139qmb

[KSR Moto]

Scoot is going very slow around 20mph and after some time it just "wakes up" and starts riding normal (around 35mph) scoot is all stock

[GrumpyUnk]

First thing I would check is the fuel supply. Limit the fuel and you'll limit the engine rpm. A sticky float could be staying closed, and allowing the float bowl to have a too-low level of fuel. Could also be chunks of dirt that flow/block/flow.
tom

[diynuke]

Aug 19, 2018 8:46:06 GMT -5 KSR Moto said:

Scoot is going very slow around 20mph and after some time it just "wakes up" and starts riding normal (around 35mph) scoot is all stock

Could indicate very rich choke so could be a clogged air filter but this shouldn't be common.

after some the choke shuts of and that could be the reason why it starts running good again. So just check the usual stuff.

If you have tools for it check the valve lash
have clean oil in it.
check for good fuel flow from the tank.
have a look at the spark plug. it should be a bit coffee brownish color.     if its black or white It is a wrong main jet or something is wrong with the air filter.

But first I would just take the spark plug out and have a look and if you aren't sure just post a picture 

[KSR Moto]

Aug 19, 2018 12:44:25 GMT -5 diynuke said:

Aug 19, 2018 8:46:06 GMT -5 KSR Moto said:

Scoot is going very slow around 20mph and after some time it just "wakes up" and starts riding normal (around 35mph) scoot is all stock

Could indicate very rich choke so could be a clogged air filter but this shouldn't be common.

after some the choke shuts of and that could be the reason why it starts running good again. So just check the usual stuff.

If you have tools for it check the valve lash
have clean oil in it.
check for good fuel flow from the tank.
have a look at the spark plug. it should be a bit coffee brownish color.     if its black or white It is a wrong main jet or something is wrong with the air filter.

But first I would just take the spark plug out and have a look and if you aren't sure just post a picture 

Here is the picture of the spark plug

[diynuke]

As far as i see it now it wouldnt hurt going a jet smaller. looks a bit rich
and how old is the sparkplug.? and personally i would just get one of those mainjet kits and try going a few sizes up or down and see how the scoot reacts to it.


on allot of stock 139QMB engines jet 80 is installed which allot of times is around the right size but yeah that also depends on the air filter

This is what i think: as the engine warm's up the engine gets hotter so the air entering the engine less compact which would mean it goes leaner when it's hot so that says a leaner jet

so a too rich jet with the choke could be the reason why it bogs when its cold/the choke is activated
(check the airfilter first tho.

[wahlman]

This is so cool, I am certainly impressed with the wisdom on this forum. O.P. just stated his symptoms and we have all these gear-heads willing to chime in and give advice for free.

I really respect you guys not just for your knowledge and wisdom but also for your kindness to render help to a complete stranger and not charging any money.

This is really Humanity at its finest. Good Samaritans....you guys are really great people. My hat goes off to you

[kagetenshi]

If it is a stock 50cc with the stock carb (usually either 17mm or 18mm from what I gather), I would assume a #72-#83 main jet should suffice depending on air flow restrictions. Though this is at best a ballpark estimation to use as a start. Jet kits are fairly reasonably priced.

[ratdog]

Wait! That plug is NOT rich. That plug is very lean. On a correctly jetted bike, the “white part” ( insulator) should be tan.



There is something else you might want to check. These little guys have a “automatic” petcock. What it really is is a vacuumoperated petcock. Trace the fuel line back to the gas tank and you will see one or two other lines back to The petcock. There normally is a lot of extra tubing there. If there is a lot of extra, I would pull the tubing off, cut about an inch and a half off and put it back on.

Here is something to try regarding the petcock. Take the fuel one off at the carb and put it in a jar or something to catch the gas. A very little gas should run out. Now turn on the ignition and crank the starter. When the engine starts cranking, there should be a sudden flow of gas. If you just get a trickle, I would look to those lines that go to the petcock! Ofcourse, a clogged fuel filter would also cause this to trickle, but a clogged fuel filter wouldn’t get better when the engine gets warm.

The other thing to check would be the electric choke. Easy to do, start the bike up cold, unplug the electric choke. If the bike stumbles and most likely stalls when it is in plugged, you are good. If there is no difference, then you have a bad electric choke