Removing carb vent lines solves starvation mystery

[Steve]

You just brought back a seven-year-old thread, so I did not bother reading it all, so I will ask you one question:

Do you have pods on your bike?

It is quite common for bikes with pods to have carburation problems (besides the usual jetting issues) that can be traced to the vent hoses ending in turbulent air.
You can re-route the hoses so they will sit in relatively still air or you can remove them and turn the fittings downward to prevent water entry.

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[1980GS1000E]

I learned about this the hard way :-)

[Jeffrey650]

No I don't have pods in my bike.

[gsinister]

Steve, If I turn the fittings downwards is there any chance of siphoning due to the fitting end being lower than the float height? GS1000G with APE pods and V & H 4 into ! pipe.

[1980GS1000E]

Steve, If I turn the fittings downwards is there any chance of siphoning due to the fitting end being lower than the float height? GS1000G with APE pods and V & H 4 into ! pipe.

I can answer that. Yes, there is a strong chance of that happening.

[posplayr]

Well, some days my brain works better than others and today my brain has the day off. 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.

Not sure if this is going to work yet. I am running Mikuni 135 mains v.s the stock 110's. I have tried to minimize the drag in the lines.

http://www.thegsresources.com/_forum/showpost.php?p=1870527&postcount=42

[Revelation22]

Got here for the 1st time in 6 months because I was having this same asperation problem and what'd a know fixed by yanking those tubes off. just got to get the Idle right and I'll be a happy camper

[gearheadE30]

So this is a really old thread, but my experience has been that removing the tubes completely can cause issues because the vacuum in the carb changes when wind blows across the carbs. This is why the vent lines are routed behind the airbox on a stock bike - this is an area of stagnant air. When I had pods, I routed the vent lines all the way to the battery box, and it made a big improvement and was unaffected by crosswinds.

[KEITH KRAUSE]

Never saw any slide type carb/pod filtered bike run better with the vent lines still attached. Dynojet tells you to remove the lines in their stage 3 jet kit.
The stock venting is inadequate for a freer flowing intake. The jets can't draw fuel as easily. Fuel starvation results. Crosswinds worsen things.
Most common way to address the problem is by removing as much restriction as possible. The floatbowl lines add to restriction. They also contribute to a vortex created inside them that compromises venting.
If you increase the flow in the intake, the bowl venting needs to be increased the same amount. If you can't easily do that by other means, then you remove as much restriction as possible. Intermittent poor venting/fuel starvation still happens, but it's much less with no lines to add to restriction.

[OldVet66]

I haven't run the vent lines since you told me to lose them long ago with my stage 3 jet kit Keith. I have never experienced any cross wind problems and have ridden in the air wash of a tractor trailer where a friend on his 1150 was having extreme problems. I think it must affect CV carburetors more than VM's.

[Steve]

So this is a really old thread, but my experience has been that removing the tubes completely can cause issues because the vacuum in the carb changes when wind blows across the carbs. This is why the vent lines are routed behind the airbox on a stock bike - this is an area of stagnant air. When I had pods, I routed the vent lines all the way to the battery box, and it made a big improvement and was unaffected by crosswinds.

My son's 650L has pods and a pipe. It also has vent lines on the carbs.
Yep, they are routed back and end up near the battery box, and the bike runs quite well. If there is a problem, I must just be too insensitive to it to recognize it.

Never saw any slide type carb/pod filtered bike run better with the vent lines still attached. Dynojet tells you to remove the lines in their stage 3 jet kit.
The stock venting is inadequate for a freer flowing intake. The jets can't draw fuel as easily. Fuel starvation results. Crosswinds worsen things.
Most common way to address the problem is by removing as much restriction as possible. The floatbowl lines add to restriction. They also contribute to a vortex created inside them that compromises venting.
If you increase the flow in the intake, the bowl venting needs to be increased the same amount. If you can't easily do that by other means, then you remove as much restriction as possible. Intermittent poor venting/fuel starvation still happens, but it's much less with no lines to add to restriction.

