Carb question for old 4 speed guy's

When I picked up a 70 400 a few years ago, the bike had a mikuni carb on it. It ran but really crappy. I had the motor completely gone thru by John at Vintage Husky. When I reassembled everything I went back to the 36 Bing. Bike runs fantastic. However, when I was going thru my parts box I found the Mikuni and the manifold for it and noticed that the carb was a 34mm. The bike was modified for racing with that carb and cross over cone pipe from way back when. It also had long travel suspension work done.
My question...for you old 4 speed guys...was there some sort of performance gain by going to a smaller carb or what? Most of the time people go bigger. Dave

I'm not the tuner I wish I was but jetting a larger carb when installing where a smaller sized carb can be tough on the first one. It's not just changing a pilot jet and main jet. The needle jet and needle matters too.

Sometime I'll post about jetting. I notice it make take two or three try's to jet it perfect. With the rest of the settings on the carb being off I set the pilot, but the rest of the setting could offset it.

Once the jetting is close you may need to rebalance it, reset the idle rpm, then move the needle to achieve perfection.

Remember to make sure the needle and seat is not leaking. And the float level is set correctly. Once you learn this it's not that hard. Plan on chasing your tail for a while.

When rebuilding the bottom end with new seals and bearings once it's all assembled do a leak down test first.

At this point I am not planning on reinstalling that Mikuni. I have the Bing dialed in good for where I ride so there is no reason to do that. However..thanks for the advise. May come in handy in the future.

Swapping to a Mikuni is normally done as they are a much better quality item and while Bings and Amals are equal in performance they do not last for long.

Still running the Bing 54 on my 70 400. Properly jetted, it can be lugged down just above idle without clutching and runs clean throughout the RPM range. I put a small notch in the bottom front of the slide just in front of the idle air/ fuel mixture port. Reduces the off idle 4 stroking (somewhere I've got a picture of the modification if you're interested).
The 2mm smaller bore size on the Mikuni would not likely be a concern on a stock(?) 400, since it is not a revver (max HP at 5500 RPMs). The Mikuni is more precise at metering fuel.
In my opinion, the biggest downside to the Bing 54 is worn slides, the #1 slide used on these bikes is no longer available, except as a billet machined item at over $100.00. Everything else is available.
Sorry for the long answer to a short question, but I personally would stick with the Bing, since you have the jetting right and are happy with the performance.
Steve