FT9.9ELJ weird running issue???

dohcdelsol93

Petty Officer 3rd Class
Joined
Jul 13, 2010
Messages
97
I'm looking for a yamaha 4 stroke expert to chime in and help me!

I have had this motor for about three years. It's been super reliable for me.

Last year while running it sounds like I am losing a cylinder. Then it picks back up. Its very random.

I specialize in outdoor power equipment repairs and have rebuilt the carb several times so I can rule it out.

I have adjusted valves, new ngk plugs, always kept stabilized pure gasoline.

I have pulled the thermostat since it can stick and affect the auto choke system. It was sticking. The oem thermostat is obs nla and the aftermarket equivalent was too long and would not fit into the housing. I was able to clean the old one and verfied it was no longer sticking via a pot of hot water.

Since the thermostat cleaning it does run better but will still lose power and then pick it back randomly.

I have a brand new water pump and a strong pee stream.

Any ideas on what I should do? Any ideas on where to find another source for a thermostat?
 

99yam40

Fleet Admiral
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Sep 7, 2008
Messages
8,851
too much fuel as well as too little will cause a cylinder to not burn properly.
have you checked the fuel pump to see if it is leaking fuel into the crankcase?
 

dohcdelsol93

Petty Officer 3rd Class
Joined
Jul 13, 2010
Messages
97
I have not put a fuel pressure gauge on the fuel line to verify proper pressure ( I'm assuming it would have less than 4psi) but I have inspected to verify I have good strong flow as well as keep close tabs on oil level. The oil level is good and not getting any fuel inside the crankcase.

I specialize in carb rebuilds so I am confident the carb is in top shape.

I, however, am not familiar with the vacuum operated choke. I believe this issue is in the auto choke system. If I could do away with this sytem and just install some sort of butterfly operated choke or primer life would be easier

I have heard of the 15 hp carbs installed on 9.9 motors....I would not be opposed to this modification in order to get an easier operating choke
 

99yam40

Fleet Admiral
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Sep 7, 2008
Messages
8,851
sorry i forgot it was a 4 stroke and not a 2 stroke.
not sure how that pump or what operates that pump.

have you watched timing light flashes when problem shows up to make sure spark it steady.
Some timing lights read RPM also and that might indicate if one cylinder is loosing spark compared to the other

did you test to make sure stat was opening and closing at the specs list to make sure the motors temp is getting high enough?

I have found jets drilled out or deformed by someone shoving wire through them before causing too rich a mix at high RPM before. hard to find some of these problems

Testing voltages into and out of CDI might show weak spark or better yet would be meauring the secondary voltage to the plugs if you have the proper test equipment.

One carb and 2 cylinders means fuel to air mix should be the same to both, so valve sticking would be the only way to chance fuel mix to the different cylinders unless the was a vacuum leak
 
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Dizz

Petty Officer 2nd Class
Joined
Jun 12, 2007
Messages
148
I did not understand,why did you say you can not find a factory OEM thermostat on a motor this new?Sounds like you`ve really been thru this motor!
 

dohcdelsol93

Petty Officer 3rd Class
Joined
Jul 13, 2010
Messages
97
It's a pulse driven fuel pump running off of crankcase pressure.

At least I think it is. I'm not at the boat this moment. We'll call it 90 percent sure. Dealing with carbs and motors daily...I haven't had it apart in two months...the memory bank is not a clear as it used to be.

I have not tried a timing light but I was thinking a possible cdi issue.

I don't believe it too be mechaical because I can unplug one of two cylinders (one at a time) and the engine will still run at roughly half power

The carb appeared to be unmolested upon removal. Looking at the plugs they don't seem overly rich. Granted looks can be decieving. It will lose rpms and power at varying rpm ranges. It idles smooth as can be. At wide open throttle it runs just as smooth as half throttle. Being in a sailboat full rpm isn't used other than testing.

If someone wasn't familiar with the motor heard it while it's losing power and rpms they would think it sounds fine. It doesn't load up or lean out or bog or any performance hindeing noise. Its as if a gremlin flips a switch and cuts my power.

What makes me suspect the cdi is the cdi controls timing and rev limiter. However unless I'm 100% it's a 225 buck gamble.
 
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99yam40

Fleet Admiral
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Sep 7, 2008
Messages
8,851
get your hands on a DVA adapter or make one so you can test the peak voltages into and out of the CDI.
check timing to make sure it meats spec and stays there when problem shows up
 
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