Much needed help with a carb mixture screw

[Nicknackzz13]

Hey yall, installed the Mikuni VM26 today on my 125cc Pitbike (apollo ADR)

i currency got everything working alright i think just takes a bit to heat up..

wondering if anyone on here has the mikuni carb OR what there fuel/air mixture screw is set at? from tight in how many turns is yalls out to make the bike perfect? so i can mess with mine again and get it nearl perfect - perfect..


i currently have it 1 and a half turns out from tight twice.. so i got it turned it once half turn then another half turn out so one full 360.. anymore than that and i notice the bike likes to bog (still does a tiny bit before warming up)

how about yall?? which setting is best?

now that i have installed the carb and tested it, the bike is much faster and does wheelies waaayyy better, before i couldnt get the wheel up, now the wheel lifts up by itself with barely any backward thrust.. :b_grind:

 

[my67xr]

The pit bikes i've worked on with the china Mikuni 22/26mm have all benefitted from a bigger Pilot jet, from the factory they have a #15, sometimes a #17.5
fit a #20 to it, it will start easier, and the mixture screw is normally around 1.75 turns out with the #20.

These make them a bit easier to tune too- Flex Jet, mixture screw

 

[Nicknackzz13]

whats 1.75 turns? lol..

ive got my fuel screw turned out 1half 1 half so one full 360 turn from tight...and once the bike is warm it runs good now with NO bogging whatsoever before i warm it up it will bog if i quickly twist the throttle after about 15mins on choke/off choke just warming the bike, the bike wont bog and itll run great so i guess im okay, and it just needs warmed up.. tho before ive heard it pop a little and a tiny bit of smoke came from the air filter?

the mikuni carb i got is from japan..not china

 

[my67xr]

The mixture screw usually needs to be about 1 ¾ turns out, from done up.
With yours being 1 full turn out, that is on the verge of being too lean, it will be slightly hard to start, may need choke, and will take a while to warm up.
The backfiring and vapour coming from the filter is also a sign it's lean.
Have you checked the valve clearances, from the factory they are done up too tight.
This makes the bike slightly harder to start and once it's warm it can run a bit funny, idle speed changes, harder to start etc.
So set them to .003" on the inlet side, and .004" on the exhaust.




If your carby is the china copy, fit a #20 Pilot jet into it, it will start a lot easier, and will take under a minute to warm up, when you give the throttle a quick rev it wont hesitate or backfire.
If it's the genuine VM26-606 go up one size in the Pilot jet (#22.5 standard) to a #25



Does your carby look like this one ? China copy's, the slide is 22mm wide and 26mm tall oval shaped.

Or does it look like either of these ? genuine Japan, slide is 26mm round

Here is a quick video that explains how to adjust your valve clearances

[video=youtube;QJciyJRJh8Q]https://www.youtube.com/watch?v=QJciyJRJh8Q[/video]

 

[Nicknackzz13]

It actually looks exactly like the one you posted first so i guess youre right it is a china copy? wil it be okay tho? :/ paid 60bucks for it delivered! i was gonna get tht mikuni one with the circulated holes but i didnt wanna have to buy the inlet gaskets and all for it to fit right :/ .. i dont know how to do valves? or how to set them/measure them..

My bike starts first kick everytime? i dont think i said it doesnt start good? if i did sorry but yeah my pit bike fires up first time everytime, it fired up 3 kicks after i installed the mikuni...

i think maybe then i should tighten my mixture screw up about 1 turn and 3 quarters? one turn to me is 180 so if i have to adjust it to around 1 1/3 is that a 180 turn for a full turn, then 3 quarters half?? (very confusing)

Does the valves have to be adjusted? the bike is about 3 weeks old and not rode hard yet.. the mikuni i put on it is working good now and it does take a while to warm up but its freezing here in NC! so that plays a part too, this new card i got tho has made the bike faster and has made it idle, when i got the bike it had a ****ty ole PZ26mm carb on it and it never idled when it was kick started.. now i put on the mikuni it idles perfect and it doesnt bog at all when i quickly throttle it, and it wheelies good now.. im gonna turn the mixture screw in tmo morning in about a quarter of the way? (2 turns out) so i guess around 1 1/3 is a tiny bit turned in? ill try it and see how the bike runs...


sometime ill look into these valve clearances at the moment i havent got the right tools for that stuff..

but thank you all who has replied... ill get back to yall tmo about how the bike goes from there..


is them mikuni carbs alright? i never knew it was china i thought it was japanese :/

 

[my67xr]

Yeah those Mikuni copys are fine once set up properly.

