mercruiser 120 - idles poorly won't go above 2500 rpms - carb swap??

F

fizzle

Cadet
Joined
Apr 19, 2011
Messages
10
howdy,
i've got a mercruiser 120 in a 1979 - 16 foot boat.
the engine has been rebuilt and compression is awesome across the board.
spark seems good - had the distributor rebuilt and bought all new components - coil, cap, rotor ect.
rebuilt the carb - cause it still wouldn't idle well.
and now it still doesn't idle well.
i don't know what the problem can be.
my latest idea is to buy another carb?
i don't know how a carb can go bad - but i really don't know what else it can be.
therefore my next question is.
what kind of carbs can i put on this 120?
will a carb from a newer 3.0l engine work?
any ideas?
thanks for your time
ajk
 
L

Leedanger

Petty Officer 1st Class
Joined
May 29, 2011
Messages
323
Re: mercruiser 120 - idles poorly won't go above 2500 rpms - carb swap??

fizzle said:
howdy,
i've got a mercruiser 120 in a 1979 - 16 foot boat.
the engine has been rebuilt and compression is awesome across the board.
spark seems good - had the distributor rebuilt and bought all new components - coil, cap, rotor ect.
rebuilt the carb - cause it still wouldn't idle well.
and now it still doesn't idle well.
i don't know what the problem can be.
my latest idea is to buy another carb?
i don't know how a carb can go bad - but i really don't know what else it can be.
therefore my next question is.
what kind of carbs can i put on this 120?
will a carb from a newer 3.0l engine work?
any ideas?
thanks for your time
ajk
Click to expand...

I wouldn't buy "a new carb", but if you did any of the Rochester 2 jet will work. Chances are it was rebuilt incorrectly and/or your not timed right. Try this: http://forums.iboats.com/showthread.php?t=168855

If it will start with the warm up lever engaged than both the choke richness and idle set screw need a little dealing with. Carbs in there entirety don't go bad, but the guts sure do hence the "rebuild kit".

Good luck.
 
O

OldNBold51

Petty Officer 2nd Class
Joined
Apr 19, 2010
Messages
169
Re: mercruiser 120 - idles poorly won't go above 2500 rpms - carb swap??

Have you tried advancing the timing? I borrowed a buddy's mid '80's 120 boat once. That thing was a dog. It could barely get out of the water with just me onboard and wouldn't rev past 2800 rpms. I advanced the timing until it kicked back (once warmed up) during starting, retarded it a few more degrees and that woke the boat up. It came out of the water great and rev'd to about 4K after that.

Just and idea.
 
F

fizzle

Cadet
Joined
Apr 19, 2011
Messages
10
Re: mercruiser 120 - idles poorly won't go above 2500 rpms - carb swap??

yeah i've set the timing and the carb to spec.
the thing still idles like crap - and i haven't even taken it out on the water to put it up to see how fast it will go.
i'm just so upset about the whole thing.
would it run if the timing chain was off by a tooth?
thats my next move - to check the timing chain.
 
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