Hey OP, I had that exact exhaust system on my '91 and it always ran strong, even up to this date. I sold it to one of my sons and he's about to sell it off to one of my grandsons with about 95k miles on it. I ran the stock '91 Keihin CV40 with a kit from Gary Bang (who knows maybe out of business by now). The kit as I recall included at least one jet, wish I could remember which one and the size, plus a softer slide return spring and instructions to drill out the transfer hole in the slide to a larger size. Unfortunately, all this happened in 1994 and my record keeping has let me down. All I can offer is encouragement to get your bike's carb tuned in, you'll love it when you do.

 

G'day Since 1968.
The CVP kit has a 7/64ths drill for the slide as well as jets and emulsion tube. I'll leave that for later. Just want to ride and tune as I go (when this bloody rain stops).
One thing I will rectify sooner rather than later is the miss match of the manifold to head. the head port is a bit smaller giving the incoming charge a speed hump. An hour with the Dremmel is cheaper than a pair of later model heads.
Cheers

 

Do not drill the slide or cut the spring! just causes problems with the slide fluttering, use oem Keihin or CV Performance jets, emulsion tube, and spring

 

If you cut the spring you're making it stiffer, which then would contradict the drilled out hole. The move to a drilled out hole and a softer spring is meant to provide quicker throttle response, at least to my limited understanding. When I made those changes back in the 90's it resulted in an extremely crisp running EVO. I also recall doing a number of throttle chops and reading the plugs to tune further. I believe this resulted in moving the needle, don't recall which way and don't recall a jet change. I also took the time to index the plugs and it ran really well.
Maybe my results won't transfer to another EVO. I read in a magazine that the '91 Springer had a slightly different cam which I never confirmed. I put it on a dyno at a bike rally back in 1996 or so and the operator wanted to know what cam I was running. I told him it was stock and he said that a previous owner must have changed it.... but I was the original owner.
The owner (now retired) of Head Quarters told me the stock EVO headers with the fat cross-over pipe that runs under the pipe gave the best results on these engines based on his dyno testing. I had the exact exhaust as seen in the OP's picture, stock headers with Cycle Shack side dumps. The picture brought back these memories. It would be fun to have another EVO for a project bike.