1993 Yamaha 40 C40MSHR Bottom carb starving for fuel intermitently.

SparkieBoat

Captain
Joined
Aug 17, 2009
Messages
3,643
I have a 1993 40 HP yamaha outboard. C40MSHR. I have repeatedly cleaned both carbs and paid particular attention to my bottom carbs needle valve assembly and passage. When I put it all back together and take it to the river it runs great for a while then begins to run on 1 cylinder. I pulled the plug wires and when I pull the bottom one there is no change when it is acting up. so it must be my bottom carb. It seems like it is starving for fuel. but when I get home and take it apart the valve does not seem to be stuck. sometimes I can pull the choke a few times while it is running and it may go back to running perfectly again. I cannot find any signs of trash in the fuel, fresh fuel in a clean can. I cannot understand what is going on. Any help appreciated.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
water or trash is getting into the carb and plugging up a passage. When you choke it ,it may suck it on through and clear up for awhile until something else does it again.

I have had stuff from the lines or pump hit my carbs after cleaning, looked like a piece of film that would get sucked up to the main jet and plug it off

Maybe the pump is leaking and letting fuel into the crankcase, causing the bottom cylinder to be too rich to fire properly.
But the choke would not fix that.
 
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