I've used aftermarket main jets several times and they work fine.  What you can't do is "mix" the jets during the tuning process, for example an original Mikuni jet will likely not flow the same as a one marked with "AB" so as long as you always stick with only "AB" jets during the tuning process you'll be okay... 

For example if you could have a situation where it's a little lean with an original Mikuni 200 main.  So you try an "aftermarket" 210 main.  Now it's even leaner!  Or maybe it jumps to way too rich with only one size different!   It's easy create a whole story of some crazy tuning issue but the more likely reason is simply that flow rate of the different brand jets are different.  (MIkuni jets are calibrated in cc's per minute flow, not hole size...  and the original Mikuni hex mains most of these bikes used even flow slightly different depending on the flow direction through the jet)

In this case a swap to a 300 "AB" brand main jet might have worked just as well?

P.S.  Someone might be able to confirm but I think the "AB" might stand for "All Balls"?  I think they used to make jets and a complete set was a lot cheaper than Mikuni.  I got a whole assortment with matched pairs of main jets that came with an RD350 Yamaha that I think I passed on to the owner of the bike.  I prefer to stay with Mikuni.

Great uncle, I have used main jet/needle combos in the past, and think that is the easiest. I've not heard of anyone that makes a kit for these old bikes though.

Markt, yes I believe "all balls" sounds right.  I know Dennis Kirk sells that brand.  I agree if I had the whole kit, knowing there is a difference now, I could tune it out.  That was the only "310" main jet I had at the time, and i was using that mixed in with actual mikuni jets.  That's why it just felt lean when I went to the lower sized jets.  It was actually lean lol.

I’ve not had good luck with DK when getting specific about the fine working of a carb and engine. They are down the road from me. The body parts and standard mechanical needs are fine. At least working on some old Honda’s and snowmobiles…
Sneezles61

My favorite thing about Dennis kirk, is that I usually have parts in a day.  They are in rush city, and that's only 2 or 3 hrs drive from where I reside.

I first heard of Dennis Kirk back in the 1980's when my friend Sheree got a catalog someplace and showed it to me.  They had a tremendous selection of riding gear and accessories that we didn't even know existed.  (This was way before the Internet)  Prices were great and shipping was fast.

Not long after we discovered a company called Chaparral opened up a huge store in the old 7-Up bottling plant in San Bernardino, Ca...  it was very crude...  literally a gutted manufacturing plant they threw some racks and shelves into...  it almost seemed dangerous to shop there!  I think their primary business was mail order so a pretty storefront wasn't that important. 

Since we used to race a few miles away at Glen Helen, Chaparral became a common place to visit...  Super friendly and if they didn't have something, they would try to get it.  I think they might have been one of the first/only places to start selling riding gear for kids? 

It's great to see good companies like Dennis Kirk and Chaparral are still around...  (Chaparral has a HUGE very modern dealership a few miles from the original old warehouse store...  they came a long way.)

Yep, Chapparal was our version of the internet back then. Lust through the catalog, write down the stuff you want and give them a call. Boxes of goodies appear on your doorstep a few days later. Good times!