Modern Vespa

I've put about 150 miles on a recent Bluebadge project. Doing fine until day before yesterday. After about a four mile run at around 50 mph, it stalled while sitting idling for a few minutes. Would not start back. Had spark and compression. Bone dry plug. Removed plastic tool box to look at carb. Left fuel cap open. Finally started back up and made it the 1/2 mile ride home.

Died again in the driveway while idling. Put gas in spark plug hole. Fired up, rode it into shop. Died again while idling.

It acted like vapor lock, but a smallie has a downhill all the way fuel line, so bubbles should rise up and out the tap vent. Black fuel line, so no way to watch flow.

I replaced the fuel line with translucent, and there it was. With the fuel line routed ABOVE the tap rod, there was a long bubble trapped in the line. Only a small trickle flowed along the bottom of the bubble even while revved. It would not go away. Not quite completely downhill. Duh, the tap rod is in line with the fuel tap, dummy.

Removing about two inches and routing the fuel line BELOW the fuel lever rod caused the bubble to disappear. It appears the vapor lock was my fault.

So far so good

Bart,

Thanks for the post. I was/am having fuel starvation issues in my BB and posted on this topic last week -- Smallframe Fuel Line Length

Still have not sorted it out and was beginning to think it might be a coil issue with the high rpm fluttering.

I am running the black fuel line, though, and your post has inspired me to switch out to clear just to make sure that I am getting good flow before jumping into electrics. I might also try re-posititioning the banjo to get the it as vertical as possible also to avoid any unnecessary bends or possibility of going uphill at any point between tap and carb.

On a possibly related note, I was worried about vapor lock so I put a safety pin through the tank cap to make sure there was no obstruction last night. Anyone have any idea if there is supposed to be something inside the cap between the smaller pin hole on the bottom and the larger hole on the top of the cap? I would imagine not, but it felt like I was pressing through cork or something, so that might be my problem if the tank cap is supposed to be hollow metal. Thanks.

I route a clear fuel line just like blackbart under the fuel tap rod because when it is placed above it, the path travelled looks 'up and over'. This can be a problem when the tanks drops to reserve level or below with not enough gravity push to reach the carb.

When reaming out the fuel cap to I would be careful using a drill bit because of a steel mesh gauze that is inside the cap that could get wrapped around it. I used a slighty oversized nail to insure a clear passage and cover your fuel opening with a rag to prevent any stuff dropping in the tank.

I also have the fuel length long enough so the tank will sit sideways on the frame when I need to get to the carb.

I think that the cap is supposed to operate as a check valve to prevent fuel coming out the vent in unhappy positions. Safety. I have drilled through with a very small bit once. You could give your scooter a big old sloppy kiss and try to blow through the vent. Or spray stuff with the little red straw.

My problem above was more of an air lock than true vapor lock. The long bubble provided an air pocket that sealed off much easier when heated than the typical fuel boiling in a full line that occurs with genuine vapor lock. The drop from the tank to the carb only provides a fraction of a psi fuel pressure. It doesn't take much of a bubble to seal that low pressure off. Winter gas formulations can boil at a little over 100 degrees.

It happens with gravity type water drain lines. Runs up and down grades, inside buildings, in the ground, or inside a scooter. Boring. Page 6 on down talks about air locks. The fuel tank vent acts as an air release valve.

http://www.agf.gov.bc.ca/resmgmt/publist/500Series/590304-5.pdf

Non-smallframes have a low point before the fuel line rises up to the carb, which has no vent for releasing a bubble.