cant get this merc to idle

tydog

Cadet
Joined
Apr 10, 2008
Messages
12
got an 77 inline six 1150hp and ive tried everything i know to get this motor to idle. things ive done checked comp all seem to be around 120,cleaned carbs once then twice off the motor..,checked ign timing,did the linc and sinc,replaced all corroded wires,changed all the gaskets,exhaust was leaking water into cyl 3 and 4 checked spark plug wires,cleaned dist cap and rotor,replaced fuel pump diaphram and fuel lines,and probly more than that but cant remember. now to what its doing, motr starts,runs and shifts good while on the muffs,but as soon as i put it in a pail of water thats whern the problem starts. it will start and idle good in neatral about 800rpm but then when i shift into gear she wants to die,and the only way to keep it running is to give it gas. now like i said before ive done the linc/sinc and i am still have a hard time with this motor. as far as i can tell the carbs are working as they should because itl stumble when mixture screw turned in and then smooth out when turned out. now i keep reading all these post people say merc sydrom/bad reads and what not but i dont realy want to give up on this motor as she runs good at high speed and at low speed but seems to be a little fast while in gear idleing.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: cant get this merc to idle

At the bottom of the transfer ports are small bleed holes. A small plastic 'restrictor' should be in each hole... If they're missing, you get bad idle....

Part# -66517 (they're a $1.01 each from mercruiser.parts)

Chris........

This is a scan from the Merc manual....
 

Attachments

  • 115 restrictors (412 x 450).jpg
    115 restrictors (412 x 450).jpg
    82.9 KB · Views: 0

tydog

Cadet
Joined
Apr 10, 2008
Messages
12
Re: cant get this merc to idle

curious what do these plastic restrictors do and why would they not be there i just had this motor apart and i dont think i can remember them being there. and can i re use the gasket on the cover if i decide to take apart and inspec angain as i just replace the gasket a few days ago can iget away with it as i would hate to have to buy more gaskets.
thanks
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: cant get this merc to idle

The restrictor have a habit of falling out if the engine gets a bit warm... They are there to restrict the flow of gases back to the crankcase. The reason for the restrictors is because the length of the hole limits the minimum diameter of a bore hole, hence the need for a fitting after manufacture...

If you don't damage the gaskets, yes, you can use them again...

Chris..........
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,102
Re: cant get this merc to idle

Tydog, I had a motor identical to yours for 25+ years. It finally blew up last summer. The bleed inserts as Achris describes may be missing. Mine were missing for 20+ years, and although that may have had some impact on the idle quality, it did not stop the motor from idling quite well. The key is to adjust the carb mixture just rich enough to accelerate well, but lean enough to idle well. The top carb on that motor usually runs leaner that the mid carb. The bottom carb runs richer than the mid carb. The key is to synch the carbs to the cylinders, and then richen them properly.

I recommend you set all carbs to 1-1/2 turns out from lightly seated. Now with the boat in the water, warm up the motor. Put the motor is gear if you can, however, neutral is OK too, but run the motor at idle speed 800-900RPM, and adjust the idle mixture screws to get the most RPM and smoothest running. Alternate the adjustments, until you stumble on the best setings on each carb. You will likely find the mid and lower carbs mixture screw is turned in more than the top carb. Ok now they are synched with the cylinders.

Now try to accelerate. If the motor stumbles, open the top carb mixture screw 1/8 turn and try again. if it still stumbles, open the mid carb 1/8 turn and try again. If it still stumbles, open the bottom carb 1.8 turn and try again. Repeat the sequence until it idles reasonably well and accelerates smoothly.
 

tydog

Cadet
Joined
Apr 10, 2008
Messages
12
Re: cant get this merc to idle

ok thanks chris ill try that is it just me or do these motors have to be set just right to idle properley.
 
Joined
Sep 5, 2007
Messages
1,790
Re: cant get this merc to idle

An old merc man told me that often when one wont idle and the cause cant be found it the small brass tube inside of the carb that gets blocked or develops a small hole. Sometimes even a good cleaning and rebuild can miss this.
 
Top