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Why do my 78 GS1000 carbs have 3 bowl vent tubes???

  • Thread starter Thread starter Anonymous
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Anonymous

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I know the tube on the 3rd carb runs the petcock. I know that 2 & 4 are vent tubes for the bowls and that the tubes should be open if running a DynoJet kit. For some reason, however, there is a vent tube on the 1st carb. Is this a vac. tube or a vent? I don't feel any suction when the bike is running, and the other two sets of identical carbs for my bike don't have this tube. Any idea what gives? They are the carbs that came on the bike and have not been molested.

I'm confused...for now I'm going to leave it open and see what happens. If anybody can set me straight, I'd appreciate it.

Thanks,
Jay
 
the bowl vent tubes are not vacuums.

the vacuum hoses (and there should only be 1 for the petcock) are attached to the little nipples on the cv diaphragm caps.

this is all the help i can provide as i've never even seen stock carbs (plenty of flingflops though)
 
Re: Why do my 78 GS1000 carbs have 3 bowl vent tubes???

KnuckleBuster said:
I know the tube on the 3rd carb runs the petcock. I know that 2 & 4 are vent tubes for the bowls and that the tubes should be open if running a DynoJet kit. For some reason, however, there is a vent tube on the 1st carb. Is this a vac. tube or a vent? I don't feel any suction when the bike is running, and the other two sets of identical carbs for my bike don't have this tube. Any idea what gives? They are the carbs that came on the bike and have not been molested.

I'm confused...for now I'm going to leave it open and see what happens. If anybody can set me straight, I'd appreciate it.

Thanks,
Jay
Click to expand...
#1 shouldn't have any nipple, vent or vacuum. You say they are the carbs that came on the bike, but are you saying you had the bike since new?
If it was a vacuum port, left open, the bike would run badly. If it's a vent, you have a #2 or #4 carb installed where a #1 belongs. You should have noticed a fuel flow problem because the float bowl venting passage to what is your #1 carb wouldn't be connected to a vent. The #2 carb provides venting for #1. Their passages/chambers connect. Two #2 carbs side by side wouldn't connect. A #4 carb in place of the #1 wouldn't work either. Does the side air screw and float bowl drain screw point outward?
I remember seeing some E-bay carbs recently that were mixed up similar to what you're describing. I could see at least two bowls pointing the wrong way. This stuff can happen.
 
It's seems a mystery where this carb came from because it can't be a #2, #3 or #4 carb because the #1 carb is the only one that has a single fuel inlet on the right side. 2 and 3 have two fuel inlets while 4 has only one on left. All 4 carbs are actually slightly different and not really interchangeable. I suppose 2 and 3 could be swapped if you also swapped vacuum and breather hoses but that's all.
 
Here's a crappy picture of the left-most carb. You can see that the fuel hole is plugged, so it couldn't really be in any other position. I didn't buy the bike new, so I can't say that these are the ORIGINAL carbs, but I don't think that anyone ever had them apart before I got the bike - all the screw heads look like brand new....

number1carb.jpg
 
Sandy said:
It's seems a mystery where this carb came from because it can't be a #2, #3 or #4 carb because the #1 carb is the only one that has a single fuel inlet on the right side. 2 and 3 have two fuel inlets while 4 has only one on left. All 4 carbs are actually slightly different and not really interchangeable. I suppose 2 and 3 could be swapped if you also swapped vacuum and breather hoses but that's all.
Click to expand...
For another reason you described (fuel inlets), the carbs can't be swapped. To operate correctly, the carbs can't be swapped at all, not even 2 with 3. The vent passages between 1/2 and 3/4 must connect. They can only be connected in that order to be vented correctly.
I suppose you could modify the carb though. Maybe drill out an existing plug or passage to make things work. Never tried it, but may work?
 
Jay, you've got a bonafide freak carb there. :lol:
I think it's a factory goof. Looking at the fuel passage/inlet, it suggests it was meant to be a true #1. But the presence of a brass nipple suggests it was meant to be 2/3/or 4. I'm guessing it was meant to be a #1 and the brass nipple was added by mistake. I don't know. What matters to you is how it's actually operating.
The nipple is mounted in the same place if it's meant to be used for vacuum or venting. Inside, where that nipple leads is the question. If the carb was drilled out to provide vacuum, it would lead to the main bore and you would feel vacuum. Unplugged, the bike would run badly. You would have to notice it. So it's not drilled out to provide vacuum.
On the other side of this carb, where we can't see in the photo, there should be a 3/4" or so rubber tube connecting #2 vent with this carb. If so, then I would guess it's vented. If you blow into #2 nipple, it should go through to #1 bowl? If not vented, this carb would show fuel starvation symptoms. Some people notice it more than others, but it is noticable. Some people think it's a lean condition and they try to re-jet it out. Can't be done.
I would guess if you blow into the nipple on this carb, it's plugged? If it goes through, then taking off the bowl should show you the air coming into the float chamber. Could be double vented? At least you can determine what it does.
 
My guess was freak carb. I had my valves adjusted a couple of weeks ago and finally got around to setting my float levels precisely and changing the needle clip as Keith suggested in one of our earlier discussions about this bike. I bench synched the carbs, started it up, and it runs absolutely wonderfully! It transitions from pilot to needle to main without any kind of noticeable changes - just one smooth acceleration from first gear right on to fifth. The air screws now make a difference in idle where they weren't having any effect before. If this were a vacuum nipple, I don't think it would run this well. This carb had actually been running rich before I set the float levels and bench synched it - so I doubt that it's got a vacuum leak. I also don't feel any suction when I put my finger over it.

Thanks for the input guys. As soon as I have a nice day to properly synch the carbs with my manometer that will get done. Then I'll break out the ColorTune and really get the sucker dialed in.

Later,
Jay
 
If they plugged the fuel hole, then they probably plugged the vacuum hole. It is a #3 carb someone adapted to be used as a #1.
 
The fuel hole wasn't plugged, it was never drilled out. It's kind of tough to see in the picture, but that hole ends with a contoured casting of the carb body - no filler.
 
I too have the VM carbs that came with my bike (in a box disassembled) and they have three apparent vacuum/vents out of 4 carbs. :?
 
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