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i have a 1985 vulcan 700. run good when the gas tank is full and down to a 1/4 of a tank, at this point the motor boggs down. the motor will run but it acts like it is not geting a enough gas. know matter how much you open the throttle. the bike will only run at 50 to 55 mph. if you take and fill the gas tank back up the motor run fine. untill it get back down to a 1/4 tank and start bogging down again.this happens after i ride it for about 10 miles, turn it off,let it setfor a while and then take off again. i have check the gas tank by cleaning it out. check the pitcock filter for blockage. check the vent in the gas cap. which is clear.i put a whole can of seaform in the gas tank 8oz at each fullup. the gas fiters are good. it has me stump. need help!!!!!!!!!!!!!!!!
 

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This sounds like poogs (phantom out of gas syndrome). You need to clean the tank vent that goes from under the cap to the hose that vents at the bottom of the tank under the seat. Remove the cap, cover the tank with a rag, and spray some carb cleaner and compressed air down the small hole. You should see cleaner coming out under the bike. Also with the red spray tube spray into the 3 tiny holes on the under side of the cap. Let it soak and then repeat. If you don't get cleaner to come from the tube under the tank you will need to find something to push down through it so it is free of blockage. One thing to try to see if it is plugged....when it does this again shut the bike off and open the cap. If you hear a sucking sound the tube is plugged.

Some folks on here have taken the cap apart and removed something but I can't remember how that goes though.
 

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Here is a writeup on cleaning the gas cap vent and vent tube;

I just went through this on my ’06 (non-CA model), exact same situation too (stall at ½ tank gas). Try this (again '06 non-CA model, other models and CA emissions may differ):

Step 1: Clean the gas cap vent tube:

1. Place a piece of paper in front of the REAR tire
2. Open the gas cap
3. Spray carb cleaner into the vent tube located on the tank, near the base of the gas cap hinge. Use the straw attachment for the spray can.
4. Observe the drips onto the piece of paper. Continue spraying until it runs clear.


Step 2: Clean the gas cap vent mechanism

1. Open the gas cap and remove the key.
2. Cover the tank inlet tube with tape or a piece of paper to prevent dropping things in the tank. I used a large PostIt note, which worked well.
3. Remove the two brass screws that hold the latch mechanism on the bottom of the gas cap.
4. Remove the latch mechanism. Note: there is a loose component in the base that covers the latch spring. Be careful not to drop it into the tank during removal. Inspect the latch, clean as necessary (chances are this is NOT where the problem is), then set aside.
5. Careful not to loose the 5 small springs mounted on posts under the latch mechanism. You can leave them where they are or remove them for safe keeping.
6. Before proceeding remove the small collar from the male latch connector (look for the round key cylinder). The collar acts like a bearing when opening and closing the latch using the key. It is loose and easily lost. Slip it off and put it aside for safe keeping.
7. Remove the two short screws at the base of the gas cap vent, but DO NOT yank the vent off yet.
8. The vent gasket is glued to the cap (at least it was on mine) and the vent assembly might be stuck to the gasket. GENTLY, wiggle the vent back and forth to loosen it and prevent damage to the gasket.
9. With the vent assembly removed, inspect the gasket for tears or bulges. Spray with carb cleaner (just a little) and wipe with a clean cloth. Inspect the cloth for signs of debris.
10. Now for the root of the problem; Remove the small round plastic check valve/liquid separator located on the underside of the vent assembly. GENTLY ply it out, careful not to damage the silicone valve covering the base of the check valve. Remove the gasket as well and place it aside. Inspect and clean the check valve, ESPECIALLY the tiny 90 degree elbow which leads to the vent hole. NOTE: there is another silicone valve cover permanently mounted in the gas cap vent itself (part of the pressure release part of the valve). DO NOT attempt to remove it or you will damage it. Just make sure it is clean and functioning.
11. Now check the vent hole. Hold the vent assembly up to the light and look up from where the plastic check valve was mounted. See any light where that tiny 90 degree elbow fits? Follow the vent channels on the opposite side of the vent assembly. Any blockage? Unplug the holes and clean the channels.
12. Reassemble.


Step 3: Repeat Step 1 frequently (at least once a year) to prevent this from happening again.

UPDATE 5/5/2010: My Phantom has been uncloaked! I took a closer look at where/how the rubber vent tube exits in front of the rear tire. I found that the tube had been (and still is) rubbing against the rear tire. The 'black fluffy carbon' blocking my vent was probably rubber shavings caused by the vent tube rubbing against the tire. I've temporarily rerouted the vent tube to exit along side the coolant overflow tube to eliminate the vent tube/tire rubbing. I still plan to flush the vent once a year to reduce the possibility of dirt and dust reaching and blocking the actual gas cap vent.​
 

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Step 2 may not be needed to be done if you do as I did by just spraying through the tiny holes with carb cleaner. As you spray you should get cleaner coming out of the other 2 holes. Now...... if you do not get that result then disassembley of the cap will have to be done.
 

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Step 2 may not be needed to be done if you do as I did by just spraying through the tiny holes with carb cleaner. As you spray you should get cleaner coming out of the other 2 holes. Now...... if you do not get that result then disassembley of the cap will have to be done.
Skipping step #2 in favor of spraying cleaner into the gas cap vent opening is ok when performing preventative maintenance (i.e., yearly) and POOGS is not evident. However, if POOGS is suspected, then I do not recommend skipping step #2 for several reasons;

  • There are two vent channels carved into the gas cap vent (at least on the '06 non-CA model); a primary and expansion only. Spraying may clear the first, solving vacuum related POOGS, but it is unlikely to clear the second channel which can lead to an over pressurized tank and carb flooding.
  • There are two silicone check valves that should be inspected. The high pressure of the spray may be sufficient to open the valves, but the vacuum causing POOGS may not.
  • The gasket could be damaged or sagging into the air channel. Again, the high pressure of a spray can may overcome a sagging or damaged gasket, but the vacuum causing POOGS may not.
IMHO using spray cleaner only, without dissembling the gas cap vent should be done once a year to prevent POOGS. However, if POOGS is suspected, then verifying that the entire air path is clear and all vent components are functioning should be done. That means disassembling the gas cap vent and performing a visual inspection of all components. It only takes about 15 minutes, is time well spent and worth the piece of mind knowing that the entire system is clear.
 

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Discussion Starter #6
i cleaned the gas cap. the there are no vent tube on the tank at the back end by the seat. the diagram show the tubes but they are not on my gas tank. the hoses are there but not hook up. the gas tank must have been replace.
 

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Hello, I've had the same problem and there are NO vents in my tank either. This is a frustrating situation which I have not had any luck fixing up to this point.
 

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i cleaned the gas cap. the there are no vent tube on the tank at the back end by the seat. the diagram show the tubes but they are not on my gas tank. the hoses are there but not hook up. the gas tank must have been replace.
It appears that the '85-'87 models have a different tank venting system then the later models. However, no one seems to know how the earlier system works. So, how did you clean the gas cap? Did you disassemble the gas cap and if so, what is inside? Is there some kind of vent inside the cap? Where does it exit? Can you snap some pictures of what's inside the gas cap?
 
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