Removing carb vent lines solves starvation mystery

[Steve]

I have no arguement that the bike probably runs better, I just don't understand the popular reasoning why.

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[Gorminrider]

blowing more smoke here, but suckin or blowing through a long narrow tube is harder than a short tube of the same diameter,isn't it? I haven't got any handy, but I recall it is so,
or perhaps the longer airlines tubes get squashed,collapsed,dirty.

[Agemax]

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.

maybe someone who has a perfectly running bike, set up with pods/DJ kit/4into 1 and the breather pipes removed could fit the tubes, routed as they should be and then go for a ride?
would be interesting to see if the bike runs worse with the tubes fitted!

[KEITH KRAUSE]

maybe someone who has a perfectly running bike, set up with pods/DJ kit/4into 1 and the breather pipes removed could fit the tubes, routed as they should be and then go for a ride?
would be interesting to see if the bike runs worse with the tubes fitted!

That's basically what I did years ago when I completed my restoration.
My first test ride was a ride to work. Calm day, nice out. I left the tubes on because I doubted the tubes could cause any problems. The first couple miles seemed fine though the bike was still warming up. But just like that I experienced fuel starvation. I pulled over and pulled the tubes. The bike started fine and showed no more fuel starvation the rest of the way to work, about 10 miles.
I talked to a friend who was also jetting his bike at the time (same model and mod's). I told him what I did and he said he tried pulling the tubes and the bike smoothed right out. He was sure it was lean jetting but it was just poor venting.
There have been others who have had the same thing happen. So I passed it on here. If it helps, great. If not, I can't say something about a bike I had no part in tuning.

[GregT]

If I can put my oar in here, I'll certify that fuel pressure at the carbs with gravity feed never exceeds 1.3lb. Measured on many bikes.

When the airbox is removed on a GS or GSX you're left with a rather large cavity in the frame. It's not closed to atmosphere by any means but IMO when the bike is in motion a positive pressure develops in this area.

If the float chamber vents finish up in an area of positive pressure it doesn't take much pressure to stop the flow into the float chamber.

[850GT_Rider]

After our near-1000 mile trip over the 4th, I'm about to go through all the routine stuff with my bike again. Valve adjust, carbs synch (with new Morgan Carbtune!), plugs etc. Also replacing the carb boots this time around. Noticed last time they were getting pretty stiff.


As bought, my bike had pods and VH 4-1 and flattened out at ~4500 rpm. When I went thru the carbs, found out they had DJ Stage III kits. Left those intact, as well as the pods. Took off the 4-1 and put on stock exhaust. No more flat spot. I have also retained what appears to be the stock vent tubing that just kind of hangs down by the swing arm.


After I go thru and make sure everything is kosher, I'll run the bike with and without vent tubes and see if it makes any discernible difference.

[Steve]

I have also retained what appears to be the stock vent tubing that just kind of hangs down by the swing arm.

The only "vent" tubing that runs that low would be the safety drain from the fuel level sender. The stock carb vents come from the T's between carbs 1&2 and 3&4. In stock arrangement, they route over the right side of the airbox, through a loop that is on the back side of the airbox.

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[850GT_Rider]

The only "vent" tubing that runs that low would be the safety drain from the fuel level sender. The stock carb vents come from the T's between carbs 1&2 and 3&4. In stock arrangement, they route over the right side of the airbox, through a loop that is on the back side of the airbox.

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Could be they aren't stock then. Like I said, bike came with pods and I just never had the desire to do all it would take to go back to the stock carb internals & airbox, although I have one sitting on the shelf.

When I get back to the ranch and before I start tearing it down, I'll take some pics. I'm in the Czech Republic on business til next week sometime.

[rudeman]

I'm on this kinda late in the game but while in the "PRI" position, the '80 petcock only requires momentary vacuum to initiate constant fuel flow. It will continue to flow freely until the control is turned to the "ON" position. That is how it functions on my GS1000ST.

This may be a dumb question but here goes. Regarding performance improvement following removal of the vent lines...had any of you guys checked for obstructions? Insects sometimes pick the strangest places to set up a nest or to just "explore" and get stuck. That would restrict or otherwise clog a vent tube quite effectively. Just a thought.

I jetted my 1000ST after installing a Yosh pipe; upped the pilots, shimmed the needles and upped the mains to 120. Ran the stock airbox with a K&N and I enlarged the breather hole. It ran perfectly for years and I never had to pull any of the vent tubes.

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.

[Agemax]

removing the vent tubes is required for individual pod filters. it is not an issue if you stick with the stock air box

[rudeman]

I've tuned many bikes equipped with individual filters and not one of them required the removal of the vent tubes to run properly.

removing the vent tubes is required for individual pod filters. it is not an issue if you stick with the stock air box

[Darkspeed]

For the vent tubes I always make sure they are pointing down and slightly back never facing forward. Zip tie securely so they won't drip on the exhaust and then cut diagonal with the longer edge facing forward. Someone who use to race and wrench alot told me that was the proper way. Never questioned it but that's how all my bikes are run.

[posplayr]

Thanks for bring this up; it may have solved a self inflicted problem I managed to create. I got some starvation problems after I did this.

http://www.thegsresources.com/_forum/showthread.php?208339-Cross-wind-power-loss&p=1870527#post1870527