qmb139 loses fuel when riding for ~30min

[chris23413]

Hello all! I have recently ran into an issue with my tao tao moped with the qmb139 motor. It runs just fine when first warmed up, but after a short ride (30-45min) it will lose fuel and stall. It will fire after it does this, but it won't stay running unless you blip the throttle or hold it at WOT. I've encountered this issue on three separate occasions over the past week or so. My first thought would be vapor lock, since it only happens when the scoot is at operating temp (240-270deg F). I did recently go through the fuel lines and put a fuel filter in-line coming from the tank that is a size too big for the application, but it seems to work so I stuck with it, maybe that's the cause of the issue. Anyone else have any ideas on what the issue might be? It's odd that it runs fine for a while but then stalls out

[FrankenMech]

Crack open the fuel tank cap and see if that helps. Sometimes the fuel tank vent gets clogged. Some tanks vent through the cap and others vent through a tube. A clogged vent will prevent fuel flow.

[chris23413]

Thank you for the suggestion FrankenMech. I have inspected the fuel cap, as that's where the tank vents from, and I tried your suggestion, didn't seem to make a difference. (Cap leaks anyway, didn't expect it would be an issue)

[FrankenMech]

What year is your scoot? Is it Carbureted or fuel injected? Where is your fuel tank located? Is it above the carb or below the carb? If it is below the carb you will have a fuel pump to deal with that may be vacuum operated with a carb. If the vacuum line that pulses the pump has any restriction or fittings/splices in it the pump will not work with prolonged full throttle operation.

[chris23413]

The scoot is a 2015 Tao Tao speedy50. Carbed, not fuel injected, fuel tank is above the carb and runs out of the tank, through the in line filter, to the vacuum petcock, directly to the carb. I did inspect the single vacuum hose going to the petcock and I didn't notice any cracks/ splits in the hose. I plan on deleting that vacuum petcock in the next couple of days so the fuel can be manually controlled coming from the tank instead of constantly relying on engine vacuum to provide a consistent fuel flow.

[GrumpyUnk]

I would check the fuel flow out of the tank, through the petcock & filter to the inlet of the carb. You could have a buildup of 'stuff' in the tank that slowly migrates to the screened outlet and disrupts flow. That could take some time running which fits with the reported delay before it starts to fail.
The heat could affect the vacuum operating the petcock if intake or carb get loose when heated. Check the intake elbow bolts and carb mount bolts for tightness or evidence of leaking. They have O-ring styled seals that could flatten with age/heat and allow air to leak in, leaning the mix and reducing vacuum.
Most times a vacuum petcock will work or not, nothing in between. They can develop leaks in the diaphragm or the diaphragm can get stiff with age, but there likely would be other symptoms. Fuel leak would make the mix rich, etc.
You could check the outlet screen by disconnecting the fuel line at the petcock inlet and letting fuel run out for a while to see if it reduced flow over time. Marginal flow due to any of the above could get too low for proper operation, so check that the flow is full, not a trickle, at the carb inlet.
tom

[snaker]

I had a China GY6 with a similar issue, but it was intermittent and hard to track.
I looked over the hoses, replaced fuel hose and filter.
I tested the petcock through the vacuum hose.
All good.
Then one time it stalled on a ride.
I was looking things over and just happened to notice, at just the right angle, the petcock vacuum hose was kinked where it routes around the carb.
Apparently, it would soften up and kink when warmed up from engine heat and open up enough when cooled down.
I didn't have the proper size hose handy, so I took an old spiral notebook wire, spiraled it tighter to fit and positioned it on the hose where it wanted to kink.
The spiral worked to keep the hose in a round shape and not kink.
Problem solved and still on years later.

[averagehillenjoyer]

I had a similar issue due to my own mistake when changing fuel hoses; the seat bucket pinched the fuel hose between frame tubing and the seat, and whenever the hose got warm enough it was so soft it would get pinched down. Bogged my mind for a few days lol

[classacted]

I have a tao tao and that vacuum diaphram misbehaved early on. I was lucky that it happened at home. replaced it with an inline valve (don't use plastic because every one I've used became hard to move the lever) and haven't had a problem in almost 6000 miles. I just stopped up the hose at the intake.