Newer Yamaha 50 HP 4-stroke question

[outbdnut2]

A relative of mine has a 2004 50 HP 4-stroke Yamaha he bought new and it has always ran great until now. I’m going to take a look at it in a few days and thought I’d ask if anyone knows of any issues this model is known for that could cause it to not rev up all the way. He said his boat had always done 24 MPH and now he only gets 17 MPH.

I suspect he’s only running on 3 of the 4 cylinders, so I’ll start with a spark plug inspection to see if it’s obvious one is fouled, wet, or other signs of not firing; and do a spark check, and determine if it is runnin’ on 3 like I suspect. It also has 4 carbs – so could be a problem with one of those.

Any insight/experience on this model before I go look at it is appreciated.
Thx,
Dave

[billw]

With Yamaha, the problem is fuel related 99.99 percent of the time. We are just waiting for the carb motors to go away. They are horrendously sensitive to dirt and ethanol related crapola. They are also pretty complicated to get on and off the motor. (Hint: remove the entire intake manifold and carbs and hold in a vise.)

Long live American manufacturing!

[outbdnut2]

Bill,

Thanks for the insight about fuel!

I’m hoping it’s a simple spark plug problem. I put new ones in for him last fall before storage – maybe got a bad one. I’ve visually looked at the carbs and manifold in the past, and yes – it looks like a terrible thing to disassemble! If it’s carb-related I may tell him to take it to a shop, and I’m a guy that’s never been afraid of working on any carb. The dealer he bought it from only fixes motors up to 10 years old now – that sucks too!
Dave

• This reply was modified 3 years, 10 months ago by outbdnut2.

[billw]

I have to admit that I have seen an NGK plug “go bad” or at least, terminally foul, every once in a great while; so it is worth a shot…but it’s quite rare.

Those carbs are also difficult to trouble shoot. With a two stroke, if you have a lean carb, you can just take a pump oil can full of two-stroke mix and shoot it down the throat of each carb. If the speed picks up, there’s your lean carb, and proof that a carb job is justified. That trick is nearly impossible to do on the Japanese four strokes.

Long live American manufacturing!

[fleetwin]

Wish I could help but have zero experience. I have also heard that these four stroke carbs are complicated and get plugged easily though. Try not to dig too deep, the problem is most likely something simple, use basic common sense inspection techniques that would apply to a two stroke multi carb outboard.
Bill: Why is it that you can’t use the “oil can injection technique” on these four strokes? Do they have some sort of complicated intake that won’t allow access to each individual carburetor throat?

[outbdnut2]

Yes – access to those carbs is not service-friendly. Getting that motor ready for winter the first time, I was going to use some fogging spray since being a 4-stroke, there was no oil in the gas to leave any coating in the cylinders. It was impossible to spray it into the carbs with that intake shroud vertically over them all, having the intake air opening at the bottom, hanging almost to the floor of the power-head compartment, and no obvious, easy way to remove it. I maybe could have fogged if I had a u-shaped tube on the fogging spray.

If I can’t find a simple solution, and/or it looks like a carb problem, I’m going to tell him to take it in somewhere because I don’t want to be “learning” on his motor. I’ll be looking at it on Thursday and let you all know what I find.
Dave

• This reply was modified 3 years, 10 months ago by outbdnut2.

[dave-bernard]

for winter fogging just run some 25-1 oil mx at the end should be fine.

[billw]

For our big EFI engines, we use a recommended quart of TCW3 to three gallons of gasoline for storage mix, with some stabilizer….What’s that? 8:1? We drain the VSTs down and make sure the mixture gets all the way through the motor. The plugs get a little fouled but there is no noticeable smoke. It is kind of amazing.

Long live American manufacturing!

[outbdnut2]

A quart to 3 gallons is 12 to 1. I could probably use the 8 to 1 mix I run in my PO-15 Johnson.
Dave

[fleetwin]

Do these carbs have chokes? If so, I have another idea to find a lean carb at high speed, but it will require an assistant. Disconnect the choke linkages from each other so each choke valve can be closed independently. Have the assistant drive the boat while you manipulate the chokes paritally closed one at a time…. When you choke a normally functioning cylinder, you should lose RPM. RPM will pick up on a cylinder that is lean. If you get no change in RPM, that cylinder is dead for some reason…Perhaps no fuel is getting to that carb bowl…..Perhaps the cylinder has no spark, or some internal problem.
Hopefully this engine is on a boat where you can get near it while running at higher RPMs….

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