there.
Would like to try but wanted to ask first. cj

 

[tconroy]

What do you do with the opening that the vent hose connected to?i dont have this problem on my 650 but if it might run better why not try it?

 

[ddaum]

BOY, am I glad that I just came across this thread!

I just rebuilt the carbs for my 1980 GS550L which did not have those vent lines installed since I got the bike. I noticed the vent tees and looked in my manual which had a picture showing how the vent tubes are supposed to travel back and above the airbox.

I was going to get some hose and install them as pictured in the manual, but to hell with that now, my bike ran great before and I'm going to listen to you guys and keep those suckers off!

 

[duaneage]

I pulled mine and it didn't seem to make any difference. The only issue I have is insects or dirt could make their way into the carbs. Spiders build nests all over motorcycles, I found spider silk in light switches and fuse boxes, so Maybe some form of filter or screen over the open tubes might be in order/.

 

[KEITH KRAUSE]

The vent lines should only be removed for bikes with pods or major air box mods.
Removing them shouldn't effect a stock bike at all. If it did, I'd have to think the lines were blocked by something or kinked, or even hooked together as I've seen/heard, believe it or not.
I've never had a problem with things getting in the open ports, but it could happen obviously. A screen may work, as long as the venting isn't compromised.

 

[IanFrancisco]

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)?

Yup, measure with no gasket. I couldn't get mine to the exact 22.4mm. Setting them there made the carbs overflow big time, had to back off a bit, but the fuel level in the bowls are about as high as they can get and the bike liked that, reduces that on/off action I was getting at about 6.5k rpm at mid-throttle, what you need to cruise at ~70mph. There is (or was) a thread on carb specs here and apparently there are two setups for these BS34SS carbs - one is using 115 main jets and 22.4mm float height and the other a slightly higher float height (lower fuel level) and 120 main jets.


Here's the GS450 carb specs:
http://www.thegsresources.com/_forum/showpost.php?p=286530&postcount=17

 

[bj]

The vent lines should only be removed for bikes with pods or major air box mods.

Keith, why does it make a difference only when using pods?

 

[DaveDanger]

Keith does seem to be one of the sharper tools in this "shed", but I'll argue a tiny bit about the wisdom of permanently removing the vent lines from the carbs. Pulling the vent line to troubleshoot a starving fuel system is fine as far as it goes, but if that solves your problem, I'd be willing to bet that the hose causing the problem is kinked, or has a dirt dauber nest in it or something along that line. A good, clean, open line isn't going to cause the starving problem alone. Replace the vent line(s) with new lines and route them so they can't kink. If you run the carbs with those vent lines permanently removed (vent nipples pointing straight up with nothing protecting them) they'll end up with rain water, dirt and grit in there. Incidentally, I usually see fuel flooding out the rear of the carb into the airbox if those vent lines begin to clog.

Addressing the question about the fuel tanks rusting out in the left rear corner... Water is heavier than gasoline and will settle to the lowest point in the tank. Most bikes spend much more time on their side stands than in any other position, making the left rear corner the lowest spot. Water can come from many sources, including contaminated tanks or fuel pumps at the station, and condensation from inside your partially filled tank when the temperature changes.

*Sigh* After posting the above a few minutes ago, I exited back to the forum and glanced at this same post and realized that I'd not read the last 2 of 3 pages before shooting off my piehole. AFTER re-reading the remainder of the post, I saw that the things I'd mentioned had already been more or less covered. Forgive my eagerness to share my admittedly minimal knowledge And yes, the vent lines off the nipples on these carbs are supposed to be open to atmosphere. If they are connected together, they absolutely close the vent system and the carbs will starve almost instantly. They will usually flood out the back end into the airbox as mentioned before.

 

[tconroy]

I know what you mean dave,my piehole opens more than it should sometimes too!Its the concern that counts though buddy!!

