yamaha 1992 150 hp 2 stroke engine will not get above 4,000 rpm at full throttle

3stooges

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Jul 20, 2016
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3
1992 Yamaha 150 hp 2 stroke engine won't get above 4,000 rpm. Engine compression good at all cylinders, primer bulb new, fuel filters and seperaters new/good, fuel pump diaphragms not broken but not sure if the pumps/diaphrams are weak and they have never been changed, gas doesn't appear to have water, hull vent open as gas came out when filling tank, new spark plugs, ran great when first put in water in May but hadn't been run for 1.5 months when max 4,000 rpm found even at full throttle when we can usually run to 5,000 rpm and plane off fine with 4 adults onboard. Could only get to 3700 rpm with 3 adults onboard but tides and wind could have been the difference here, starts immediately and idles well, a little rough when running at low speed/idle. Any ideas or suggestions on where to go from here?
 

3stooges

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Joined
Jul 20, 2016
Messages
3
Used a separate gas tank and ran fuel directly to the engine at input to the small screen/filter on the engine, cleaned bottom of the boat, and sprayed carb cleaner into carbs while running under load. Engine RPM got to max of 4100 so still 800-900 RPM short. Leaning towards carburetor or timing issue??
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
You're apparently loosing power on at least one cylinder and best not continue running motor until problem indentified and fixed....my first guess would be a dirty carb jet.....remove each carb float chamber drain screw(hold a clean white rag underneath to see what water or debris comes out)....and prevent dropping drain screw down in cowling. Also remove each spark plug and inspect for any differences in colors or carbon which may indicate which cylinder is the problem. May have to remove, disassemble and lean the carbs to make motor run right....a Yamaha outboard technician can do this for you...
 

3stooges

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Jul 20, 2016
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3
Carbs were rebuilt 6 years ago. With an acquaintance/mechanic who rebuilt the carbs, we removed the carb drain screws using white cloth to catch any particles (maybe a few fine black particles came out), sprayed carb cleaner in each carb, with timing light checked the spark to each plug - everything fine, didn't pull any plugs this time, ran boat and picked up 100 rpm so now maxed out at 4200 RPM. Didn't remove or disassemble the carbs at this point. I can get about 22knts full throttle with engine trimmed properly but when we put it in the water in May we got 28knts. Will check plugs again.
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
May be a partially plugged carb jet and require removal and cleaning....do you have a fuel/water seperator installed on your boat to help remove water and trash that can plug jets?....A Yamaha technician can clean your carbs a whole lot cheaper than replacing a burned piston due to lean fuel mixture...
 

gap1949

Cadet
Joined
Jul 17, 2003
Messages
13
Check the throttle cable. It can come out of its hold down bracket or the cable it`s self can separate not allowing full throttle.
 
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