Twostroke VM jetting, just want to think out loud.

[John Orchard]

I am amazed, 50 years after owning and tuning, aircooled twostrokes, with VM Mikuni's, and thinking back then, that I knew what I was doing ...... ha! I think I may just get all this sorted before twostrokes are banned ;-)

After 40 years of building racing fourstroke fours, I am back playing with aircooled twostrokes again, the thinking, and methodology, of a 65 year old is so much different from when I was 15.

I'm enjoying juggling the various VM fuel circuits, in particular the needle straight section v needle jet size, many considerations that had never entered my head before, I feel sorry for my young naive self, I guess also all the young kids tackling life with minimal life experience.

I wonder if I will learn as much in the next 50 years? :-)

[PeterC]

Well John,

I think this opens the door for you to share your knowledge. Perhaps you wouls be so kind as to post your thoughts pn jetting the Mikuni VM carburettor i a series of articles.

The Mikuni VM series may be old technology but I enjoy the ease of obtaining new stock and parts. I find them easy to work on, compared to other model Mikunis.

[skypig]

And I couldn’t ask you for tips on the internet 50years ago.

I found some extensive info on modifying a YZ80F, 44 years too late.

[John Orchard]

Well John,

I think this opens the door for you to share your knowledge. Perhaps you would be so kind as to post your thoughts on jetting the Mikuni VM carburettor i a series of articles.

The Mikuni VM series may be old technology but I enjoy the ease of obtaining new stock and parts. I find them easy to work on, compared to other model Mikunis.

Yeh it would be a 'War & Peace' story :-)  I have learned a bit more recently, but it makes me realize that there is so much more to learn? I'll break-down my recent 'realizations' .....

On my wildly ported, high compression (probably too high), VM38 carb'd Yam DT1;

My throttle slide has had numerous attacks with the file to open, or close, the cutaway, my current cutaway equals a 1.5, but due to the many times I have modified it, the 'needle retaining floor of the slide' is now 2mm lower, I had to make a small relief to the under side of the slide, so for it not to bottom-out on the needle-jet shroud, and also make sure the needle does not protrude so far as to hit the main-jet (which it doesn't).

The 2mm lower needle capture means that, with the needle in groove number one (top), it is really -2, heaps leaner, but, because I dropped from 2.5 cutaway to 1.5, the richer cutaway means the needle is not as lean as it might normally be. Plus the filed bottom of the slide has reduced the size of the cavity under the slide, further reducing air pressure (or increasing air-speed and hence vacuum) above the needle-jet, giving a rise to the enrichment, further limiting how much leaner the needle had become.

Now, the many fuel circuit maps of VM carbs are correct but incorrect, they are not as clear-cut as they appear, there is so much overlap (at times) with 'slide-cutaway', 'needle-straight-section' and 'needle-jet'. The many offerings of Mikuni needles, give very little range in diameters of the straight portion of the needle (2.50mm to 2.538), so, to get the air/fuel ratio correct at 'JUST off idle, you have to select the right size needle-jet, but, if your engine needs plenty of fuel at that point, and you go big with a needle-jet, it can also give a lot of fuel all through the needle effected range (half & three quarter throttle), in many instances, it is hard to find a useable needle taper to deal with the amount of fuel the big needle-jet is giving, once off the needle-straight-section.

Reducing the slide cutaway can richen; idle and part throttle mixtures, so if you need a richer mixture at 'just off idle', less cutaway can get you the richness without going to a large needle-jet (that could spoil things once off the straight-section), but, less cutaway will richen idle, so a smaller pilot-jet will be needed.

To a lesser extent, raising & lowering float level can have a small effect on idle & part throttle, but there is only so far you can adjust before you're spilling out fuel, or draining the bowl on those long main straights.

In capping-off this 'just off idle' point; pick a needle-jet that won't send the half & three quarter throttle out of range, then use the slide cutaway to get the 'just off idle' where you want, then match the pilot-jet to the slide-cutaway.

I have read many times; "start your jetting from the main-jet downwards", never really understood that but maybe I do now?

As I fully understand (or think I do) other points, I'll share my thoughts.

[Dungar Pilot]

Hey John,

...........After 40 years of building racing fourstroke fours, I am back playing with aircooled twostrokes again, the thinking, and methodology, of a 65 year old is so much different from when I was 15.

I wonder if I will learn as much in the next 50 years? :-)

My maths puts me about 5 years younger than you and I am more confident about sorting problems now than I ever was. My only concern is which decade did you forget about, was it the psychedelic 1960s? On second thoughts, maybe it was the Naughties?

Look forward to reading your findings.....

[pokey]

The venerable VM Mikuni has been the answer for many problems for years but I guess someone said that about the Amal whose technology the Mik was licenced from. Carburation is a painful process as its always a compromise  of where do you want to win the most without losing too much elsewhere. Sometimes its as if you need a needle shaped like Marilyn Monroe and thats where Dell Orto are great as they almost do.