1985 Mercury Outboard V-300 V-3.4L Shop Service Manual, Page 48Get this manual
8Synchronize carburetors as follows: aRemove bolts and remove fuel line con nect or from mount, then remove sound box cover from engineThi is necessary to be su re that all carbu ret or throttle utters are closed and that they open toget her bLoosen s:arburetor synchron izing screws (Figure 5) and allow all throttle hutters to close completel ycPosition thrOllle lever so that idle stop screw is against topMove roller arm until roller (Figure 5) just touc hes thrott le camWithout moving roller from this positionretighten carburetor synchroni zing screwsdLook into carburelor bores and check that all carburetor shutler plates close co mpletely and open exactl at the same time when thrott le lever is actuated aSlash MarkThrottle CamRollerIdte Stop Screw (Adjust so that Slash Mark Is at Point of Contact with Roller)
Figure 3Idle Stop Screw Adjustment
aSynChronizing ScrewsRoller