Hi Enduro enthusiasts,
This is driving me crazy!!! I have a brand new VM30-83 carb that I installed on my 74 DT360. Want to fine tune the jetting. EVERYTHING I read says the jet number equals its size in millimeters. I have some older Mikuni jets and a 70 main jet is in fact .7mm in diameter. BUT my 250 main jet in my VM30 measures 1.40mm and the stock carb's 180 main measures 1.15mm.
What am I missing??? I have very accurate pin gages so it isn't my measurements.
PLEASE help!!!
Roland

You read some bad info someplace.  Mikuni jets are precisely calibrated by flow rate under very specific conditions.  Even the direction of flow is actually important for some types of jets.  The standard small hex main jets (4/042) are rated with the flow from the threaded end to the flat end as that's how they were originally installed in the early carbs with the jet holder in the side of the float bowl (and the Amal carbs that the Mikuni was copied from). 

Reversing the flow by threading the small hex main jet into the needle jet... flow from flat end to threaded end... like was done on the later carbs actually reduces the flow through the jet.compared to the flow in the "calibrated" direction.  

 

I should have said main and pilot jets are flow calibrated.   

Needle jets are measured by diameter...  Air jets and starter jets are sometimes identified by the drill number...  like a #60 is a #60 drill bit.

I'm pretty sure slide cutaway is measured in 1/16th inch increments...  a 3.0 would be 3 x 1/16 or 3/16 inches

Mark is correct with Mikuni Hex jets being flow calibrated . However , I believe the round Head main jets
are numbered to size , so perhaps that's where Rolands confusion stems from .
Yamaha , to my knowledge didn't use round Mains , but some older Kwakers and Suzi's did .

I wish I could find my old book that had info on the Mikuni main jet types.  I don't think any Mikuni main jets are calibrated by diameter.  But if I remember correctly the round jets are machined symmetrically so that flow direction doesn't matter.

I was having doubts about my recollection .
I enclose a page from period  book " Motorcycle Tuning for Performance " by Carl Shipman .  

WOW, thank you Mark T and Swoop56 for that information. Made it so much clearer!! Especially the excerpt from the tuning manual.
I just Googled "Mikuni jet size and diameter" and everything I could find just stated jet number equals internal diameter.
Thanks again so much!!!!

Thank you Swoop.  I've had that same book since I was a kid...  probably read it 1000 times in the 1970's.  And that looks like the chart I remember.  I didn't remember the part about the Mikuni round jets being marked in diameters.  Haven't read that book in decades but occasionally refer to it.

I do remember seeing "conversion" jet charts between the round and hex jets more recently.  It was someplace on the Internet.  The author claimed the design of the jet was responsible for the flow differences.  He was correct that the Hex jets flow rate is different depending on flow direction. 

But I think he was wrong on the round jets... my understanding is a round jet flows the same in either direction...  his chart showed a massive flow difference between round and hex in the larger sizes which he attributed to the difference in flow direction for the hex jets.  The difference in flow depending on direction would likely be fairly small...  it makes sense that the round jets are measured by diameter which would account for the massive flow difference in the larger sizes between a hex and round jet. 

I don't know if any of that made sense...  I'll see if I can find the article on the Internet still. 

Lots of good info in this thread!

I would like add my 3 cents worth of experience on this subject. When it comes to jetting and tuning I recommend using new components from a sealed package. I've encountered needles that have been cut and shaped and jets that have been drilled.

Here is that article I remember from a few years ago.  I believe Swoop's info that the round jets are marked with diameters is correct.

www.ozebook.com/compendium/t500_files/mikmnjt3.pdf

Somewhere in an old book I had a chart that showed the difference in flow for a hex jet depending on flow direction.  It was more like a 200 main flowed at 200 cc/min from the thread end to the flat end (old flange carbs with jet holder in float bowl) but flowed slightly less like 196 cc/min if flow was from flat end towards threaded end (newer spigot carbs with main jet screwed directly into bottom of needle jet)