Where is the air screw on the Mikuni VM26?

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Kourt

Kourt

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My bike is running like shite since i changed from the Ducar 138 to the Lifan 138, my plug is all sooty and there is stuff all top end power? I want to try and ajust the air screw i think its running to rich? Unless anyone else has got any idears why theres no power at the top end? Cheers
 
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its the screw undernearth the opening to the manifold.
 
Thank bro, thats a pretty crap place for on the fly ajustment though! how do you adjust yours, by taking off the carb?
 
dont really, but yeh its a pain, you either have to get a special screwy or take the carb off, both are a long process especially when you have to do it several times to get it correct
 
Now with the ajustment what does screwing it in do- is that rich setting and screwing out the lean setting?
 
The screw at the bottom rear controls fuel (when the slide is nearly closed ... the strong vacuum signal from the cylinder pulls fuel up from the float bowl) ........ screwing it in leans out the engine and screwing it out enrichens it ....... try screwing it right in then backing it out 2 turns ....... OR

The proper way to do it is to turn the idle up ........ then screw the mixture screw in until the revs drop ........ then while counting the number of turns ....... screw it out until the revs drop again .......... now screw it back in by half the amount of turns ........

Lower the idle speed and repeat the procedure until you get a nice , clean , steady , plonking idle ......... with no flutter or lurching ...... Once you've got it right ...... you could then screw the mixture screw right back in counting the number of turns then return it to the same point ......... then WRITE DOWN the number of turns for future reference as it will save you a lot of messing around ........

By doing what I've said you are setting the idle mixture to burn with a neutral flame (dead center between rich and lean) ....... this is the safest mixture that will remain stable on either hot or cold days .......once you find the neutral flame you can go either side to fine tune in winter or summer ...... wherever works best for you ...... :)
 
Get a small 90 degree bend flat head screwdriver for on the fly adjustment. Bunnings should have them
 
If its got fark all top end the screw wont make any diff.

Fuel screw affects the bottom end only.

If your plug is sooty try a smaller main jet, or drop your needle.
 
Yeah it sounds like he's done something wrong with the carb (needle clip position ?? ) during the engine swap because there wouldn't be that much of a tuning difference between the two engines ........

I've taken a 22 mm Keihin carb straight off a worked XR75 (80 cc) and put it on a worked 110 Lifan and it ran spot on with the same jets and idle air mixture adjustment ...... (#38 pilot #105 main) .......... the plug reading was right ......... and that's a 27 cc capacity difference .........

Carbs are mainly affected by atmospheric pressure since that's what pushes the air into the cylinder .........
 
Thanks cactus worked a treat mate- the bike moves now and has got a brownish sparki- might be a bit to lean tho as there is nothing on the porcelin or the middle electrode?

The Air screw is two turn's out and the needle is full rich- so to me that means most air + most fuel- seemes to work quite well.

Sill not to happy about bottom end power on these Lifan 138's not half as much as the Ducar 138's
 
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that screw is it located at the manifold end or air filter end??
mine needs abit of a tune and ive screwed it right out then right in and made very little differnce... maybe im doing something wrong?

are there two differnt screws one for air one for fuel??
ill try and get the model of my carbie up soon =)
 
On the mainfold side, just screw it all the way in, then two complete turns out. I was shocked it works realy well.
 
so you rkn all the way in and two turns out? seems easy enough =P ill give it ago 2morra :) its dark now =O
 
yeh just went for a ride then =) would have to say it worked for me =
i had it all the way in and bringing it out 2 turns was the icing on the cake =D runs nice =)
 
hmm, we have recently made adjustments aswell because we have been having problems with kick back when kick starting it.
Dad seems to think that doing it that way: all the way in and bringing it out 2 complete turns, is if you dont know what your doing?
we pretty much turned it in about 1/4 and went for a quick ride.
before we had made any adjustments the spark plug would be black but after this short ride it was immediately noticeable that it was much leaner.
does anyone know whether this sounds too lean?
like should it be that immediate?
and do you think that this could help with the kick back issue?
 
