Need help tuning my carby

[Shing]

Hey all,

I just recently finished building my midsized pit bike and im running a Lifan 140 motor and Mikuni 26mm carby.

As for tuning it, i have moved the clip on the needle to the highest position and it was better but still holds an inconsistent idle (occasionally will build up slightly for no apparent reason then die off again) but when accelerating (like idling in second then gunning it) the 1-1/2 throttle seems ok. but 2/3-full throttle it will just die off and not rev at all. I'm not sure what to do. I know I need new jets, will these do?
NEW MIKUNI CARBURETOR SLOW & MAIN JET KIT - eBay Other Makes, Bike Parts, Motorcycle Parts, Accessories, Cars, Bikes, Boats. (end time 07-Sep-09 11:24:58 AEST)
which jets should i be using/replacing and how do i do it in the carby?

I am new to carby tuning so be nice

cheers, Shing.

 

[Cactus Jack]

That's a good range of sizes ... Just buy the kit ... Then tune the top end revs first ... then the bottom end ... then sort the midrange out ... Consider it a learning curve ... You'll be better off for it in the end rather than having someone tell you what they're running ... Once you've got your engine running schmiko , you can post your experience and findings in a thread ... What suits one bike in one part of the country may not suit another in a different part of the same country ...

 

[thump*140]

^^^^^^^ yeah what Cactus said...
a wee hint to get you started... sounds like you're a bit rich on the main jet...

 

[Shing]

Yeh i figured i was rich on the main jet. But I dont know which jet is which and where it goes. thats what I mainly need help with.

Thanks anyway though Cactus.

 

[creole]

the clip should ideally be in the middle.
Higher is lean, lower is rich.

Pull ur carb off, and look at the main jet number. Also clean the main jet, u might have something stuck in it too.

If you wot it, and it stutters it may be too rich, or blocked, or even lean. Check ur plug color too.

 

[Shing]

Alright spark plug was back and so was the carby in the spot where the air filter goes.
So I have cleaned both
Im assuming this means that my mixture is too rich? So I moved the needle clip one spot off the bottom and i have made the idle mixture leaner 1/4 turn. Is this ok?

So now Im thinking that my problem lies where the jet isnt big enough to supply enough fuel at higher revs/larger loads (engine laboring). Would this be right?

I have also ordered the jet kit. But where the hell are these jets? I dont know where abouts to look for them. Is the smaller jet the main jet and the longer one is the idle one?

Cheers, Shing

 

[Shing]

Never mind, I pulled the bowl off and found it all!

So the narrow long needle is #20. And the short fat one is #100.

Is this big enough for both? Which one should I be changing?

 

[thump*140]

hmm... the force is strong with this one...
clearly you've done a bit of reading, and your understanding of jetting is on the right track...
look at jets this way... the higher the number, the richer your jetting, and vice versa....
you have done a plug chop, and seeing the black plug, you have correctly deduced you are running too rich, and you have stated that your bike runs ok down low, but at full throttle it dies in the ass...

so..... black plug+dies at full throttle = too much fuel at full throttle, which equates to too rich on the main jet...
which would mean you need to go to a smaller main jet...
so now you put a smaller main jet in, and try running the bike at full throttle again... test it, do a plug chop, and make your next deduction as to how your jetting is going from there...
but you're definately on the right track...

 

[thump*140]

oh, and the long narrow one, that's your pilot jet... the short fat one, that's your main jet...

 

[Shing]

Alright thanks for the help and confirmation there thump!

When I get my jet kit I will put in a smaller main jet and have a bit of a run and see how it goes.
Should I be changing my Pilot jet??

Cheers.

 

[thump*140]

nah pilot jet sounds pretty right, but see how it goes once you have your main jet sorted.

 

[Shing]

Just an update. I have my bike running great now!!! Almost perfect, Only it dies in the ass under load in 0-1/2 throttle which is making me think the pilot is too rich, But its on #20 jet which is the smallest jet. So it cant be that. So Im now assuming its the idle mixture. I have been turning the screw out 1/2 a turn after every run, but havnt noticed a difference yet. I'm just adjusting it now and I will take it for a run and see how it goes.

Anti clockwise makes the mixture leaner right? Or do i have it all wrong?

Cheers.

 

[Shing]

just got back from a quick run. Basically it just dies in the ass under load. So I have turned the idle mixture out 1/4 turn. And ive decided to go down one jet size to 92.5 on the main jet. This is the smallest jet size for the main jet.

Now for another run...

 

[mrbrnz]

if it's running bad at 1/2 throttle try adjusting the clip on the needle, putting the clip up will lean the mixture, down will richen it. The needle affects fueling from 1/4 to 3/4 throttle.

 

[Shing]

Ok, so it runs better now. But if Im idling along in second, for example, and i just give it stick, it will just die in the ass. The plug is a burnt black colour.
I could make the fuel mixture one notch leaner but then that would mean its on the bottom most notch. And I would rather not have to do that. But is that the only way I can fix it??

I'm not sure what else to do. Please help!

 

[Shing]

if it's running bad at 1/2 throttle try adjusting the clip on the needle, putting the clip up will lean the mixture, down will richen it. The needle affects fueling from 1/4 to 3/4 throttle.

I'll have a look at the clip. I think I may have been moving it the wrong way... >_<

 

[mrbrnz]

clip down, moves needle up, letting in more fuel.

 

[Shing]

Yeh I get it now, thanks.
Well I moved it one notch up and it made no real difference... I CBF screwing with it anymore today. I will move the needle up another notch another time.

 

[mrbrnz]

92.5 seems a bit small if anything you might need to richen it up, if you're idling a lot and putting around the plug will be black, to get a proper plug chop you need to be going flat stick in as high a gear as possible, hit the kill switch and remove the plug to get the proper plug status.
with the air screw, turn it in all the way then start backing it out slowly until the idle speeds up, one turning the screw doesn't speed it up anymore turn the screw back clockwise to the point where it first reached that peak idle point and that should set your pilot up correctly.

if you hear it backfiring when you let off the throttle it's running lean.

 

[Shing]

I'm running it with a Lifan 140 BTW. I cant reach the idle mixture screw when the carby is on the motor. Its a real pain. I havnt heard it backfiring at all.

Who else has a 140 with Mikuni 26mm carb? What specs are you running? I really CBF with this POS carby anymore...