Need help tuning Lifan 140

[locko90]

Im having alot of trouble!! with tuning my pit bike.
Its a lifan 140 and i have a mikuni 26 carb on it with a 20 pilot and a 95 main.
And i just cant seem to get it to run right!
I cant get it to idol (just revs high with coughs and spluters) and when reving it up it backfires and wont really gain that high rpm

Does anyone know where i should have my idol screw and mixture screw set to ?

 

[my67xr]

With it running fast, have you checked that the slide is around the right way? the cutaway on the bottom goes to the air filter side.

Otherwise the revving high i'm guessing there could be a vacuum leak ?
Start it and spray some wd40/degreaser etc on the join where the carby meet's the plastic insulator plate, and where the manifold meet's the head,
If the engine run's any different then there is a vacuum leak there.
These china copy carbie's can warp easily if they are over tightened where the carb meet's the insulator/manifold and cause a vacuum leak.

Mixture screw should be about 2 turn's out, idle speed screw should be screwed in enough to just lift the carby slide off the bottom of the carby bore about 1-2mm

Do you have a good, oiled, foam air filter on it?

 

[locko90]

I got some vids on how it runs. Mixture screw is at 2 turns out.
I have to pump the throttle for it to fire and hold it at about 1/4 throttle or it just dies.

[video]https://youtu.be/Ri5eDhf7bNA[/video]

[video]https://youtu.be/S-aba5nTzOo[/video]

 

[locko90]

I had a different carb on it from a 125 engine that was running fine and it still ran really bad wasnt able to get it to idol and at high rmp back fire and splutter.

 

[my67xr]

Have you checked the valve/rocker clearance's, it sound's like compression is coming back out the air filter ?

 

[locko90]

I did set the valve clearences to .003 intake .004 exhaust. But ill check again.
And yeah some times it back fires through the carb and the air filter smokes a bit

 

[my67xr]

Make sure it's at tdc on the compression stroke when you check them, line up the flywheel T marking at 12 o'clock and cam gear marking at 9 o'clock marking at the front of the head.

.003" (.08mm) intake .004" (.1mm) exhaust


Did you try spray degreaser or wd40 onto the carb/manifold joint's with it running to see if it changed the way it idled ?


Post up a couple of pic's of the carby and manifold, air filter etc too

 

[locko90]

Nah haven't tryed the wd 40 yet ill try it tomorrow. And ill also post some pic.

Just wondering.. my spark plug goes in to the head funny like it was the wrong thred and it striped the spark plugs thred. So there could be an air leak there ?? Ill post a pic of the striped spark plug aswell.

 

[locko90]

Nah haven't tryed the wd 40 yet ill try it tomorrow. And ill also post some pic.

Just wondering.. my spark plug goes in to the head funny like it was the wrong thred and it striped the spark plugs thred. So there could be an air leak there ?? Ill post a pic of the striped spark plug aswell.

 

[my67xr]

Stripped spark plug thread in the head is a head off job, then Helicoil it for around $30 or use a Champion thread insert to repair it for around $12 if you can DIY it.

 

[mikkh]

I had similar backfiring coughing etc problem and it was caused by sheared woodruff key (the magneto was still good) and the ignition timing was off because of that.

 

[locko90]

I got it running better today i done the valve clearences and cleaned the spark plug up and made the spark clearence smaller and also tightened the spark plug more then i usually do (hand tight)
Seems to idol fine now but is still back fireing at mid range rmp.

I have the mixture screw at a bit more then 2.5 turns out. to rich ?

Was running worse at 2 turns out.

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[locko90]

Also i think my float level could be wrong because the carb leaks fuel out of its overflow when fuel is left on and sitting for a few minutes without engine runing.
I think this is causing it to run to rich.

I have bent the tab up to close the needle and seat valve quicker but it still seems to over flow after about 5 mins

This is the carb i got on it atm.



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[my67xr]

So with it leaking fuel out the overflow it might have dirt in the needle and seat.
When did you last clean out the tank and replace the fuel filter ?

Take the carby off and remove the fuel bowl.
keep the hose from the fuel tap connected to the carby, turn the fuel on and gently lift the float, it should stop the fuel coming out when the float is parralel to the base of the carby
Turn the fuel tap off when you work out what it's doing.
Adjust the tab to correct the float level then refit the fuel bowl and carby to the bike.


Once the float is set and it no longer flood's then adjust the idle screw back to 2 turn's out and try the bike out

The tab in the centre of the float arm should only just touch the tip of the needle when the bottom of the float is 25mm off the fuel bowl mounting face

 

[locko90]

[video=youtube;G2svMlNQwpI]https://www.youtube.com/watch?v=G2svMlNQwpI[/video]

this is the bike running at around 2.5 turns out.

[video=youtube;4C2ebM383EE]https://www.youtube.com/watch?v=4C2ebM383EE[/video]

and this is it at a bit below 2 turns out. seem to be running better. I took it for a little ride in the yard and it has no torque it just wants to die when accelerating. and when riding along and reaching a higher rpm it splutters and back fires.

ill have a look at the float again tomorrow and ill try get it to the right level.
do you think that this might be the problem ?



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[locko90]

ive had the carby off like 30 times man and I have checked that the needle isn't getting blocked by any dirt the tank and fuel filter has be cleaned just recently.

and ive checked to see that the needle stops the flow of fuel when float is raised which it does. with the overflowing I thought that the float might be getting stuck open on the fuel bowls wall, but don't think so.

im not sure if I have the floats the right way around on the pin as I might have put them on the wrong way when I first cleaned the carb a while ago. that would explain why I had the bend the tab so far to stop it overflowing fuel.

do u know what way there supposed to be on this carby?

when I take the carb off ill take some pics of everything any way.

thanks for the help so far mate

ill try everything u have said tomorrow but I don't quite understand what u mean by.

"The tab in the centre of the float arm should only just touch the tip of the needle when the bottom of the float is 25mm off the fuel bowl mounting face"

 

[my67xr]

Pointy bit's face down

 

[locko90]

I got it going pretty good today I bent the float arm down cuz I think the fuel level was to low and it runs a lot better. still got the abit of backfiring up in the higher rpm when engine is under load. when engine is in natural and reving to wide open throttle seems to get there with little hesitation and slight misfire in high rpm... so think it needs a slight tune still.

heres a little vid on how its running now.

[video=youtube;z_3desDNsHs]https://www.youtube.com/watch?v=z_3desDNsHs[/video]

yes I suck at wheelies haha.. still gota learn to cover the rear brake, new rear brake pads would help also

 

[locko90]

any idea on what I should do about the spluttering at high rpm my67xr ?

and where should I have my float tab bit at? I might have lowered it to much

 

[my67xr]

Float level may still be a touch high.

The tab in the centre of the float arm should only just touch the tip of the needle (needle and seat) when the bottom of the float is 25mm off the fuel bowl mounting face

Pic borrowed from TBolt, different carby but show's how to measure it.

You could also try raising the clip on the needle up one notch, to the 2nd notch from the top if adjusting the float level doesn't do anything.