1998 mercury 150 idle mixture screw adjustment

[PROLINE201]

Just did a fresh carburetor rebuilt on my 98 mercury 150 offshore and after assembling back on to motor i turned the idle mixture screws out 1 1/2 turns as suggested in seloc repair manual and she was running ruff. I started backing out the screws to about 2 1/4 turns and seemed to be running much better, now my question is, is this running the motor to rich? When i dissembled the carbs the idle mixture screws were all set at the suggested 1 1/2 turns, just don't want to do any damage to the motor. Thanks in advance for any feedback

ian

 

[Chris1956]

Re: 1998 mercury 150 idle mixture screw adjustment

Idle mixture is intended to be adjustable. Set it where the idle is smooth and adjust throttle for proper idle RPM. If she accelerates smoothly, you got it right. Idle mixture will never harm an engine.....

 

[Faztbullet]

Re: 1998 mercury 150 idle mixture screw adjustment

Make sure when you adjust them that the thermal valve hose is disconnected as if hooked up and engine not at temp the adjustments will be off. The thermal valve shuts off air to idle mixture circuit to make it run rich at warm up.

 

[PROLINE201]

Re: 1998 mercury 150 idle mixture screw adjustment

Thanks for the feedback guys but i just can't seem to get it right, i have adjusted them from 1 1/4 out to 1 3/4 as recommended. I thought i had it adjusted right and took out on the river but engine still is coughing at idle and stalling out. Ran fine at wot just keep coughing and stalling out when in idle, thinking maybe the reed valves not sure, anyway gonna take it to my local marine mechanic to see if he can figure it out. I'll post results when i get them, thanks again for the feedback guys!

 

[Faztbullet]

Re: 1998 mercury 150 idle mixture screw adjustment

just keep coughing and stalling out when in idle,

Reeds wont cause the stalling. Did you remove the caps off the adjustment screws? Did you mess with the calibration screws? Are the air idle jets installed?

 

[PROLINE201]

Re: 1998 mercury 150 idle mixture screw adjustment

reeds wont cause the stalling. Did you remove the caps off the adjustment screws? Did you mess with the calibration screws? Are the air idle jets installed?

yes i did remove the caps off the adjustment screws, when i rebuilt the carbs i broke it down completely before soaking in carb cleaner. When i reassembled i set the screws to the same setting they were when i broke them down, also put all the air idle jets back. The reason i did the carb rebuilt in the first place is because at low idle the engine was coughing and stalling out so i believer there is another issue and i just can't figure it out. I blew out fuel lines, checked all hoses, pulled plugs and cleaned, and did the carb rebuild. I hate to take it to the mechanic i have done all work to the engine myself since owning the boat but just can't figure this one out..

 

[Dukedog]

Re: 1998 mercury 150 idle mixture screw adjustment

"Float test" (trash it) tha idle stabilizer. Set idle timin' ta what tha motor wants. Do this with it in tha water, on tha trailer, in gear......

 

[PROLINE201]

Re: 1998 mercury 150 idle mixture screw adjustment

Well after driving myself crazy trying to fix this rough idle situation, i get a call from my mechanic telling me i have a bad tachometer. So basically when i had the motor idling where it sounded good but the tach read 2000 rmp it was actually at 900 - 1000 rpm's. So gonna put a new tachometer in today, again thanks for all the feedback this is a great site and hopefully i can give some advise of my own in the future.

 

[Pdevito]

I have this cover over the screw. Can't pull off. Do I cut down the side. Don't know what in there for space

 

[QBhoy]

Hi. I’m sure you already know…but thought I’d mention. You need to adjust the idle setting when the engine is warmed up and normal conditions. Otherwise you are fighting other influences