Need help diagnosing an engine problem please.

[Kenai In July]

'06 Yamaha F50 4 stroke.

Starts and runs fine at low to mid rpm/throttle. In the video you can hear/see there is a problem when I reach about 2/3 throttle - the engine starts missing.

Motor was fogged and fuel stabilized for winterization in October. Spark plugs are fine. I have an inboard gas tank and above deck/external gas tank setup. I put fresh fuel in the external tank. October 2010 gas in the inboard tank, but I added stabil. Tried every combination of tanks - inboard tank only, external tank only (with brand new fuel hose), and both hooked up at same time. It's not water in the fuel or a problem with filters.

I am guessing it is a weak fuel pump? Anyone else seen this before? Thanks!

 

[MacGyver]

Sparkplugs are cheep replace them. Yes i know there ok.

 

[jrogers]

Did you drain the float bowl? On a small motor like this it would take a while to go through what is in the carb, and this is an easy way to get fresh fuel in there. I wonder how you are sure it is not some water in the carb.

If this is not the case, then it could be a plugged jet. Carbs run the fuel through different jets depending on where they are at in the range, such as the idle jet and the main jet, so it could be as simple as some junk in the main jet.

 

[jrogers]

Another thing, running it with no load at higher throttle settings may be overreving the motor. I would do your testing under load (in the water) to be sure you really have a problem. It may be as simple as you are hitting a rev limiter in the motor which is keeping you from blowing it up.

 

[Dupont Spinner]

Sounds like the rev limiter to me and watching the exhaust makes me think this also. It doesn't neccessarily need to be wide open throttle to reach the upper limits.

 

[BigSuBoater]

Air

Air

I had that happen once and the ventalation/breather tube for my fuel tank was partially clogged, when I hit the throttle all the way it just couldn't couldn't breathe. Maybe loosen the gas cap and try? Just a thought

 

[Kenai In July]

Thanks for the replies. I will try it on the water to see what happens. I was told there was not a rev limiter on this engine, but I am sure hoping that's all it is!

 

[MARV1]

Doesn't take much throttle to get to 6-6300 rpm and hit the rev limit on outboards with no load like being run out of the water and cooling...

 

[Dupont Spinner]

Thanks for the replies. I will try it on the water to see what happens. I was told there was not a rev limiter on this engine, but I am sure hoping that's all it is!

Most engines with electronic ignitions all have a rev limiter. Even my little '02 15 hp 2 stroker has a rev limiter on it.

You can do a quick test. Get it in a barrel of water you will get about 10 seconds before it blows all the water out or find someone that will let you put it in there tank.

 

[Kenai In July]

On the water I was full throttle with no problems, so I am thinking it might just be a rev limiter. My tilt/trim motor quit on me however, so I wasn't able to trim the motor to get max performance, so there is still a small chance there is still a problem. $700 for a new trim motor - I took mine apart and cleaned it (should have taken photos - it was a mess!) and it works great again. Think I'm ready to fish! Thanks again for the replies!