Keith, you are the recognized master of carb jetting, but that highlighted passage still has me puzzled.

It is my understanding (please correct me if I am wrong) that there will be a little bit of air that moves through the vent tubes as the carbs fill with gas the first time. After that, the floats and inlet valve needles will keep the fuel level quite constant. If the fuel level remains constant, just how much air needs to move through the vent tubes?

The purpose of the vent tubes is to provide an atmospheric reference for the float bowl. It is the difference between that pressure and the air moving through the venturis that draws fuel through jets.

I will not argue that (most of the time) removing the vent tubes helps, but, like gearhead30 and I have done, moving the ends of the tubes to calm air, where they have a steady pressure reference seems to work quite well. Stock-length vent tubes might end in turbulent air, I installed longer tubes to be able to run them back farther.

I am still convinced that the problems are because removing the airbox disturbs the airflow immediately behind the carbs. Leaving the stock vent tubes in place has them ending in that turbulent air. Removing the tubes leaves only the nipples on the Tees, which are in relatively calm air between the carbs.

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[KEITH KRAUSE]

I'm no master of jetting, I just pass on the results of my trials and errors. If something works, either from my experimenting or from someone else's advice, I pass it on.
Dynojet first told me about removing the vent lines on slide type carbs when running pods. Since DJ doesn't explain, I've tried to state what I think is happening. I have no idea why some bikes are effected more than others. Most people say removing the lines improves performance.
Better venting improves the jets ability to draw fuel from the bowl. If you increase flow, then it figures you should increase the venting or the stock/inadequate venting will regulate fuel flow in some conditions. That's why DJ says removing the vent lines is especially important in California and on highways where crosswinds are common. Those are DJ's words.
Removing the lines minimizes restriction. It also appears to help minimize a vortex effect in the vent passages. That improves the venting.
I know trying shorter lines doesn't help. I've never tried longer lines. I've tried using lines that reach to the battery, laid in different places, with no improvement.
I guess you do what works. I just wanted others to know there's options.

[Steve]

I just don't understand the origin of the vortex. A vortex will happen when you have relatively large quantity of a fluid (liquid, gas, etc.) moving in a circular pattern. Once the bowls are filled, there is virtually NO air movement in the vent tubes.

As mentioned earlier, removing the hoses moves the atmospheric reference point to the end of the T between the carbs, which is relatively calm air. My son's bike has longer tubes that end over the battery, under the seat, again, in relatively calm air. I think that "calm air" is the most important part of the puzzle.

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[Gorminrider]

... a vortex on a bike would seem to be as a result of bits of hardware in the (apparent) wind where a vortex is created by the drag of these.
At the tube,
If air blows acrooss the top of a tube,air- pressure is reduced in the tube and creates suction...The floats+needle will stop gas and maintain its level,but it will take slightly MORE float pressure given there is a vacuum of air above the gas level ...Don't know offhand what other passages inside the bowl and above the fuel level and it may affect

If air blows INTO the tube, then there might be an opposite effect.

apparent wind is important. a crosswind of 30 on a bike doing 60 creates an apparent wind of 67 from an angle 26 degrees to the bike.

a vortex can become a kind of battering pump. Look at flags as a gross example. but I wouldn't expect pieces on a bike to "flutter" so pumping would be more an effect of changing relative wind. hmmm.


not that this happens to me. It'd be the last thing on my troubleshooting list put it puts me off pods,I think...that just rely on screens and filters for shelter.hmm.

[KEITH KRAUSE]

Steve, DJ did mention in a second phone call years ago that a vortex forms. How smart the guy was on the phone, I have no idea. He was also busy and didn't want to explain.
I'm no expert on vortex's or any of this. I can't explain why my bike ran really bad with the tunes attached and cleared immediately when they were removed, and why some bike don't. It offers another option when tuning. Easy fix if it helps. Some guys said they were going nuts trying every possible jetting combo and getting nowhere. Then they pull the tubes and the bike runs like they think it should.