Scooter idle drops to very low RPM after giving throttle By linkmodo (Read 2,845 times)

[linkmodo]

Howdy folks,

My first post here.. I have a 1998 Yamaha Majesty 250cc scooter, that I've done a complete restoration to bring back to good ridding condition.. But there is one problem that I was unable to fix and it's getting furstratring and don't know the cause...

The scooter starts and idles just fine at factory idle speed on the kickstand (1500rpm)... But when I actually start to ride and give it throttle and then suddenly drop the throttle like in stop & go traffic, the scooter's idle would sometimes drop to below 900rpm and sounds like the engine is gonna stall any moment.. When this happens I usually give it some more throttle to counter the effect of low rpm, and it usually would bring the idle back up to 1500rpm.

And to make the situation even more complicated to diagnose. Sometimes when I rev the throttle and release it, the rpm would be well over idle speed (2000rpm) before it drops down to 1500rpm in less than a min or so.

The scooter rides long distance just fine, and hits 70+mph on highways also no problem... I have just recently cleaned the carb, and adjusted valve clearance. Also checked around for leaking tubes and carb and have found none.

The scooter however does have a fairly big leaking point on the exhaust where it is leading to muffler, that previous owner did a horrible job at welding. Could this hole be the cause?  There is no one selling replacement exhaust for this scooter any more and welders I've talked to all said they can't patch the hole...

I'm wondering if anyone experienced similar situation as me and know of the cause to this issue? Your help is greatly appreciated!

[jct842]

I would go over the carb again. Watch for a small tear or hole in the diaphragm. Pay attention to a vacuum leak, gaskets, cracks in the rubber manifold and hoses. A vacuum leak can be found by spraying a mist of carb Cleaner, ether, even water all around the carb and manifold. If you hit a leak the engine speed will change...could go up or down depending.

it is posible the leak in muffler has a very small effect. As long as there is steel around the hole a good welder can patch. Many good welders have no experience with real thin metal, but there are some at muffler shops that could fix you up. Bad portions can be cut out and replaced.

[linkmodo]

Ok, will check again on the Carbs when I get the chance, thank you.

[linkmodo]

Checked the scooter and spraying carb cleaner around automatic choke caused the engine to stall.. It was the automatic choke that's failing, the worn out o-ring does not seal it well in the grove and I can even hear the air gets sucked in from the leak..

Ordered replacement auto-choke hopefully get the problem fixed once and for all.