 

[Jackyl]

I knew I wasnt the only one

I knew I wasnt the only one

I have a 1980 GSX750E, mikuni carbs and ](*,)

I have been having the EXACT SAME PROBLEM as described in the first post in this thread, I recently cleaned the carbs, and, because I hadnt found this thread yet, capped those vents. It wouldnt even idle, now I have 2 tubes, one on each side about 8" long that go behind the transmission, I still have the dying out/starving out problem, now though it is worse, not just at high speeds but anytime its under a load, let it drift down to about 30mph, or just get off the throttle for a minute or so and it runs great again. So now Im wondering if maybe I have a clog between the nipple and where-ever-the-hell those tubes go in the carbs, I looked at the schematic at Old Bike Barn and it shows the other side of the carbs, not the side that these 'vents' are on. any ideas would be great, like this guy, I was ready to give up and just find another bike, I dont want to, I like this one more everyday, and if it would just run right, I would love it .

 

[KEITH KRAUSE]

The brass float bowl vent nipples simply lead to the float chamber. Each hose supplies venting to two carbs. You should be able to blow into them. But blowing into them wouldn't prove they are 100% clear. Flush and use high pressure air as you would in a rebuild/clean up.
The jets need to draw fuel easily. If the float chamber isn't vented well, the jets can't easily draw fuel from the bowl.
I don't know if this is your problem but it could be. You may also have a clogged gas cap vent or a petcock issue.

 

[Jackyl]

I feel like such an idiot now

I feel like such an idiot now

Ok, after 2 yrs and a new charging system, re-jet, pods, a year w/o riding, 3 times in the shop, kicking, screaming, cussing, and hair-pulling, I checked and cleaned everything, twice ! ! So there I am trying the last couple things suggested, I have the fuel cap off, still does it, vent lines off, still does it, turn the petcock to pri.

fixed.


If you see me out riding, please be so kind as to not point when you giggle, snicker or laugh. Thanks.

 

[KEITH KRAUSE]

Keith, why does it make a difference only when using pods?

Well, some days my brain works better than others and today my brain has the day off. :lol: I wonder why I didn't see your post for all this time?
I've tried to give my opinion on this a few times in the past and I'm sure someone could use the right words to explain better than me or actually knows the entire sequence of events that explains what's happening.
I just know that when you greatly increase air flow into the stock carbs, you're also changing the vacuum and the pressure. These changes will effect the venting and the fuel flow. These changes effect some models more than others but it effects them all. Some models only seem to have problems in crosswinds or other windy conditions, while others will have serious fuel starvation just cruising down the street on a calm day.
I called up Dynojet many years ago to ask why they suggest removing the float bowl vent lines when running pods (part of their stage 3 and 7 jet kit info). They said "a vortex will compromise the air flow into the vent if the lines are left on." That's about it.
I don't know how much of this problem is related to air travelling across or around the pods or how much is related to increased air flow INTO and THROUGH the carbs but on a few of the bikes I've tinkered with, the problem was there regardless of outside wind speeds.
I can see how a vortex could be created at the line end, such as blowing across an open tube. A vortex would create a resistance in the tube and any resistance could compromise the venting and this would make it harder for the jets to draw fuel from the bowl. I don't know why this same vortex wouldn't be as bad when acting upon the vent nipple.
I've read where this vortex can be strong enough under some conditions to actually create a vacuum at the vent line and this will draw on the bowl and defeat the venting to at least some degree, if not completely.
I've also considered that the stock venting system, just a float chamber and tube, could be inadequate to allow for the increased flow the carb is experiencing with pods. If more fuel is exiting the bowls then the stock venting may not be adequate to keep up with it under some conditions. In this case, the best thing we could do is remove the vent hose and that would at least MINIMIZE resistance to best venting. Line/tube length obviously factors in but I've had no luck by "shortening" the lines either. It's either remove them completely or don't run pods on many models.
It could be a combination of things or something else than the above. You'd be amazed at how much air your bike sucks in and it relies on proper venting.
I do know that even with very good jetting, most pod equipped bikes will show some mild hesitation and fuel flow problem while in stronger crosswinds. It's not a true mixture problem. It's because the jets can't draw fuel easily under this condition. Fuel starvation.
I remove the lines because it usually must be done if you want the bike to run well. I or my friends have never had a problem with the open nipples getting bugs or dirt in them or water either. Obviously, it could happen though. I've never had a serious problem from crosswinds either, but I've felt hesitation and even got a few "spits" out of the filters at the time. Just a minor annoyance in my case. I know what it is and that it will pass quickly. Even if expecting it, it can be difficult to tell if any hesitation is from the pods or just the wind hitting you and buffeting you around.
Hope this helps.

