I use to do alot of work on the old Formula MX`s The Best thing that I would do is raise the exhaust port 1mm and widen each side of exhaust 2mm for a total of four. Clean the exhaust port to a chrome like finish and sharpen your intakes where they meet the base almost knife like.
I used to use a 207 rotary plate with a pipe from a 1987 formula plus,raise each main jet by 20 and put in
a set of 144 ramps. I don`t remember which spring,You won`t believe the difference.you can then twist the rpm`s to match a 87 plus.
You should see about 9-10 mph difference. Good luck
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