Modern Vespa

Hello, I just brought a P back from the dead. It starts fine, runs and idles great. The problem comes when I am riding and pull in the clutch, the motor starts reving like crazy and will not stop until I cut the bike off. I am planning on changing the cable due to fraying. Any help would be greatly appreciated.

Thanks
Rover1992

Sounds like either an air leak or a clogged idle jet.

When it's running, spray some carb cleaner around the base of the carb box. If the idle changes at all, you have an air leak. Make sure the carb to case bolts are torqued correctly.

Have you rebuilt the carb yet? Maybe your carb slide is warped and sticking? When you say you're changing the cable, do you mean the throttle cable? Take off the top of the headset and lube that cable up as well as you can.

Make sure your jets are CLEAN.

Check your float needle, make sure it's clean or even better, new.

Definitely an airleak. Could be lots of things: carb gasket, fly-side oil seal, clutch-side oil seal, headleak (headleak is unlikely, IMO).

Easy checks:

-First, when it revs high (not a normal idle, it probably idles ok when cold, then idles super high and lean after it warms up) you can adjust the speed of the high idle by slowly and incrementally pulling out your choke. I wouldn't recommend riding like this unless it's your only way to get the bike home. Also check that your head is properly torqued down, sometimes they can rattle loose just enough after being put back together to give you an airleak.

- Spray carb cleaner around the base of your carb and airbox. As its sucked into the engine through any leaky spot it will rev high.

- Remove your gear oil fill screw and hold your finger over the hole while idling. If you feel suction, clutch-side seal is bad. To confirm, drain a tiny bit of gear oil, see if it smells like gas from your fuel leaking around the seal.

- If none of these help find the problem, try changing your flyside seal, as you only have to remove the flywheel and stator to get at it.

- More than that, you'll probably end up splitting the cases (or lapping the head if it's a headleak). I don't know what "bringing it back from the dead" involved, but I'd guess a rebuild? In that case (recent rebuild), it may not be your seals, could be the head.