Fuel mixture WOT 1986 Yamaha 30 hp 2 stroke

[formula88]

I have an older, well cared for Yamaha 30 hp engine that is giving me trouble at wide open throttle. It starts fine, idles, fine, picks up with some hesitation, but then never really winds up at full throttle. From looking at the plugs I am thinking that it is running too rich, but I don't believe there is any adjustment for mixture at the high end.

Odd behavior: If I pinch the fuel line for 15 seconds or so when running at WOT the engine finally kicks into full power and revs up. Of course, if I keep pinching it cuts off all the fuel and the engine dies. But if I release a little bit it will continue to run a full power.

I thought it might be the float in the carb, too high or too low, blocking the needle inlet valve. Could it be that the main jet is just too big?

Background. Carb has been cleaned and all replaceable parts replaced a few years ago. The main jet is the correct size, 135. All fuel lines have been replaced with ethanol-resistant tubing. New membranes in fuel pump. New gas. New CDI.

Any ideas would be appreciated.

 

[boscoe99]

The idle mixture setting also has an affect on the amount of gasoline that it provided through the carburetor at WOT.

Fuel level being too high can cause the carburetor to provide too much gasoline.

Someone might have augered out the existing main mixture jet.

 

[robert graham]

Maybe check your choke/fuel enrichening device for proper operation…could be not opening completely after engine is warmed up causing the rich mixture/low power…

 

[formula88]

Maybe check your choke/fuel enrichening device for proper operation…could be not opening completely after engine is warmed up causing the rich mixture/low power…

Choke seems to be working correctly. At first I had tried raising the float level, with the idea that at normal height it would allow more gas in through the needle valve. Now I have lowered the level of the float. I will try that out in the next day or two and report.

 

[Sea Rider]

If need to re prime the bulb for the motor to spring back to full life while at speed, check the fuel pump's diaphragms, probably hardened, shot ?

Happy Boating

 

[formula88]

Someone might have augered out the existing main mixture jet.

I guess my memory isn't what it used to be. A few years ago when I put in all new fuel lines and new diaphram in the fuel pump, I also got a carburetor rebuild kit. I thought for sure that the main jet was from that recent rebuild. When I checked the jet with some gauges - it was oversize by about 25%. Somebody (me!) had drilled it out trying to fix fuel issues long ago. I didn't have a new jet available right away, so I rigged a piece of wire to block some of the opening. Back on the water it was like night and day. I now have 30 hp again instead of about 15, along with pickup in the mid range.

So, fuel is good. But too much fuel is not so good.

 

[99yam40]

back when I was fighting an over rich condition at high rpms on my C40, I never found any info on measuring the size of the jet holes.
so I bought a new one and measured it and my old ones with different drill bits to find out someone drilled out the main jets