Mikuni Carb Issues (Solved today in fact)

[Gazza667]

Ive got a Qingqi QM125 2007 cruiser. Well i say Qingqi. It say's Sinnis all over the engine. K157FMI to be exact.
My problem is trying to Identify my carb. I know it's a Mikuni VM Flange fitting. Ive kinda got it down to either a VM22 or VM26. Both have similar specs. I even went to identify it via the needle & jet markings. But they are not very clear.
So i'm kinda working blind with trying to tune.
My problems started a few months ago when my bike refused to tick over. I serviced it when i bought it 10 months ago. Did valve clearances, new NGK spark plug, fitted an inline fuel filter, oil & filter change etc.
Everything ran fine until a couple of months later when she wouldn't rev properly. I had a terrible flat spot. So decided to invest in an uprated carb. It's only recently ive learnt that there a quite a few counterfeit Mikuni carbs around. I think i may have bought one.
Anyway, after trawling Google it seemed my float was too high. So i wasn't getting enough fuel under load. I adjusted it & everything was great again.
The a few weeks ago, for some reason while i was out, she started cutting out on tick over. Not always. Mainly when i pulled up at traffic lights. I didn't really have to go for very long when as soon as i allowed the revs to idle she would just cut out. She restarted straight away & accelerated fine.
So i decided to investigate. But because i work quite long hours i didn't get chance to investigate deep enough.....
Until today. I understand the air screw is critical for idle, along with the needle jet. After a little trial & error with some spares i have lying around. It turns out the needle was way too long for jet. A full 9mm too long. So she was eventually fading out at tick over. I put a 41mm single taper needle in & now she's running a treat.
So the moral of the story is to make sure you get a genuine carb from a reputable outlet.
Just one question i would like some advice on though. Which size carb would you recommend i eventually upgrade too? I have been looking at a VM30 with a spigot & rubber flanged mount. Do you think this is worth looking at or should i just stay as i am while she's running ok?

Sorry it's quite a long winded post. I shall shut up now :toetap05:

 

[itsid]

For all I know the K157FMI engine is derived from the suzuki GN125
(essentially a clone version of)

And with that it's rather low power (12horses? 15 perhaps?)

Aaand with that I doubt there's any need for more fuel really.
And while there are so called 'racing' cams (*cough*) and big bore kits and whatelse
in the tuning parts bin...
As long as you do not modify the engine I doubt you'd need a bigger carb.

Sure if you increase airflow (intake and exhaust) you could push more fuel through as well,
in which case a bigger carb might be a good idea already
(if it's now on the choking side of things.. usually though a slightly larger jet does the trick if you need more fuel at all)

But keep in mind that even slide carbs rely (at least up to a point) on the venturi effect
and if you scale the carb up to or even beyond the intake diameter,
the passageway might not be narrow enough to create the desired flowspeed.

Slide carbs are less sensitive than butterfly carbs ..
still at full throttle you need some sped up airflow to pull enough fuel up the jet.

Or short: a too big of a carb can cause you ending up in a fuel delivery issue even quicker than a smaller carb would.

If the engine runs fine and revs up quickly.. don't bother changing the carb.
Only if you have the feeling it chokes as soon as you pass say 3/4 of the throttle
you might want to consider n upgrade.
(again maybe just jetting)

'sid

PS I've seen 125cc with 24mm carbs being outperformed by 19mm carbs
and while that wasn't a fair test in any way.. (different engines)
the fuel delivery on the 19mm wasn't slowing things down let's say