50 cc runs but won't accelerate past 1/3 throttle

[gabo1486]

Hey everyone,

i just bought my first scooter a little while ago and was hoping someone might have a suggestion to help me fix a problem i recently encountered. i'm not afraid to get dirty and i'd like to fix it myself, that's how you learn after all.

here's the problem

after having not ridden my scooter for a week i went to give it a spin and the battery was dead. no big deal, i kick started it and got it running. here's the weird part, it idled at an unusually low rate (i had adjusted the idle higher for the cold weather.) i had to pull the throttle to keep it the idle going (again, no big deal, it often starts like that), but if i pulled the throttle too far it would die. now after having run it a couple of times hoping to clear out any gunk that might be clogging the air intake? or the fuel line? (this is based on fairly limited automotive knowledge.) i've got it so it will idle just fine and accelerate up to almost 20 mph, that is if i accelerate very slowly. if i pull the throttle anywhere past about 1/3 of the total throttle capacity it dies down to a low idle or kills the whole thing. also, if i slowly accelerate it up to 20 mph it'll run at that speed for a little bit but then slowly starts loosing power.

i opened the top of the carb and the drum/blatter (not sure what the correct term is) was clean and moving just fine. i also checked the air filter which was clean.

anyone have any ideas?

thanks!

[kyle401]

Try changing your spark plug.

[kyle401]

What kind of scooter do you have?

[mean54]

Have the same problem! someone help us!

[mean54]

i have your run of the mill 139qmb

[kyle401]

Or you could adjust the fuel mixture screw on your carburetor

[mean54]

ive tried every air fuel adjustment possible, itll run fine one day then the next its right back to driving then all of a sudden bogging right down to the point where it will shut off if i dont let it idle down

[wile]

I think it is time to adjust your valves, tight valves will cause hard starting and wanting to die at LOW rpms. And a lot of times will die when coming to a stop after the motor is nice and hot. Also will cause low idleing. REASON is it is looseing Compression because one of your Valves usually the Exhaust valve is a little open TO TIGHT.

Re-read your first post, HAVE you checked your PETCOCK to see if it is working and allowing fuel to flow to fill your carb bowl.

Take the Fuel Line off the Carb and put it in a container and CRANK THE MOTOR. FUEL should flow out NOT squirt out and STOP flowing when you stop cranking.
If it just dribbles out you need a new PETCOCK.

Wile

[mainepeace]

Wile is right on. Adjust the valves.

The next culprit would be a clogged passageway.
The next would be a vacuum leak.
The next would be something else.

Greg

[Stash]

Also check the diaphragm in your carb. a hole or tear can also cause this issue.

[jonnienumberfive]

plugged main jet??

[gabo1486]

great, thanks for all the ideas guys!

the valves have been adjusted pretty recently so i'm going to try a few of the other things first. i got a chance before it got dark to do just a little bit, i messed around with the f/a screw which helped but didn't fix the problem, checked the carb which was in great condition as well as the auto choke. tomorrow i'm going to tackle the petcock and if that's not the problem i'm going to check the rest of the fuel/air passageways for clogging.

thanks again, i'll keep keep you posted

[regz]

Hey, everyone. I have the same problem with my recently purchased Super Cub. Takes forever to start and when it does, it won't accelerate past 30 until dying unless I let it idle. I have ZERO automotive knowledge. Any and all help would be greatly appreciated. Thanks!

[justmopeds]

Hey everyone, dont go to adjusting your valves so fast this is a common problem
where the air intake is taking in too much air, take off the end of the airbox and put a
piece of foam to block some of the air intake, should solve problem!

[katastroff]

Jul 3, 2012 20:37:56 GMT -5 justmopeds said:

Hey everyone, dont go to adjusting your valves so fast this is a common problem
where the air intake is taking in too much air, take off the end of the airbox and put a
piece of foam to block some of the air intake, should solve problem!

Rejetting would make a little bit more sense me think

[Pony66]

Jul 3, 2012 20:37:56 GMT -5 justmopeds said:

Hey everyone, dont go to adjusting your valves so fast this is a common problem
where the air intake is taking in too much air, take off the end of the airbox and put a
piece of foam to block some of the air intake, should solve problem!

If they didnt adjust the valve lash, it needs to be done.

If blocking half your air intake works, you have a fuel delivery problem. Start by properly taking your carb apart and running a wire through the jets and passages.

[viral]

Jul 3, 2012 21:18:50 GMT -5 Pony66 said:

Jul 3, 2012 20:37:56 GMT -5 justmopeds said:

Hey everyone, dont go to adjusting your valves so fast this is a common problem
where the air intake is taking in too much air, take off the end of the airbox and put a
piece of foam to block some of the air intake, should solve problem!

If they didnt adjust the valve lash, it needs to be done.

If blocking half your air intake works, you have a fuel delivery problem. Start by properly taking your carb apart and running a wire through the jets and passages.

I bet he's a "mechanic"

[wile]

Jul 3, 2012 21:18:50 GMT -5 Pony66 said:

Jul 3, 2012 20:37:56 GMT -5 justmopeds said:

Hey everyone, dont go to adjusting your valves so fast this is a common problem
where the air intake is taking in too much air, take off the end of the airbox and put a
piece of foam to block some of the air intake, should solve problem!

If they didnt adjust the valve lash, it needs to be done.

If blocking half your air intake works, you have a fuel delivery problem. Start by properly taking your carb apart and running a wire through the jets and passages.

Right on Pony66, blocking the air is just compensating for the real problem.
Wile