100 merc 2+2 rough idle

[joeb]

hey guys i have a merc 100 2+2 . I had the carbs rebuilt last year. i recently took the boat out and can't get the boat to stay running. I would take it back to the place I the carbs were rebuilt by but they dont know anything more then i do.. I checked the carb needle screws the top carb was set at 2 1/2 turns and the second was 1 1/4 turns this does not seem right. the two bottom carbs dont have needle screws this seem odd. i turned to idle up it runs at about 1200 rpms but has it's rough spots any ideas what might be my problem.. the compression are all around the came so i think it's a carb problem . never worked on anything like this before. just cryslers so i'm a little shy. boat idled fine before i took it in, but would take forever to plain off . any sujestions would greatly be apreciated thanks

 

[MURX]

Re: 100 merc 2+2 rough idle

this I know for a fact: the number 3 and 4 carbs differ from the 1 and 2 carbs 1 and 2 have adj screws 3 and 4 do not thats why it's called 2+2 it means it runs on two under 1800rpm after 1800 rpm the other two carbs come into play and you're now running on four cylinders the fuel mixture supplied to carbs 3 and 4 is only sufficient to lubricate the cylinders but there is not enough to promote combustion below 1800 rpm, idle speed should be 700 in gear..... a pair of std WME carbs from a 70 or 90 merc may be in order to cure this strange set up I'd check charging system output and timming, if it dosen't produce addequate mid to top end check the switch box

 

[joeb]

Re: 100 merc 2+2 rough idle

murx, thanks for reply. charging system works ok (gauge is about 14 volts) motor runs fine just will not run at idle...

 

[Red De Braal]

Re: 100 merc 2+2 rough idle

I'v got the big brother (115 HP) This is a real touchie motor, carbs need to be synchronized. The good Mercury dealers have carb synchro equpment, It's small tubes that he sticks into the carbs that checks vacuum, then he adjusts so the two low speed are the same. The high speed carbs don't have much adjustment. Good luck <br /> Red !!

 

[ODDD1]

Re: 100 merc 2+2 rough idle

JOe, you should have the same idle mix setting on both carbs.....usually around 1 1/2 turns....having one throttle plate hanging open at idle will cause your problem....any idle mix setting you do should be done in the water,warmed up...they are pretty picky but run well when setup right....

 

[joeb]

Re: 100 merc 2+2 rough idle

odd1 Thanks guys for reply.. I new that they should be the same.. you said something about throttle plate might be hung up , how can i check.. i got the motor idled up around 1300 rpms when ingaged it runs at 800 rpms . this boat as sat since last july. I took it to the sea ray dealer ship here they did 2300.00 to this motor rebuild carbs twice (claims the jets were drilled out. ) i'm the second person to own this boat. this motor never had a problem idling but took forever to plain off. The boat plains off a lot faster but has a fludder sound tell it get around 4000rpm. (motor tops out at 4500rpms with a 19 pich prop. Is this something i could work out myself. I not used mercs so i'm a little shy. i am currently rebuilding a force 125 88 model so i know alot about them motors but this 2+2 system has got be thrown for a loop. please reply. and thanks to all who read this .

 

[joeb]

Re: 100 merc 2+2 rough idle

bump

 

[ODDD1]

Re: 100 merc 2+2 rough idle

Joe...you can adjust the throttles with the plastic tie bar that hooks the throttle plates together...loosen the screws and make sure they are all closed...check the throttle roller too, it should be just not touching the throttle cam at idle...there is an accelerator pump that feeds fuel into the lower 2 cylinders when the throttle is opened...this may be causing your bog.....the noise I cant help you with, but you do need less prop...you should run to 5250 max rpm...good luck with the force, they are one of the easiest motors in the world to make run well...