Thanks for that,
Yeh we have just been taking the carby out to reach the fuel screw.
We are pretty sure we are on the right track.
Now all I need to do is after every ride, take the spark plug out and have a look at the colour of it.
Then go from there.
I know in that pitsterpro link it shows a diagram, but am I looking for a very light tan colour?
Because I have been told I am looking for a tan colour but that pitster pro website just looks white and black?
Thanks, Mitch.
 
:p manifold adaptors:p

just use a nail thats been ground down into a blade shape, wrap some tape round the end so you can get a grip. or even:eek: use one of those hex drive screwdriver bits... cheap n easy:) yes, youll burn your fingers...ha ha ha! i revel in others misery.... and have a bevel gear screwdriver just for adjusting the things anyway....

but as cactus said, you ususally wont have to tune a carbie between engines... even with fairly large capacity differences.(a 90 and a 150 still require the same air/fuel ratio) unless youre really pedantic and like playing with jets and the hassle of stripping the carbie everybloody time you adjust it....or theres something wrong..... and im not saying theres no point in tuning a carbie, im just agreeing that theyre usually pretty well much right for just general useage...

black sooty plug, take out the slide, pull out the needle, and raise the lil "c"(e?) clip by one notch. thats leaning it out.(needle sits lower in slide/blocks main jet more/reduces fuel air ratio) should help. if you run out of notches, time to go down a jet size....ooops. tuning...my bad... but the colour youre aiming for is yes, a light tan...sorta like the lightest shade on my avatar actually...

then refer to all them links that have been posted and youll get the lil sucker hammering along...
 
I think the best way to describe the colour is light coffee.
I've tried hex screwdriver bits, a ratchet screwdriver, all sorts of bent up bits of metal and just found nothing would let me easily turn the screw. got the adaptor for $22 and ground it out to fit my engine and now I can just stick a normal screwdriver in there with plenty of room to move, it just makes it so easy and no burnt hands and 1/2 turns that didn't connect
 
When I said to screw it right in and then turn it out 2 turns ... That was just a starting point for beginner tuners so you know where you're at since the fuel screws are hard to get at on the el cheapo ebay type mikunis ...

And as Pitster states it should never need to be screwed out more than 2 turns ... IF you have to have it out that far then either your float level is too high OR your pilot jet is too big ...

Kick back is caused by TOO MUCH IGNITION ADVANCE IN RELATIONSHIP TO THE COMPRESSION ... and has got nothing to do with the carb ... although too lean a mixture can make it worse as in kick back harder ... but no engine is supposed to kick back in the first place ... for the mixture to ignite too soon ... the spark must be occurring too soon ... PURE AND SIMPLE ... If you don't fix the ignition advance problem you're going to end up breaking something in the long run ...

If your engine has an IRK with an adjustable pickup ... undo the screws and move the pickup anti-clockwise as far as possible (to the top) to retard it ... IF it still kicks back or has NO adjustment ... then the ignition itself or flywheel position on the crank is faulty and the ignition needs to be replaced ... If you have a standard type flywheel ... same thing ... Read SnItChY'S thread on how he solved his YX engines kick back problem ... The REAL problem is most likely a result of the YX factory raising the compression and leaving the ignition advance where it was ... when they should have retarded it to suit the raised compression level ... A hell of a lot of engine "Guru's" told people to fit a high compression piston or BBK to their engine and advance the timing and it "appears" that the YX factory has followed the advice of those "Pied Pipers" ... LOL ...

Now the big mod is to fix the factory f'up by grinding part of the flywheel tab off to retard the ignition back to where it should be ... LOL ... And I noticed that it involves several different TYPES of flywheels so someone in control of "Engineering" at the YX factory needs to do some retraining ...
 
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