MJN

Yoshimura "MJN" carb kits. If they matched your engine's requirements, they worked well - But if the holes weren't drilled exactly right for your bike's small throttle opening, it was unfixable, short of starting over again with different holes at different locations. And if the high rpm fuel curve wasn't right, you were essentially stuck because the MAJ was blocked off. And because the MAJ was blocked off, each MJ change was a huge fuel rate change. Like 2% CO per MJ.

We had one of those situations at Mid Ohio with "secret carbs from Japan"..... that ran awfully. We did everything that They said to do until it became apparent that they (wasn't the Yoshimura guys or a Suzuki) didn't know how to change fuel delivery at any point and wished that we, while endeavoring to not look like smar***es,  would stop sending the bike's mechanic to ask them what should we next do to fix the fueling.

In other words, they said that if we continued to fix it, we were on our own.

So, despite a warning of cessation of tech support and other thinly veiled, dire consequences, we restored the blocked MAJ (so we could address the high rpm fuel curve issue) and put in a 75 MAJ. The mixture didn't lean out very much but power picked up about 1% - indicating that fuel atomization was improved and that that that nifty catalog picture of a fog around the MJN needle wasn't reality - If you have a plethora of .020" holes, fuel droplets tend to be about .020".
So, we added air to the .020" droplets and fuel state improved and power increased despite the dire warnings.

But the fueling and fuel curve at high rpm was still too rich. So we tried a 110 MAJ and an even larger 160 MAJ - and got "even more more power", but still the fuel delivery curve was too rich at high rpm - And the mixture still hadn't changed much, but now power on the slow motor "B bike" was 3% to 4%  better and better than the "A" bike by a few %.

Because the fuel delivery curve wasn't leaning out, we gave up on doing any more to try to address the MAJ fuel delivery curve and turned to the midrange - which was too rich.

So - I asked the mechanic to ask if would be ok to plug up some holes to try to lean out the midrange as we had already tried lowering the MJN and nothing happened to the mixture. Raising the suspicion that there just wasn't a whole lot of fuel flow through each of the plethora of holes. The response was (in comedian Ron White's voice) "Apparently you don't understand how an MJN works".

Vic Fasola had a better response. Something like "Well, try it!" (and tell me how it goes.)

So, with that admonition from Them, but with the team's blessings, we superglued fully 1/2 of the holes shut. The midrange finally leaned out just enough (lucky guess on the number of holes!) and we ended up 4-6 hp better in the mid and about 4% better peak and still the fuel curve was too rich - making the "B bike" a monster as comp to their "A bike".

In one afternoon, we managed to make one 600SS rider and mechanic very happy.
And learned a lot about "the MJN".
And managed to embarrass at least 3 southern California AMA team people who didn't talk to us for many years after that.

Marc Salvisberg