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why is my F4 acting like it's starving for air?

Megunticook

New member
2008 F4, I've owned it since 2013. Well-maintained, I do work myself including total carburetor disassembly, cleaning, inspection, etc. every winter.

I removed the metal cap/welch plug over the mixture adjustment screw so I can fine-tune the adjustment. Before launching boat the week of Memorial Day, I dialed in the idle speed and mixture using a tach. and she was running super.

Ran perfect first few times I used it.

Then one day I fired it up, pushed the choke knob in halfway as usual, and she wanted to stall. I pushed the choke all the way in and she recovered but still sounded stumbly. I removed the cowling and she came back to life. I tried leaning in the mixture needle some (I'm 1-1/4 turns out from lightly seated at this point, factory is 1-7/8), but something is way off.

Is she starving for air? Why would having the cowling on or off make that much difference? I've always set the idle speed/mixture with the cowling off, then replaced the cowling, and it never seemed to make much difference. I inspected the cowling and the vents at the top aren't clogged.

Any thoughts on why this is happening? Thanks.
 
Start motor.-----Squirt some oil into the carburetor.-----The smoke may make it easier to find an exhaust leak !
 
Good idea, I'll try that. For some reason I can't quite envision how the exhaust gas is routed from the combustion chamber down to the exhaust port in the lower unit and the port just a bit below the head. Which seals are the possible culprits?
 
Do your test first.-----I do not believe there is an exhaust leak.-----But I am not there to do any trouble shooting.----I think there might be a simple fix for this.
 
Just talked to a local Yamaha mechanic, he thought more likely it was leaking carburetor fuel bowl. Possible I suppose, but I just rebuilt that carb. this spring and put in a new float, needle valve, adjusted everything to spec., etc.
 
I finally had a chance to bring the motor to my shop and have a look I pulled the carb. and saw this:

throttle-plate.jpg

carb.jpg

fuel bowl.jpg

The stuff on the throttle plate was a brown/red sticky goo. The stuff in the bottom of the fuel bowl was same color, but more like pieces of grit. Oxidized gasoline?

I disassembled and cleaned this carb before launching around Memorial Day. It was clean as a whistle. Ran great the first few times I used the motor, then one day it didn't/

Fuel was purchased around Memorial Day and I added Yamaha fuel stabilizer.

Fuel filter is new. Water separator is new. Fuel lines have all been replaced in the past 3-4 years.

What's going on here?

Thanks.
 
So I put the carb in the ultrasonic cleaner, reassembled, checked the float adjustment against spec.; then I decided to try some ethanol-free gas (only thing available here is aviation gas, 100LL). Ran great on the bench with cowling off. But as soon as I put the cowling back on it wanted to stall. Even if I just placed the cowling over the motor without latching it shut tight.

Exhaust leak then?
 
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