1999 Yamaha 115hp surging

Lyndal

Recruit
Joined
Oct 26, 2017
Messages
2
Yamaha 115 carbureted. One of the cabs was gummed up so took in to service both carbs. Mechanic also replaced in line filter. He lake tested it and said it was OK. I took it out and at low and medium speeds it's fine, but open it up to about 5000rpm and it starts surging. It will drop down to about 3000rpm, then up to about 4000rpm then back up to full power, then will lose power again. Mechanic replaced fuel pump. Still the same. I am bewildered. It was not doing this before I took it in to have the carbs serviced.
 

ClassicAQ

Petty Officer 3rd Class
Joined
Jun 2, 2017
Messages
78
I'd contact the mechanic and ask him what his carb cleaning included. You're hoping he says he pulled all of the jets from each carb, soaked, and cleaned out each's orifice so they are completely see through. If he did not do this, well then you got screwed because you've probably got one or more clogged high speed jets. Also, verify float height. Sounds like a fuel issue so I'd address all possible fuel related components before moving on to spark system.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
could be a restriction back to the tank or sucking air somewhere causing the float bowls to run out of fuel at high RPM
 

Lyndal

Recruit
Joined
Oct 26, 2017
Messages
2
Well it is finally fixed. After changing the fuel pump (not the problem) and some gaskets (not the problem) lake testing it using a portable fuel tank and hose (so the hose or tank was not the problem) I asked the mechanic to re-visit the carburetors, since it wasn't surging before the rebuild and it is surging after (granted, that is probably the first thing he should have done.) There is a float inside each of the carburetors (not visible from the outside) that were part of the re-build parts, but something was defective with them and they were sticking and restricting the fuel flow. The good news is Yamaha picked up the cost of all that since it was their defective parts. Thanks for all of your suggestions.
 
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