Advice please: 49cc won't go over 20 mph

[bjh517]

Hi

My wife and I have identical chinese 49cc 4-stroke scooters. Several years old very low miles, kept inside. Have not riden them in a while, so of course they would not start. Long story short: Drained all old gas. Had the carb cleaned(by a "pro"), put new gas in. It cranked up and ran like a champ, but when I hit 20mph, it just bogged down and sputtered until I eased up on the gas and got just under 20 mph. Played with the mixture adjustment - did not help.

Went back to scooter mechanic, he said I needed to get a new carbeurator and new PET valve - so I did. Again.... the EXACT same thing happens: Cranks up and runs like a champ, but when I hit 20mph, it just boggs down and sputters.

Went back to the mechanic, who is now sick of giving free advice, and says he has no clue but if I want to have him tear it apart for $65.00/hr he could probibly figure it out in about 4 hours!!!

Does anyone have an idea of what the problem might be? I'm no mechanic, but I can handle a wrench.

Thanks in advance for the advice....

[keetarider]

i think i'd start with a valve adjustment, if it hasn't been done already. it's easy, fast, and doesn't cost anything. surely with all the fuel related attention, the filter was replaced?

rule out the easy stuff first

[felliott]

I have had this happen when the gas cap vent got plug up. Try going down the road with the cap loose and see what happens. You could also have diaphram trouble not letting the slide to move upwards. Take out the 2 screws on top of the carburetor and carefully remove diaphram and check for any pin holes in it. Make sure the slide moves up and down freely.

[bjh517]

Thanks for the input. Yes, the filter was changed. Where could I find information on the valve adjustments? I will also check the gas cap and the diaphram.

Thanks again for the thoughts.

[keetarider]

here's one. i just searched youtube.

www.youtube.com/watch?v=3gwoe1xPiZE

[D-cat]

My favorite valve adjust video (since it covers the whole process start to finish):
www.youtube.com/watch?v=LSL3dhg96m4

Notes: You can also use the kick-start lever to move the engine to TDC.
Some of the body instructions only apply to the Super-8 style, you'll have to figure your own scooter's body out. The rest of the video applies universally to GY6 50cc.

Edit:
If you lack valve feelers, a cutout from the side of a cola can is within tolerance. Make sure it's clean.

[z28sb400]

Old stale gas does the same thing,Is it still the same tank of gas from when sitting?The new gas goes bad in a month.It adsorbes moisture from the air and then it settles to the bottom of tank,to determine it's the gas,take a small piece of shop rag or paper towel and gently stuff into the airbox intke on the air cleaner,stuff it in there loosly,it will not get sucked in the engine if filter is in place,that paper will slighly "choke" it ,if it picks up in speed then it's the gas is bad or main jet is clogged again,but if they cleaned carb and put new filter it should not be dirty,unless the fuel lines are starting to desinigrate which is a common thing from the inside out.