 

[KEITH KRAUSE]

Ok, after 2 yrs and a new charging system, re-jet, pods, a year w/o riding, 3 times in the shop, kicking, screaming, cussing, and hair-pulling, I checked and cleaned everything, twice ! ! So there I am trying the last couple things suggested, I have the fuel cap off, still does it, vent lines off, still does it, turn the petcock to pri.

fixed.


If you see me out riding, please be so kind as to not point when you giggle, snicker or laugh. Thanks.

Many have done the same thing. The '80 petcock is infamous for failure. Actually, the factory manual says the '80 petcock still needs engine cranking to initiate fuel flow EVEN IN THE PRIME POSITION. Almost like it has two "ons". Never really understood its operation even after reading it. Those petcocks belong in hell.
They should make a diaphragm/rebuild kit for your valve but I like the Pingel fuel valves much more. When they're on they're on and when they're off they're off.

 

[ericox]

Answer me this, does this apply to the vent tubes coming off the "T" fitting between the 1-2 and 3-4 carbs on my 82 750T? In not having any problems to speak of, but im just curious. Bone stock bike.

 

[Jackyl]

Actually, the factory manual says the '80 petcock still needs engine cranking to initiate fuel flow EVEN IN THE PRIME POSITION. Almost like it has two "ons".

When in the 'Pri' position, mine is definatly "on", after just a few moments I have fuel on the ground under the bike. I am going to replace it but Im not sure wether to go stock (OEM) or the pringel (sp) petcock. Im not so much concerned with cost at this point (too late for that lol ) but I know I have to get one ordered today as I dont have the self discipline to not ride now that its running .

Thanks for all the help, as me and my antique start racking up the miles and long trips Im sure we will be here a lot

 

[KEITH KRAUSE]

Answer me this, does this apply to the vent tubes coming off the "T" fitting between the 1-2 and 3-4 carbs on my 82 750T? In not having any problems to speak of, but im just curious. Bone stock bike.

I can't see your carbs of course, but yes, on a stock bike, any vent tubes should be attached and routed so there are no kinks.
Any bike with pods/major air box mods should remove the tubes and leave the ports open to breath.

 

[KEITH KRAUSE]

When in the 'Pri' position, mine is definatly "on", after just a few moments I have fuel on the ground under the bike. I am going to replace it but Im not sure wether to go stock (OEM) or the pringel (sp) petcock. Im not so much concerned with cost at this point (too late for that lol ) but I know I have to get one ordered today as I dont have the self discipline to not ride now that its running .

Thanks for all the help, as me and my antique start racking up the miles and long trips Im sure we will be here a lot

If you have the stock petcock, I believe it's the one where you turn the slot. There's only an on and a prime on this type. Most people hate 'em.
I suggest a Pingel valve regardless of your vacuum petcock type. Mine was about $90 shipped, with adapter plate. The dealer wants about the same or more for the stocker. Money isn't the issue here as you said. Peace of mind is.
Only "con" about the Pingel valve is it should be turned on and off in my opinion. I know the float needle valves SHOULD seal against excess fuel flow, but I just don't take the chance of fuel entering the crank. I always turn it off after each ride. It's not an annoyance to me.