Bike cuts off at low revs...???

[Rider01]

Hey, i got my first dirt bike a couple of days ago, it has a lifan 140cc engine and have noticed that the engine coughs and cuts off when i release the throttle... so basically to keep the engine on, i have to keep revving it ( I CAN'T EVEN LET THE FREKKIN THING GO FOR A SECOND WITHOUT REVVING IT). Its really pissin me off coz everytime i go to change gear it cuts off. I dont have much experience with bikes so i need help...

Why is it doing this and how can i fix it??????

Thanks.

 

[Little Brucey]

hey mine does that a little bit too, not as bad as yours tho by the sounds of it. i dont know much about bikes but i think our idle screws on our carbs need adjusting. and possibly the e clip on the throttle pin inside the carb. not sure where the idle screw or e clip should be, and is different on every bike i imagine.

 

[SnItChY]

hey guys ,if you look at where your cable comes into your throttle on your handle bars ,there will be a lil screw same as on your push bikes to adjust your brakes

wind that screw out and you should see the slack in your throttle tighten up a lil

on these cheaopy bikes there aint always an idle screw on the carby
so its near your throttle on your bars try that and should work out

the Eclip is more for when you open full throttle and the bike bogs down in revs ,this adjusts the needle in the seat to allow more or less fuel

im no expert but works for me most of the time

 

[pbk]

had the say problem with my bike there is a screw on the back of the carby not the bottom screw it in just a little and you should find your revs pick up otherwise as snitchy said you can screw out the adjustment on your throttle but i tihnk you will find adjusting your idel speed on the carby will do the trick i had to do it a couple of times to find a good spot

 

[Rider01]

Where exactly is the idle screw located on the carby and how do i know which screw is the idle (like what does it look like)??

Thanks

 

[liljohn_92]

hey mate idle screw is on the side of the slide on ur carby should be a phillips head with a spring you can see
just wind it in a taad

 

[Rider01]

Hey, the only screw i can see with a spring is right at the bottom of the carby??? Is that the one???

 

[numroe]

Start with raising the idle speed. That's a REAL simple fix for your current solution with your throttle hand. Look around and you can work out which screw adjusts the slide height. Be it on the carb or up on the throttle.

But you said "coughs" then dies. So maybe the mixture is out. Which means your AIR screw might need adjustment. See this thread for some related details. http://www.miniriders.com.au/forum/tech-talk/18352-oko-26-flat-slide-choke-screw-controls.html
There is heaps of info in this site of you search hard enough. I'd try winding it out a little (leaner) first. But try both ways.

It is also possible that:
Your valve (to rocker arm) clearances are wrong. Tight or loose.
Your air filter is clogged.
Your carb float level is wrong.
Your spark plug is unhealthy.

If you really get the shits with it, then just by another carb. Try the OKO24 flatslide on a 125cc or smaller engine or a OKO26 on a 140 or 150. If in doubt then get the OKO24. Or the Mikuni 24. Note these are all round bore carbs, so the sizes are "real".

FYI: my OKO26 runs real nice with main=K102 or 105, needle pos = raised 1 clip from mid (richer), slow jet = K38. screw = 1+1/8th out. But it's hard to compare jetting. My stock float level was wrong as well (too low).

 

[Cactus Jack]

Good post Numroe ...... the very FIRST thing people should check and adjust if necessary are the valve clearances ...... the valves hold in cylinder pressure and the cam timing tells the carb what to do ..... so IF the valves aren't seating ...... the signal at the carb will be drastically altered making it "appear" that the carb is at fault ...

IF the intake valve isn't seating you get a high pressure leak blowing back thru the intake tract which will cause exaggerated reversion pulse waves ...

So now you should be able to see that intake air flows over the jets the first time picking up fuel , then when it reverses it picks up fuel again for the second time , then it picks even MORE fuel up when it flows back past the jets again ...... the intermittent alterations to the mixture can cause the engine to run too rich and splutter or bog ....

That is a genuine FACT known by race tuners and automotive engineers and is one of the MAIN reasons why carburettored engines use more fuel than fuel injected engines ... yet produce LESS power ... One of the ways engineers got around the reversion pulse problem was to use sequential tuned port fuel injection ..... then it doesn't matter if the air flow oscillates since it has NO effect on the rate of fuel delivery and fuel is ONLY injected while the valves are open ..... Fuel injected engines also don't rely heavily on well shaped high velocity ports either since injection is not controlled by vacuum from the cylinder or high air speed to pull fuel from jets .......

Other methods used to block reversion pulses are ... steps in the intake tract , reed valves and anti reversion ribs in manifold runners ...

Incidentally , the back head of intake valves moving back and forth (acting like a fan) is also a source of reversion pulses .....

You can learn a lot from the likes of Bill Jenkins and Smokey Yunich ........