Weird Sound and Inconsistent Throttle

[Guamae]

My bike has been out of commission for almost a year now due to engine failure, but a friend of mine and I just finished rebuilding the top end of the engine and putting it back together. On Thursday we attempted to clean out the idle-screw but couldn't as the tiny screw preventing it from being removed was loosing its head and couldn't be taken out. Instead we just adjusted the idle to 1500 (according to the built-in tachometer) and then I finally got to take it for a short trip around the block and it ran better than it had in years. However, on the way home, after a stop a light I decided to open up the throttle and test the acceleration, and something happened...

After that a bup-bup-bup-bup sound started coming from under the seat and the throttle would reguarly cycle up 5 mph faster all the time (half second at 30, half second at 35, repeat) and even with my hand off the throttle it would be alternating between 25 and 30 miles per hour. The engine was racing so much that I would have to flip the kill-switch in order to slow down for turns and stop lights, and even after I did that, the sound would continue for a few seconds.

Our guess is that a bit of gunk must have fallen somewhere it's not supposed to be when I opened up the throttle, and I'm wondering if that sounds feasible, and where would be a good place to start checking?

Thanks in advance for all the help!

[Ralph M]

Check the inlet manifold is seated correctly and as no leaks or cracks.

[Scootereno]

The idler screw should be adjusted up and then back down to the desired rpm level but should be cleaned first. To clean it without removing the adjustment screw you can spray intake cleaner down the air intake and that will get it about 90% clean. The first time I adjusted the idler I just adjusted it up from 1200 rpm to 1400 rpm and after about 50 miles it started idling at 1600 rpm.

Clean it then adjust it up and back down and it should stay there until it needs to be cleaned again. Also make sure the scoot is warmed up before you adjust the idler and the high idle/"choke" has cut off.

I hope you solve your issues with the over-revving!

[Guamae]

Thanks for the advice, it was the inlet manifold. Turns out the O-ring i used was too thick (just one i picked up at the hardware store) and became unseated and was blocking the butterfly valve from closing all the way. Got it fixed and it's working great! ... forgot to spray the cleaner in at the idle screw, but at least it's working now.