YZ125 H 1981 Carburettor Jetting

[Ji Gantor]

Hi fellow Yami guys and girls,
I have a great little 125 H.
I have replaced all the jets back to original
Main      280
Pilot      60
Needle   6f22-3
But the engine is still running rich.
I believe that most riders dropped the main 2 sizes and the pilot 1 then lowered the circlip one groove to compensate for the other leaner jets.

Does anyone have a 125 H that blasts off the line.
If so can you please advise jetting.

Thanks
Ji

[Viper666]

Make sure your float level is correct, needle & seat in good condition & slide is correct cutaway before you start messing with jets.
My 400 was spluttering down low and I went from an 80 pilot to a 50, it helped but not perfect. Reset float level and replaced 3.0 slide with 4.0 and now gone back to a 70 pilot and runs like a dream.

Viper666

[oldyzman]

Hello Ji,
Is this a roundslide carby, if so check the air bleed jet inside the bell of the carby is there, I couldn't make head or tail of my YZ250 1979 recently as this jet was missing..
OLDYZMAN

[Ji Gantor]

Hi Oldyzman,
The bike has I believe a Round slide Mikuni VM 32 as per manual.
I will check to see.

Ji

[mainline]

Ji, I think it should have a 34mm

[Ji Gantor]

Hi mainline,
My Yami manual lists the bike as having installed a VM32ss/mikuni and a VM34ss/mikuni for Canada.
The 34 has a main jet of 340 and a pilot of 80 which I tried but the engine ran really really rich.
I tried measuring the throat but it is oval shaped, and when I discussed this with Tom at Hatricks he told me you can not measure them that way because Yamaha made them oval shaped.


Ji

[mainline]

I see, now I'm getting confused.

I put a 34mm on my F model as I picked up a brand new one for $50 US, and I couldn't be bothered rebuilding the 32mm. I also figured there'd be a performance gain.

I'm currently running it with a 330 main and 70 pilot and it's nearly there, I'm just trying to improve the midrange a bit. Having said that I haven't ridden a sorted bike so mine could be "running like pus" and I wouldn't know

[Ji Gantor]

Hi Mainline,
From my reading of tuning dirt bikes if you use the original carb size you should have the best mid range power and if you use a larger carb than this gives better top end power.
My little 125 really needs more mid range so I will stick to the 32mm.

Ji

[YZ250H]

Lukeb1961 had this to offer on another thread.  I thought it was pretty good.
http://www.iwt.com.au/mikunicarb.htm

I'll be interested in seeing what you end on Ji as I have 2 of these little beasties - 1 is under resto at the moment.  What pipe are you running on yours ?  Do you still run the coolant through the head stem ?

[Ji Gantor]

Hi YZ250,
I am still running the coolant through the frame.
The pipe is standard.
I have read Ian's web before but it only talks about general tuning not about the YZ125 H.
Ian is a great bloke to speak to and he has all the jets.

I need someone that has tuned a standard H as that is what mine is.

Ji

[YZ250H]

There seems to be a lot of these bikes out there, so I'm sure someone will be able to help

[mainline]

From my reading of tuning dirt bikes if you use the original car size you should have the best mid range power and if you use a larger carb than this gives better top end power.

great news

So what you're saying is, it would need a rider who can keep it pinned, not someone who is learning to ride all over again

Top it off with the fact that the F model was by all accounts a peaky than average motor anyway and I'm thinking I might have to just wire the throttle open and learn how to deal with it

[oldyzman]

Hey Ji, I think you are on the right track staying with the 32mm, i have mucked around a fair bit with jetting mid 70's japanese 125 MXers, some with god success. 125's are quite critical as the low and mid power is hard to target. What is your final drive ratio. And is the motor fresh as a sticky ring on my RM125A gave me some grief lately.
Brett

[Ji Gantor]

Hi Oldyzman,
The engine is fresh.
The secondary gear ratio is 4.25:1 or 51/12.
Mine needs to be able to get off the line quick and have some pulling power up hills.
At the Nationals it ran like pus with a gear ratio of 3.23:1 or 42/13.
The carb was not jetted right and ran quite rich, I went down in main jet size but only had a few so could not find the sweet spot. Also there were no practise sessions each day too fine tune.

Ji

[Lozza]

One of those very expensive carbs must have fell off the shelf and hit Tom on the noggin to give an answer like that about oval bore carbs. They still act like a carb with a similar width but with greater top end as the cross sectional area is increased.In general larger carbs give more top end but if the carb is to small to begin with a larger carb often gives more power right across the rev range. Extracting bottom end/mid range drive out of a 125 is a piece of cake, and usually nothing to do with the carb.