fuel system failure

[duosport]

I decided to ride my F15 to work today. I had a pleasant ride this morning. When I started my scoot up at 5:00pm to come home it sounded like it was not going to hold idle. I headed home and got about 1.5 miles away from work and it stalled out and would not start. I wheeled it to a parking lot and saw that fuel was leaking from the carburetor non stop. I did not really have much for tools with me. Fuel was not only leaking from the drain but seemed to be flowing all aroun the carb body. I figured I had a pet cock failure and a failure at the carb too. I opinched of the the fuel line with my fingers but when I let go it flowed again. But when I walked the scooter two feet the flow stopped cold. If I turned the key to start it the flow would begine again. If I walked the scooter it would stop. I ended up walking the scooter back to my work the 1.5 miles and was able to secure it inside for the night and got a ride home. I guess I am going to try to repair it after work tomorrow at my work place, which is an office building.

Drag.

[duosport]

So I am guessing the float is stuck open and the petcock has failed. So what I question is why would the gas gush out and then I roll the scooter 1 foot and the whole thing stops? What is causing the flow to be interupted by pushing the scooter forward?

[redorchestra]

I was wondering the same thing. And why did it happen after sitting for 8 hours. I had an experience when I was riding to work and the scoot suddenly died. I checked the mixture screws and someone closed them down really lean. It wasn't me, I think It was sabotage.


How about yours?

[duosport]

Well my petcock no doubt failed some time ago. I guess I would not know it if the carb float is shutting the fuel supply. A float can stick at any time. What is weird is that it would stop flowing when rolled forward. I do not get it.

[duosport]

I am guessing the petcock is simply wide open and the float is stuck and when I roll the scooter it maybe vibrates the float enough to close it. I did not try taping of the carb or anything. I just need to get it home to examine it all.

[duosport]

I have read a number of people on forums who use this petcock on various Italian scooter have these things fail over and over. One guy said he had to replace the vacuum petcock on his Vespa 3 times in 5 years. That is a failure rate of almost one a year. Maybe since the gas is free flowing out of mine I could consider adding a manual shut off in line. as opposed to replacing the vacuum unit.

[90GTVert]

I'd go for the manual petcock. Cheaper and I like them better anyway. They're really nice if you do jetting. Instead of waiting for the carb to prime from turning the engine over, fuel is there as soon as you flip the switch. I would suggest checking fuel flow in the process though. If the fuel flow seems weak at all, you either need to figure out how to modify the stock petcock, replace it, or find a different method to attach a fuel hose to the tank.

I believe you are on the right track in suspecting the float as well. Could be stuck kinda on the edge or you could have some sort of debris in there moving around.

[stepthrutuner]

My Derbi has a Piaggio motor. I disabled my vacuum petcock by cutting off the little vacuum chamber (called a 'hat' in the industry) and removing the light coil spring below the diaphragm. I have a manual shut-off just above the carb but only use it when I disconnect my carb. Otherwise I just let the float needle hold up the fuel flow. I've had no problems with this.

[drawkward]

Jun 22, 2010 13:12:45 GMT -5 90GTVert said:

I'd go for the manual petcock. Cheaper and I like them better anyway. They're really nice if you do jetting. Instead of waiting for the carb to prime from turning the engine over, fuel is there as soon as you flip the switch. I would suggest checking fuel flow in the process though. If the fuel flow seems weak at all, you either need to figure out how to modify the stock petcock, replace it, or find a different method to attach a fuel hose to the tank.

I believe you are on the right track in suspecting the float as well. Could be stuck kinda on the edge or you could have some sort of debris in there moving around.

I never let the starter prime the carb. Every time I jet or otherwise drain the carb, I hook up the fuel line and just suck on the vacuum line to fill the bowl. Starts right up on the first turn usually. Maybe a slight bog then it idles just fine.

I agree that it's probably the float. I adjusted mine incorrectly once and had the same problem. There is a gasket in the carb that should stop the fuel from coming out, but it usually never does when there is a float problem. The pressure from the tank is just over bearing on the little gasket. Also, it does seem possible that there could be vacuum left over in the petcock vacuum tube causing the petcock to stay open. Maybe the clutch was still stuck open and when you pushed it forward it turned the engine once releasing the vacuum to the petcock...? OR maybe the float was stuck and when you pushed it forward the slight vibration caused it to release, cutting off the flow of fuel from the tank into the bowl.

[drawkward]

Jun 22, 2010 13:18:59 GMT -5 stepthrutuner said:

My Derbi has a Piaggio motor. I disabled my vacuum petcock by cutting off the little vacuum chamber (called a 'hat' in the industry) and removing the light coil spring below the diaphragm. I have a manual shut-off just above the carb but only use it when I disconnect my carb. Otherwise I just let the float needle hold up the fuel flow. I've had no problems with this.

Good stuff. Dirtbikes do this. Why shouldn't it work for scooters? I leave the gas on with my dirtbikes all the time with no problems. It's actually a good thing cause it can tell you immediately when you have a float/needle issue.

So how do you go about taking apart one of these little vacuum petcocks?

[stepthrutuner]

Jun 22, 2010 13:46:27 GMT -5 drawkward said:

Jun 22, 2010 13:18:59 GMT -5 stepthrutuner said:

My Derbi has a Piaggio motor. I disabled my vacuum petcock by cutting off the little vacuum chamber (called a 'hat' in the industry) and removing the light coil spring below the diaphragm. I have a manual shut-off just above the carb but only use it when I disconnect my carb. Otherwise I just let the float needle hold up the fuel flow. I've had no problems with this.

Good stuff. Dirtbikes do this. Why shouldn't it work for scooters? I leave the gas on with my dirtbikes all the time with no problems. It's actually a good thing cause it can tell you immediately when you have a float/needle issue.

So how do you go about taking apart one of these little vacuum petcocks?

If I had my camera with me I would take a picture of mine for you. What I basically did was use a cutting disc in a Dremmel tool and cut the protuberance that the vacuum line enters away flush with the remainder of the assembly.

[duosport]

Update on my saga:

I had my scooter trucked home this afternoon. I have only looked at it for afew minutes to confirm what I already observed. First there was no fuel leak as it sat. Then I pushed up on the hose to see how the drain of the carb bowl was functioning. fuel came out and when I released the drain valve the fuel stopped. I pushed again and it functioned exactly as it is meant to. Then I put the key in the ignition. As soon as I started to crank it fuel began gushing from the carb. The fuel is not really coming from the drain hose. It is simply a totally overwhelmed with fuel carb. It would continue to flow with the key turned to off. But as soon as I push the scooter forward the flow is stopped. When the gas was flowing from the carb, I tried tapping it. No tapping would stop the flow. so clearly it is not the vibration of moving the scooter forward that is dislodging a stuck float. No doubt the float is stuck but the moving of the scooter is affecting the petcock to stem the flow. This is what I gather at any rate. I think I need to order either a new vacuum assisted petcock or a manual one. Then when I take it all apart again, I will have what I need to fix it. This must be a failure of that unit correct? Or is there something else that could cause such a thing?

[duosport]

how is fuel line measured? Is it by inner diameter or outer diameter?

[190mech]

ID

[duosport]

thanks.