Yamaha 15 What could be the poblem (I'm at my witts end) pls help

kodibass

Master Chief Petty Officer
Joined
Apr 10, 2010
Messages
864
I have a nice 2007 yam 15 hp 2 stroke, I have checked compression with 2 different testers. both say 125 top & bottom. I have strong spark jumps a good 7/16 to 1/2 on three different spark checkers. I replaced the carb on it from my dads 15 yam which is identical to mine (but his runs very well) made no difference on how mine runs (bad). mine runs as if it is running on one cylinder at WOT. it starts well and idles fine but when I try to power up it has no power at all. I also switched power packs from my dads that also made no difference, I looked at the reeds they look fine, I'm lost. What should I check next? good carb,,good spark,,good compression,, All the hoses seem ok, this is a very clean motor. pls help. thanks
 

Propwash79

Cadet
Joined
Jul 12, 2015
Messages
26
Could be the seal in the crank between the cylinders. Run the motor and when it starts having issues feel the spark plugs and see if one is cold. If it is then trace it out and you migh find a bad coil???. Will it run good on a hose then once a load is put onto it it Boggs down?does the it run good at 1/4,3/8'and 1/2 throttle?
 

kodibass

Master Chief Petty Officer
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Apr 10, 2010
Messages
864
Have not tried it on a hose, but I will. I'll check to see about the spark plug I had to give up on it for a while due to work, I'll get back to it soon. it seems to run fair until about 1/2 throttle then seems to sound like it starts missing, but I used in line spark testers and they show steady strong spark on both cylinders even when its running bad at the upper speeds. so I'm still scratching my head on this motor thanks
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
a leaking fuel pump can cause an over rich condition where the spark will not light off that cylinder that the pulse port it tied to.
Also check to make sure timing is where it should be at the Problem RPM
 

doghunter9

Petty Officer 2nd Class
Joined
Aug 20, 2008
Messages
101
Have you tried different fuel? I recently got towed back to dock only to find that 5 liters of the new fuel I bought the day before was water. Of course the fuel pick up in the tank comes from the bottom where the water is. I have also seen bad gas that would start and idle a engine but would stall under load!
As for the leaky seals, do a leak down test with the other spark plug out to show faster results.
 
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kodibass

Master Chief Petty Officer
Joined
Apr 10, 2010
Messages
864
Update--- problem fixed it had a bunch of mud & grass in the exhaust? so the shop that fixed it told us, I guess its possible??
 
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