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  #1 (permalink)  
Old 09-08-2012
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Default yamaha 25hp 2 stroke performance issue

guys
last weekend i bought a used 1988 yamaha 25hp two stroke short shaft outboard. its mounted on a very light 14ft aluminum jon boat.
it started up on the first pull at the guys house, and every time i tried to start it. runs good while idling and revving it up on the rabbit ears.
today i took it out for its maiden voyage. it started up on the first pull, then i got underway. taking it easy easing into the throttle. once i got going, i then went to full throttle.
the front of the boat came up, then went on plane pretty quickly. although it was going much faster than my old 15hp 4 stroke motor, to me it seemed like it wasnt performing to its full potential.
i was at full throttle for about 1 minute, then had to back off the throttle because of an under pass and bridge pilings coming up.
once i backed off to about half throttle, went under the bridge, then went to full throttle, nothing happened. it was kind of stuck at half throttle. the tiller grip was turning, but the engine was not responding. i kept backing off the throttle, and it was slowing down, then i tried to let it idle, and the motor shut off. i pulled it about 50 times and could not get it to start.
thankfully i had my trolling motor to get me back to the ramp.

once i got home i wanted to flog the carbs with power tune spray, (as it had worked for me on numerous occasions,
i checked the air mixture screws (?), and the top carb was 2.5 turns out and the bottom carb was 1.75 turns out. i took them out, along with the bowl drain screws, flogged then thought both ports, put it back together with both at 2 turns out, and got it to start.... obviously it was smoking from the power tune spray, but once i got that cleared it would start on the first pull... i then flogged the carbs again with power tune and am letting it set for a few days.
i hope to test it again next weekend when i have time.

do any of you have any ideas as to what might be my problem ? is there different passage ways in the carbs for idle and full throttle, like an automotive carb ?
and what is the correct adjustment on the mixture screws ? i have them now at 2 turns out.
hopefully its something simple, and if i have to have the carbs rebuilt, hopefully that would be all that is wrong with it.
the previous owner said it had been setting in his garage for a while, and he always drained the carbs before setting for storage.

thanks for nay help you can offer.
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Old 09-08-2012
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the motor is a 1988 25SG Code 6L2 6
the gas i am using is new and worked fine last weekend with my old 15hp engine.... i will do a compression test tomorrow.
should i check compression hot or cold ?
thanks.
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Old 09-10-2012
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Run a compression test to see if even, remove and clean carbs, check for spark on all cylinders.
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Old 09-10-2012
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ok guys,
i finally did the compression test. once cylinder, the top cylinder, was 150psi after 5 pulls of the cord, and the bottom cylinder was 135psi, with the same 5 pulls of the cord. i did the test with the other plug installed. hopefully thats the way i was suppose to do it ?
i think i read somewhere on here that the cylinders should be within 10% of each other ? if so i should be fine there, correct ?

hopefully tomorrow after work i will start it up and try pulling one plug wire at a time, to make sure each cylinder is firing.

anything else i should try while its running here at the house ? if everything seems fine, i am hoping to test it in the water again this saturday after work. im hoping that it setting for a few days with the power tune spray in the carbs will clean them up, if thats what the problem was....

oh, how many turns out from bottom should i have the air mixture screws set it ?

thanks much guys.
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Old 09-10-2012
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oh, since the compression seems to be good, does this eliminate bad reeds ?
if reeds were bad, would that make a bad compression reading ?
thanks much... i will report how it runs tomorrow after pulling one wire at a time while running...
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Old 09-11-2012
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Reeds are not in combustion chamber they are in crankcase and will not affect compression. It could have a bad reed and still have good compression.
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Old 09-11-2012
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Quote:
Originally Posted by boats.net View Post
Reeds are not in combustion chamber they are in crankcase and will not affect compression. It could have a bad reed and still have good compression.

thanks for clearing that up for me...

i am suspecting the fuel system. the guy i bought it from two weeks ago said it had been setting in his garage for some time.
i have not taken the carbs apart, but have sprayed a bunch of the "mariner power tune" spray in the carbs through the air mixture port and through the bowl drain.... that spray has worked wonders for me in the past, on my 15hp 4 stoke engine and my lawn mower and generator.

i fired it up tonight after work and its running perfectly... half a pull with my bad arm and it starts. seems to be running smoother since the power tune spray, but the real test will be on saturday when i take it out again for a test run. although i am very good mechanically, if it still gives me problems under full power, i may take it to my mechanic. i have never taken a boat carb apart before...

and while it was running at idle, i pulled each plug wire off one at a time and it did effect the running, so both cylinders are firing.

not really sure if the ignition system is failing while running, but i do know it was only under full throttle for less than one minute... i dint think it had enough time to get hot.

i do have a repair manual on order, but it hasnt arrived yet....

my brother told me to get some "startron" fuel additive, he swears by it. says it treats fuel and cleans carbs also ?
i will try it also...

how difficult is it to rebuild the carbs on my own ? i havent done this type of repair before, but have taken automotive carbs, holley and edelbrock apart and rebuilt them years back...

again, thanks so much for the input guys... and i will let you know how it runs when i take it out on saturday after work.
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Old 09-12-2012
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carbs on these old 2 strokes are simple, just pay attention when taking apart and do not mix up parts from different carbs
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Old 09-12-2012
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Quote:
Originally Posted by 99yam40 View Post
carbs on these old 2 strokes are simple, just pay attention when taking apart and do not mix up parts from different carbs

thanks for the info... i wouldnt have thought the two carbs would be different...
i will wait until i get my repair manual, and go from there.

thanks again
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  #10 (permalink)  
Old 09-12-2012
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also, i dont see a carb rebuild kit on here.

1988 25SG Yamaha Outboard CARBURETOR Diagram and Parts

can someone please point me in the right direction. i would like to order the rebuild kit

thanks

1988 25SG 25hp two stroke
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  #11 (permalink)  
Old 09-12-2012
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Join Date: Dec 1969
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not sure about you motor but my C40TLRX has different jets in middle carb than the other 2.
That is why I mentioned it

Look at #17 http://www.boats.net/parts/search/Ya...201/parts.html

Last edited by 99yam40; 09-12-2012 at 09:04 PM.
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  #12 (permalink)  
Old 09-16-2012
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ok guys,
took it out for a test run yesterday after work, and does the same thing. starts right up on the first pull. give it some throttle and it runs good for about 15 seconds, then starts slowly slowing down, no throttle response. once i got it home i noticed when i pump the primer ball, there is gas coming out of the bottom carburetor. has to be a stuck float, or needle pin, something... i assume. i aint messing with it anymore, dont have the time, so its off to the mechanic hopefully by weeks end..... hoping it only a carb rebuild..
thanks
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