Throttle plate synchronization

[enginepower]

Have a 1995 90HP 3cyl. Noticed that the top carb throttle plate doesn't open to full extent as bottom two which means this one is closed more at idle than other two which probably explains my uneven shaky idle. It has a slotted cam with screws to lock in place. What would be the procedure to adjust this? I am thinking loosen screws and let all of them close fully then tighten screws down? Also, what would be a good starting point for the fuel mixture screw (amount of turns from seat)

 

[enginepower]

^^^

 

[Texasmark]

You are onto it. If you do more than adjust one carb to match the others, aka move from the vertical carb linkage rod to the cam and all, that also changes the setting of the timing and then you need to get a manual as that is a precise alignment and is tricky. You can sign up for the Seloc online manuals (upper right of this page) for a few bucks and have copies instantly.

On the set screw, 1.5 trurns from closed, then tweak as necessary. When turning the screw in all the way, use a small screw driver and a thumb and index finger only.....don't overtighten or you will score the cone on the carb side of the screw. Why not see where your other two carbs are sitting and adjust accordingly then fine tweak as required. Proper setting is a smooth idle and the ability of the engine to take throttle coming out of the hole without hesitating. On tweaking that the tweak could be as small as 1/16th a turn.

Mark

 

[enginepower]

Thanks for reply. Talked to previous owner and he said somebody did the carbs for him and it always seemed rough to him at idle after that but it accelerates without hesitation most of the times. I find it off balance enough that I hear a rattling when it's really rough and sounds like the driveshaft rattling in the splines every now and then. Top carb that is closed more set screw was at 2 turns out and the other two were at 1.5 out. Thinking about getting that manual and going thru the carbs myself because it likes to "sneeze" at you at idle most of the times and even my dad that rode in it last weekend said that idle seemed excessively rough.

 

[merc150]

I would also take the air cleaner off run the motor on muffs and see if you have any gas spitting out back at you from the carbs if so bad reed valves. My 03 150 motor has a crappy idle and hesitation turned out to be chipped and broken reed valves. Just a thought

 

[enginepower]

Thanks. New gaskets on order since I see that whomever has worked on these carbs left them off between the air box and carb, including the lower rubber seal for air box halves and oil is leaking to bottom of motor and probably worse when I trim up for transom saver but that's a good idea to check while I have this off. Not sure if I need to disassemble carbs and check things as apparently a shade tree done the work but previous owner said the work was done about 2 years ago and it had that "sneeze" but ran good overall so he didn't worry about it.

 

[undone]

The sneeze means it's running lean on at least one cylinder. Mine did the same thing, synching the throttle plates cured the problem. Opening the low speed screw the extra turns is an attempt to overcome the lean mix. Mine would also sneeze badly when slowing down quickly.

 

[enginepower]

The sneeze means it's running lean on at least one cylinder. Mine did the same thing, synching the throttle plates cured the problem. Opening the low speed screw the extra turns is an attempt to overcome the lean mix. Mine would also sneeze badly when slowing down quickly.

Well mine is not only just out of sync, it's so bad, with throttle pegged wot, I can see the top carb isn't even open all the way. It's no where near the others. Means the others are open much more at idle and probably a good reason the idle screw is backed off almost all you could put it.