Good afternoon!
I have two 30mm Mikuni VM carburetor that came off a 1976 125mx; I am trying one on my 1972 AT2MX - has anyone done this and what was the jetting configuration?

if i can get this set up i will try the other one on a 1974 125mx I have.

It appears to me the stock main and piolet jets are in the 180 / 190 range and the piolet is 060 - as found in the bike specs for the AT2M AT3M and the 1974 125mx - 

I can get the bike to run as is with a .030 piolet jet but only on choke - it will idle so i am assuming the piolet is too small - 

do you think i just jump to a 060 and proceed from there?

also, curious of jet purchases - seems the Ebay off brands are something to stay away from - should i just buy OEM jets ?

thanks for your comments and assistance!  

Growing up during this era, I remember the "marketing trend" to advertise and use big carbs around 1976.  We used to read the magazines and say "wow, that 125 has a BIG carb!  It MUST be FAST!".

I also remember some of the performance companies like DG (who had legends like Harry Klemm on their staff at the time) would sometimes sell a smaller carb than original to improve performance on the 125's. 

That said, why are you even trying to use a 30 pilot jet?   The stock pilot on both the AT2MX and 76 MX125 was a 60. 

As far as the other jets, the original 76 MX125 jets should be close...  keep in mind you might have to go bigger (not smaller!) on the jets when using the 30mm carb on an earlier model that came with a 26mm carb. 

Also, stick with original Mikuni jets. Period.

Thank You 

I don’t care about top end - I don’t have the stock carb and NOS that I found the seller wants like $400 for it 

I have in my possession 2 VM30’s period correct 

I merely want to dial in a carb that will function off choke 

the 2 VM30’s have the smaller jets in them and that is the only reason I am using them 

probably more of a test that with spark compression and fuel the engine would run 

it does !

thank you for your quick response - I hope I explained my situation clearer 

The bike should idle okay off choke with the 30 pilot. You probably have a clogged jet or passage, very common. Yamaha went rich on the "race" pilot jets... probably so it doesn't seize with throttle closed after a full throttle run? Race bikes don't need to idle clean. On my IT175 I ran a 30 or 35 pilot for trail riding instead of the 60. Worked great, ran much better/cleaner since I was mostly using very little throttle.

If the VM30 carbs you bought are actually Yamaha spec... they will have ID numbers stamped in the carb body that will ID which bike they came from. I would start with the original jetting specs. My concern is if the carbs actually came off a bigger engine (many 250's had VM30's) they may be jetted way too lean for a 125.

Thank you 

I will clean the carb again and try to smooth things out with the existing jet but I also will buy a 060 that is the factory spec for that carb on a 1972 73 74 75 and 76 125cc Yamaha mx bikes 

i did confirm with Sudco that the carbs are factory part numbers for the Yamaha 125mx-c 

thanks for your input 

Do you need the factory jet specs for the 76 MX125c? I probably have them.

Thank you 

I do have the specs already 

Would it run a #60 because of more inlet duration on the MX than DT therefore less suck & airflow at idle & low speed. Just guessing wildly, as usual.

It might be possible to get a used VM30 carb working well on a 125cc trail bike.

Worth bearing in mind that you may need to try various different needles, needle jets, and slides, as well as changing the jetting.

If it's possible to get it running well by simply changing the jets, that's great, but if not have a close look at the cost of the parts mentioned above, before going further.

Nibbi 26mm PE Keihin clones, work very well, are easily tuned, and might not cost much more than setting up a VM30, if multiple parts are needed.