2008 Yamaha 25 hp 1 cylinder at low speed

Tampa-Dave

Recruit
Joined
Dec 14, 2017
Messages
1
I am reaching out for help, I have tried all I know to fix a problem with my 2008 25 Hp 2 cycle. The motor runs great at high speed, but a low speeds is shuts down to one cylinder. I have tried many fixes, clean the carbs, rebuilt the fuel pump, changed the tank, and fuel lines. Also I have moved and replaced coils.

Bottom cylinder is the one that won't run at low speed, but compression is good on both cylinders.

I am in Search of what to try next

Appreciate any advice.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
my guess would be dirty pilot jet/passages, misadjusted, or leaking seal on crank.
did you use a hot bath ultrasonic cleaner and have the carbs apart completely when cleaning them?
did you follow procedures in service manual to adjust everything after installing them again
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,033
Looked at the reed valves ?-------Does spark jump a gap of 3/8" ( 1 cm ) when you test it ?
 

tommarvin

Ensign
Joined
Nov 22, 2015
Messages
999
Like the guys above, if the motor starts and runs the odds favor fuel delivery problems not issues.
Good for you know it's the bottom cylinder, that means you need to completely remove the bottom carb from the motor, remove all the parts and clean them, it's on you tube watch all the carb rebuild/cleaning videos. If you damage any gaskets/parts get new replacements.If you have a welch plug remove it, lots of small holes for idling in there.
Now you need to get up and do it again for the top carb, it will be so much easier, why is the second time a no brainer easier and one tenth the time?
Replace the on motor fuel filter,
New two stroke gas.
 

Tampa-Dave

Recruit
Joined
Dec 14, 2017
Messages
1
Raczekp1.... What do you mean " CDI" Fails at low speed?

Racerone....I check the spark and it is going well beyond one inch. I was wondering how do you check the reed valves?

99YAM40 I did not see instructions on how to adjust the jets when reinstalling, I ended up with a new jet, and put in in fully and backed off a couple turns.

Tommarvin What is a "welch plug".


I appreciate everyone's ideas. Thanks to all.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
service manual should have specs as to how far to turn the pilot screw back out(not jet).
If the carb is clean then the spec should make it run well
 
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