2 Cycle Carburetor Question

[Marc14857]

Working on my 2 cycle 3 HP Yanmar outboard. The motor starts easily, and can rev up to full RPM when in neutral. Once in gear however at full throttle under load the RPMs are significantly lower than they previously were and the power is reduced from what I remember. The motor responds to throttle control but again does not get up to full power under load. I tried to adjust the single mixture screw on the carb when under load however the screw had no effect either totally closed or backed out. The RPMs stayed effectively the same. (Yes I had the correct screw and not the idle screw). I checked the needle tip of the screw and there are no nicks etc. I am wondering how the engine can continue to run at higher than idle but not at full RPM with the throttle at max and the mixture screw totally seated.

Thanks
Marc

 

[jviss]

You might have an air leak somewhere. Is the carb tightened properly, with a good seal?

 

[MrBill_FLL]

compression?
prop ok?

 

[Marc14857]

No apparent air leakage. Prop is fine no abnormal vibrations. I’m really lost as to how the engine can continue to run with the mixture needle valve full in or out with no change in RPM. I would have expected to be able to lean out or richen the fuel mixture. Would a stuck float needle valve bypass the mixture valve somehow?

 

[jviss]

Maybe fuel pump, or a constriction in the fuel path - inline filters, etc. I have a tiller that runs fine no-load, but load it up and it stalls. Was a clogged filter on the tank flop line.

 

[justsomeguy]

Yanmar or Yamaha?

 

[Marc14857]

Sorry just saw Yanmar on initial post. Starting to hate autocorrect. This is a Yamaha outboard..

 

[gdudik]

Disassemble and clean the carb. There is probably junk in the carb restricting fuel passages, which is why you are unhappy under load.

Before I did this to my 5hp 2 stroke it would rev fine, but stall when I dropped it into gear unless the choke was halfway out.

 

[Rick D]

Maybe obvious, but what about fuel freshness? Otherwise, I'd try some Seafoam in the carb jets. Otherwise, I refuse to give carb advice since I have had two fails working on my old Jag's Strombergs. Good luck!

 

[Benny17441]

Sounds like a gummed up carburetor. Sometimes you can clean them sometimes replacing them is the sensible answer. I have cleaned a number of carburetors with chemical solvents and compressed air but them smallish carburetors sometimes seem to defy cleaning. Try cleaning it but if it does not work you could be in for a long and frustrating haul. I have learned to break the bank and replace them.