Removing carb vent lines solves starvation mystery

[Anonymous]

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)?

[Anonymous]

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!

[Anonymous]

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.

[Anonymous]

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.

[Anonymous]

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.

[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.