I would be getting a friend or mechanic to adjust the valves for you, it doesn't take long to do and is easy with the right tools, 9mm spanner a pair of pliers, and a set of feeler gauges.
That video i posted above shows you how to do it from start to finish.
Adjusting the valves will help the engine last longer, help you to tune it, and let the engine run to it's full potential.



One full turn out on the mixture screw is the head of the screw rotating 360 degrees.
Like the hands on a clock when the big hand starts at 12 o'clock, then moves right around and ends up back at 12 o'clock.
With your mixture screw being only 1 full turn out, it means the Pilot jet is too small.
So the engine can't get enough fuel to work properly.
I would recommend installing the #20 jet, then retune the carby to suit.

Is your air filter oiled with decent foam filter oil?
If not get some of this and give it a spray, not too thick, and massage it in.
That will stop any dirt getting through to the carby and causing problems later.
The Foam filters need to be cleaned every so often in Turpentine to get the dirt and old filter oil off, then re oil it.

 

[Nicknackzz13]

I didnt turn the fuel mixture screw out 2 full turns.. i turned it in until it felt it didnt wanna go anymore, then i turned it 180 then 180 again so i done a full turn 12pm -12pm and now the bike starts first kick (if cold with choke on) takes a few mins to heat up as i rev it and i leave it sitting a minute or 2 then i go ride it slowly around to heat her up more then i ride hard.. when i had it turned 1 1/3 outs the bike wouldnt start and if it did the idle wouldnt work and i adjusted it but nothing... so i set it back to how i had it an i runs fine, no more backfiring neither or flames from the exhaust and i defo get more power from how i have it set (1 full 360 turn out from in)

my main jet is big my pilot is long thinnish.. when i got the carb i tightend all them parts and cleaned them before installation..

could you tell me if amazon sells any of those size 20 mikuni carb jets i need that you speak of? cause im not sure which id need... and yes i need them feeler gauges, i could do the valve clearance i think after watching videos on it i got it in my head to do it myself lol im good with these things..

if you could show me a link maybe from amazon if possible of the jet id need, thatd be awesome!

 

[Nicknackzz13]

i think this is the pilot jet i alread have with the carb i have on my bike

Amazon.com: JET-MIKUNI 4/PK 224-20: Automotive

 

[Nicknackzz13]

Also, someone told me on here that my bike is running a bit rich since its turned out 1 full 360 turn from in.. but isnt that inbetween lean and rich?.. the bike doesnt smell, it performs well , it starts first kick, wheelies, i can quickly twist the throttle as well and it wont bog no more, itll idle low but stay on without idling low and just go off... when i had it 1 turn and 1/3 turned out from in, the bike wouldnt idle right and it would have high idle then low then high repeat repeat and wouldnt stop.. now i have it on the setting i thought was best, the idle is low but stays the same rpm and doesnt get high or low, it also like i mentioned performs better and doesnt bog when i twist the throttle quickly... soo is something wrong with it? or is it fine to ride how it is now? cause i cannot find or feel an issue with it now...

 

[my67xr]

For the jetting that it has in it currently, the mixture screw is about right at 1 full turn out.
When you change the Pilot jet to a #20, the mixture screw will need to be adjusted again, most likely out another 1/2 turn


Yeah it would pay to check what jet is currently in it before you buy one, and yes the thinner Pilot jet.

The #20 VM28/486 Pilot jets are $4.65ea from Niche Cycle in Florida VM28/486 Pilot Jet

Bit dearer on Amazon, $10 and up- Amazon.com: vm28/486 20 pilot jet: Automotive

$4.50ea plus postage on ebay - Mikuni RS34 RS36 RS38 RS40 RS TM HSR 42 48 Carb Pilot Slow Idle JET VM28 486 20 | eBay


Try moving the clip up on the needle to the 2nd slot from the top too, and try that out