50cc QMB Scoot cuts off at 40mph

[pinkscoot]

So my new buddy Charlie from Taiwan that runs the only scooter shop in town. While I was returning the triple tree I bought he asks me what would cause a 50cc QMB scoot to shut off at 40mph. I didn't ask if he was kidding about a 4 poke 50cc going 40mph I figured it was an indicated not GPS. It runs fine until 40mph when it stops. When it slows it starts back up and goes. Apparently the owner doesn't want to limit his speed to 39mph. I suggested that the rev limiter on the CDI was hitting the kill switch circuit. He said he tried 4 good CDI's and it still happened. He thought the stator was bad and when it got to 40 it just stopped working, I said maybe. So I searched the inter web and here and found nothing like this. Have any of the genius' here ever run into this? 

[benji]

Over heating maybe? Im guessing 40 is top speed, maybe it has a blown head gasket that opens up when the motor gets a certain temp and closes back up when the motor cools down.

[pinkscoot]

That sounds plausible. I may see him today and investigate more.

[tortoise2]

May 27, 2019 19:20:31 GMT -5 pinkscoot said:

what would cause a 50cc QMB scoot to shut off at 40mph 

If floor level gas tank . . deficient vacuum pulse-pump is one of the usual suspects?

Might also heat-test pulsar/pickup/trigger coil.

[GrumpyUnk]

I just purchased a Jonway Victoria YY50-QQY29 that has/had a limiter. There is a pickup inside the CVT chamber, that counts pulses from the clutch bell, just like the pickup on the flywheel that the spark timing uses. A rectangular bump comes close to the pickup, and sends a signal to the CDI. When I got the machine, the wire was/is disconnnected. I assume that the pulse is used as a 'kill' to trigger the CDI into cutting spark, thus limiting top speed. When in use, the spark would stop, the engine slow, the scooter slow, and once below the target, the scooter would pick up again.
A second possibility is fuel flow. In that case, I would expect the engine to run as if it were running out of gas, puffing and snorting as it fired and mis-fired on lean mix. The engine would slow, the level of fuel in the bowl would increas(somewhat) and the engine would re-fire more regularly until it ran low and got lean once again.
If the engine shuts down, BANG RIGHT NOW, with no stuttering or other indication of misfire, I'd suspect a limit such as the Victoria Jonway pickup limiter.
There are CDI's that have limits, and there are also variator 'slugs' that limit how close together the variator plates can get, limiting how 'big' the effective gear can be, so the rear wheel rpms are limited by the variator being restricted. You can remove the slug between the variator pieces and re-assemble to remove that limitation.
One last thing to check is the fuel tank vent. Proper cap installed? Is it vented? Finally, you can disconnect the fuel line when the condition occurs, and check that the float bowl is empty/full immediately stopping after the problem. If the float bowl is empty, the engine won't run well...
tom

[pinkscoot]

Thanks Tom, all good possibilities.