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1990 Mag 2 150hp motor losing speed/surging RPM's

devinhopper24

New member
I am starting to reach my witts end with this mottor so maybe someone has had this same issue and has suggestions.

Ok, so I have a 89 Venture with a 90 Mariner Mag 2 150hp. Bought the boat back in Feb. of this year and ran great the first few months I had it. I noticed one day that one side of the jack plate was set about 1/4 inch higher than the other. I took one side down 1/8 inch and the other side up 1/8 inch to level it out. The next time I took the boat out it seemed to get on plane a little slower than normal and once it was on plane (around 3000 RPM) it felt like I would lose speed/RPM increase and the nose of the boat would quickly veer to the right. At idle there is no noticible difference and the motor runs fine, and if I fight through the surging and get above 4000 RPM the surging will stop but motor is still not running to its full potential. Seeing that this happened only after adjusting the jack plate I loaded the boat back up and went home to put the jack plate back to its orginal postions (marked original position with a sharpie and it is back to being offset 1/4 inch). I took the boat back out again and had the same problem. Not only that, while testing it out something broke in my lower unit and it locked up. I took the boat to a local mechanic and chalked the original probem up to whatever was going out in my lower unit that caused it to lock up. 3 weeks later I get the boat back with a new lower unit and a new prop. I take the boat out and still have the same problem with the boat losing speed/RPM increasing and the nose of the boat quickly veering to the right. Having a little experience with 2 stoke motors I started going through the list of things that I could change easily. I started with the fuel system and put a new kit in fuel pump, changed all fuel lines/bulb, and pulled the pick up from the tank to ensure the screen wasn't clogged. None of this fixed it so I took it back to the mechanic and had him put new kits in all 3 carbs which also did not fix the problem. After that I moved on to electrical. I changed all coil packs, rectifier, stator, both switch boxes, and the trigger. Needless to say none of these fixed the problem either. I am at a point now where I am not sure what to do. Before the problem started the boat would run 62-63 mph and around 5400 RPM fully loaded for tournament with 2 people. Now, by myself and pratically empty, it struggles to even get close to 5000 RPM and tops out around 56 MPH. Any thoughts or suggestions would be helpful, thanks!
 
That's kind of low compression for that engine.

100psi is limit. poor performance at wot throttle and poor idling at 100 psi and below. 140psi is very good.
It's time for a rebuild now that you spent all that money on new parts.
 
I don't think those have compression that high. Your compression sounds good I could be wrong but I am pretty sure your compression is fine. Check for a broken motor mount.
 
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That compression is low .. Does it have a hot foot? If so check to see if stop screw has backed out not letting reach WOT. Also check that trigger is not sticking.
 
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