Motor running question 1990 mercury 150 black max

[Duckhunter08]

my 1990 150 black max was running rough when i first bought it. it wouldnt start without starting fluid, very bad deal. I did what is called "De-carbing" the motor by fogging out the carbs with seafoam deep creep and it was a night and day difference. when the motor was running full tilt the speedometer read 53 mph my boat is a 1990 astro quickfire fish and ski 18 ft. Since then the speedometer has quit but gps claims top speed is 40 mph. Not sure if gauge on boat read wrong or if it has lost speed. the main question I have is that is runs okay but will not idle well i have to throttle advance it everytime and acclerate fast so it doesnt have time to die. idles fine out of water but backpressure is enought to kill it in the water. When I remove plug wires i cant tell any noticeable difference in how the motor sounds. boat hits plane within three seconds so there doesnt seem to be much power loss. there are two phillips screws above the top carb im not sure if i can adjust it to idle a little higher or how to test to make sure all cylinders are firing. im thinking about putting a car spark plug in each wire to see it arc since the marine ones dont have the leg on the end to arc. right now I am running 87 octane with sta-bil ethanol treatment for fuel. oil injection system has been removed. 50:1 ration pre mix
no leaks in fuel lines it will hold prime for days on end. any input is appreciated, thanks

 

[crazyhunter]

Re: Motor running question 1990 mercury 150 black max

what pitch prob are you runing? i have a 20ft bassboat with a blackmax 150 with a 19 pitch i am running 51mph. as far as your idle i ajusted my throttle linkage on the side of the motor.

 

[Duckhunter08]

Re: Motor running question 1990 mercury 150 black max

Its a quicksilver prop and the last numbers are 21p so im guessing its a 21 pitch

 

[jamesrandolph]

Re: Motor running question 1990 mercury 150 black max

I had a the same problem with my 135hp Black Max (1990). I took it to a shop and it turned out to be the low speed stator. The technician said this series of engines used a two stage stator which is why it would run fine and high speed but would not idle. He replaced the stator which solved the problem for the most part. It still doesn't like to idle sometimes, usually after running hard but that is typical of older mercs anyway...at least that has been my experience. I'm on this forum looking for a how to on adjusting the idle.

 

[msn26753]

Re: Motor running question 1990 mercury 150 black max

Wow.. I have the same 1990 boat and motor. I know what your saying abiut ideling. Mine has same issue since I got it 3weeks ago. It wasnt ideling good at all but after I sprayed some sea foam in carbs it has helped alot. Mine still has oil injection on it .. I read somewhere that the 90 models used 40-1 ratio because mercury blackmax motors were very oil sensitive. But look that up because I only remember reading it and dont know how fact it is. Also I havent tried it yet but I did it with my old evinrude was to add marvel mystery oil to it as well. It cleaned my old motor out quick. It went from 0-100% within a tank.. im gonna try it in my blackmax with next fillup. The stabilt didnt do squat for me.. anyhow, maybe this will give you a few ideas. I heard these 1990 blackmaxs are very oil sensitive so definately look into 40-1 ratio... I heard they gum up very easily and wont idle. This will also effect top end performance as well..

 

[wired247]

Re: Motor running question 1990 mercury 150 black max

Its a 50:1 motor. Use more if it makes you feel good but it wont make it run better.

My 17 foot checkmate enticer with a 2.0 liter 150 HP Merc does 60 MPH GPS with a 19 pitch prop