125cc bike running terribly!

[tyler_jrn]

i am currently riding a zontes 125cc bike, i know its some chinese junk but i mainly bought it to get to work and back until i can save for a suzuki sv 650cc when i pass my test in may and it has i suppose served its purpose but i just wished it ran smoothly.

basically it runs top end horrifically. ill explain all my problems

i can run it on choke for about 10 minutes after warming up before it decides it doesn't like it. so pretty much choke does nothing on start up.

i can rev it all the way to 60mph in 4th gear 9800rpm, but when i change to 5th gear it either starts bogging down to about 50-55mph and stays there or it starts jumping and backfiring all over the place and it feels like im riding one of those sit on bulls.

it generally jumps around all the time, can feel it and sometimes hear the sudden sharp rpm increases when going along which causes it to slow down and then increase again.

it idles fine after a good ride and it seems to only give problems at high load e.g 5th gear. runs fine 1-3rd gear normally.

sometimes on a rare occasion it runs ok, but most of the time it boggs down like crazy and literally in 5th gear it just sits there at the same speed whether at low or full throttle just the bog pitch noise goes very deep when trying to go faster.

can hear it spitting all the time especially at 4-5th gear, like every few seconds.

it basically just feels like choke is stuck on all the time, has now started to rev up slower most of the time can hardly get it into 4th gear without it slowing down.

it will definitely start after about 10-15 mins of riding cruising at about 50mph say behind a bus or something. try to accelerate and it goes a little but then it starts jumping around and backfiring and then starts slowing down.

i have replaced the spark plug, checked choke and it is not stuck.

I really just wanted some advice. I am going to check spark plug and carb at the weekend aswell as take down the main jet size as it only seems to happen on high load. im thinking the main jet may be too small sometimes if i let off the throttle it will feel to increase speed slightly? and may be causing too much air to go through.

it runs pretty well at low speeds say -30mph, a little boggy but much much much better than 4th-5th gear. on one rare occasion it went 75mph so i know it can go that fast lol.

basically id just like to comfortably run it at 60mph and know that it will go 60mph pretty easily without it jumping all over and slowing down.

Also... when it starts slowing itself down i can say in 5th gear clutch in at 6000rpm pull back throttle and it will bog straight down for a few seconds then whip straight up to say 8000rpm but then if i let go it just slows straight back down to 6-7k rpm again.


if anyone can help me that would be great, also taken it to dealer about 20 times and they haven't fixed it so i gave up and have now decided to do it myself. not particularly hard but im just a little unsure what to check first and if anyone would know the specific problem.

thanks

[-]

Fuelling?

Blocked jets in the carb? What's the air filter like? Any intake leaks?
Does it have any filters in the fuel line? Some have a filter on the inside of the connector for the hose, sometimes there's a little gauze filter where the hose enters the carb.

[tyler_jrn]

[misscrabstick]

Things I would do, check tank breathers are clear, check fuel tap flows good clean fuel at a decent rate, all filters and hoses clean and flowing free, remove carb, remove float bowl, remove all jets,( make notes/pics to ensure correct refitting) clean out any debris using compressed air, refit/reassemble to bike and test.

[tyler_jrn]

[nathan k]

When it does run well, was the weather anything in particular?

[tyler_jrn]