25 HP Yamaha 4 stroke backfire

BAI2

Cadet
Joined
Jul 14, 2004
Messages
22
All,

I have a headscratcher.... Went through the carb, cleaned every port/jet with small torch tip cleaner and spray and compressed air.

Motor starts as it should, when I throttle it up it bogs down, occasionally backfires back through the carb, and will die if I don't throttle it back down. Note this is on the test stand not under a load. If I ease into the throttle, it will perform seemingly as it should. Changing the air screw does seem to slightly help by richening the fuel air mix up, but don't fully eliminate the problem.
Also, note when I throttle it up and it coughs and sputters, I'm not "hammering" it, just a normal throttle up process.

The issue points towards a lean condition, but I can't/haven't found it. OR I am overlooking something.

The motor is in great condition and well kept - for the record.

Thanks in advance!!!
 

99yam40

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Sep 7, 2008
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9,175
is this a carbed motor or injected?
giving all motor model ID info helps
heck we have no idea what year even
 

BAI2

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Jul 14, 2004
Messages
22
I’m not new to repairing outboards in any way shape or form. What I am new to is the 4 strokes. There has to be something small I am overlooking. I have fixed these things for over 25 years as a “hobby” turned into a full time job sometimes. I know the ins and outs very well, this one just has me stumped. As I stated in my initial post, I went through the carb, it’s not fuel injected, if it was I would have indicated. Just tossing it out to see if anyone has any thoughts is all!
 

BAI2

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Joined
Jul 14, 2004
Messages
22
I battled a 15hp Evinrude 4 stroke for a few years. Ended up drilling a small hole in the throttle plate and it fixed it. Kinda the same symptoms but unsure if this is the correct avenue on this motor….
 

99yam40

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I guess you removed the plug so you could remove, clean, and adjust the pilot screw.
that plunger mechanism has me thinking that maybe it is not injecting enough extra fuel while accelerating
 

BAI2

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Joined
Jul 14, 2004
Messages
22
I guess you removed the plug so you could remove, clean, and adjust the pilot screw.
that plunger mechanism has me thinking that maybe it is not injecting enough extra fuel while accelerating
The plug had already been removed so I am assuming someone prior to me has been in the motor ;-)

The plunger mechanism from what I understand is only for priming??? I could be mistaken.
 

99yam40

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Sep 7, 2008
Messages
9,175
so moving the throttle does not move the plunger?
it was just a guess since I have not worked on that motor myself.
sometimes running the disassembled carb thru a heated sonic cleaner can get stuff out you cannot other ways
 
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