LCGB Forums

HI, when my scooter gets hot and I come to a stop, it starts reving it's self up. The revs then die off a bit when I put it in first and let the clutch out a bit, just so it gets the bite. The cable seems fine and it is new. Anybody have any ideas as to what it could be? Could it be the clutch?

Thanks for your time, guys.

typical symptoms of a weak idle mixture caused by an .........Air Leak somewhere on the Inlet.

If it was running ok before and its just suddenly happened I'd check the manifold to engine gasket and the carb to manifold joint for leaks.
Check the carb over and make sure the air screw hasn't fallen out

If you still find nothing you will probably need to do a crankcase pressure test


Remove carb and seal off the intake manifold
Seal off the exhaust port
Attach an air fitting that lets you pressurize the motor (usually through the inlet plug or the spark plug hole
Pressurize the motor to 6-9 psi via a hose that has a pressure gauge. (floor Bicycle pump would do)
check gauge for 5+ minutes to confirm there is no loss of air pressure.
If your gauge shows a leak, spray the gasket seams and seals on the pressurized motor with WD-40. The WD40 will make visible bubbles, and a leaking sound, when you spray the leak point.

Do this sooner rather than later or you will cause serious damage such as holing a piston due to rapid overheating.

Cheers gents. I'll get on the case. Problems are never ending haha

what type of carb have you got?

It's a Dellorto SH120 I think . Should the carb have a deal where it clamps to the manifold?

"Deal" ..... explain. Is that a typo?

Yeah, sorry. It should have said seal*

Yes it should, just replaced mine.
Old one was hard to get out and the carb body is very weak so take great care.
The clamp is not much better...

New seal came from Scoot Restorations.

Brilliant! Thanks for that. Hopefully that will solve the problem. I'll let you know.

Hi, not sure if you've sorted this but mine was doing something similar. Found to to be the throttle cable (carb end) on a phbh, re housed it and it's now fine. Really simple oversight on my part, it was doing my nut in until I'd sussed it.
Hope it's something as simple.

just to let you know, I sorted the problem. I changed the car-manifold reduction sleeve (and you were right, it was a bitch to do) to only find when I put it back together that there was a split in the air inlet hose. Changed that, and it is now fine. Thanks for all of your help, it's very much appreciated.