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Setting of idle/low speed screws. Merc 90

D

Donald Boyer

Member
Hi all,

Mercury 90hp
1988
s/n 0b312461

I know some or all of the idle screws on my carbs have been set too rich. This has resulted in smokey idle and smokey acceleration. Neither of these two things used to happen to quite this extent.

Carbs were linked and synced my a Merc dealer 12 months ago.

Question is:

What is the procedure for setting these correctly? I don't want to just start fiddling like the last guys did.

Thanks in advance,

Donald.
 
kimcrwbr1 said:
In the water in forward gear slowly turn it in until the engine coughs and back out for a smooth idle. Do that several times on all the carbs turning 1/8 turn at a time and wait 20 seconds between adjustments. Once you have a nice smooth lean adjustment turn the needles out rich 1/4 turn to protect the motor and boost acceleration. If you have trouble with cold starts add another 1/4 turn rich. Do you do alot of slow speed operation if so blow it out every so often maybe do a decarb with seafoam and then adjust the slow needles again. It is allways better to run the idle air mix rich rather than lean if it is smoking it is getting plenty of oil. You are mixing 50:1 using a high quality oil mix correct, hopefully not penzoil!
Click to expand...

My motor is an auto oiler. And I'm using Quicksilver Oil. Mostly mid range - full throttle operation. I'll perform these instructions and report back. Thanks in advance!
 
kimcrwbr1 said:
In the water in forward gear slowly turn it in until the engine coughs and back out for a smooth idle. Do that several times on all the carbs turning 1/8 turn at a time and wait 20 seconds between adjustments. Once you have a nice smooth lean adjustment turn the needles out rich 1/4 turn to protect the motor and boost acceleration. If you have trouble with cold starts add another 1/4 turn rich. Do you do alot of slow speed operation if so blow it out every so often maybe do a decarb with seafoam and then adjust the slow needles again. It is allways better to run the idle air mix rich rather than lean if it is smoking it is getting plenty of oil. You are mixing 50:1 using a high quality oil mix correct, hopefully not penzoil!
Click to expand...

Thanks! I'll do that when I'm next in the water. Should I begin with the screws where they are now? Or is there a factory setting I should reset them to before I proceed?
 
Hi all,

I finally got around to looking at my Merc 90 idle screws. I warmed up to operating temp by a long run at 4500rpm, then let it idle, and started windoing the screws one by one, 1/2 a turn out each. This seemed to help, as the engine stopped coughing/back firing but after another long run at which I guess the engine was a little warmer, it did it again.
The screws were at (turns out from lightly seated):
1) 1 3/4
2) 2 1/2
3) 1

The impellor was replaced (along with seals on pump) 6 months ago. Thermostat and gaskets also replaced at this time. This is a fresh water engine.

Next question is: does the timing need looked at? Or should I be more thorough with the idle screws? I was hesitant to just keep making each one richer....

Donald.
 
Those adjustments are too different from each other; they should be no more than a 1/2 turn different. That means they are still far from correct.

I suggest you use an infrared temp gun. Shoot the red dot on the metal shell of the plugs and diddle the idle mixture screws until you get tem all roughly the same temperature--about 85 to 105 degrees. This is the only way I get my older triples, with two carbs, to idle correctly.

Jeff
 
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