first i hope this is is correct, here goes, your idle jet is at the top front of carb( i think) behind washer and screw (#25&26) believe to rich'n you go up in size,because gas passes thru the jet, on later models carbs the jet is not covered, so air passes through it, like a 1987 70hp, check parts break downs for this year, to rich'n in the 87" model(i think) you restrict air flow, smaller jet, hence less air more gas,makes sense.
one other thing is the older carbs have another jet behind a screw at the top back of carb, i'm not sure which one , the front or back is idle, i'm assuming its the front because of the 87" design. need a guru to step in here!
i changed my 87" 48spl reeds to Chris Carson Marine reeds, no re-jet, and like the results. for those who like to tinker with these old motors, something to check out.