Newer Yamaha 50 HP 4-stroke question

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    outbdnut2

    US Member - 2 Years
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    #205032

    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

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    billw

    US Member - 2 Years
    Replies: 1219
    Topics: 39
    #205039

    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!

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    outbdnut2

    US Member - 2 Years
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    #205047

    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 1 month, 1 week ago by Avataroutbdnut2.
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    billw

    US Member - 2 Years
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    Topics: 39
    #205062

    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!

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    fleetwin

    US Member - 2 Years
    Replies: 3168
    Topics: 48
    #205066

    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?

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    outbdnut2

    US Member - 2 Years
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    #205069

    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 1 month ago by Avataroutbdnut2.
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    dave-bernard

    US Member - 2 Years
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    #205097

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

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    billw

    US Member - 2 Years
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    #205102

    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!

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    outbdnut2

    US Member - 2 Years
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    Topics: 65
    #205103

    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

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    fleetwin

    US Member - 2 Years
    Replies: 3168
    Topics: 48
    #205104

    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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    outbdnut2

    US Member - 2 Years
    Replies: 1254
    Topics: 65
    #205116

    No chokes. It has an electric primer – part #34 in link below. Also – I must be remembering it wrong as a 4 cylinder. Parts list shows 3 cylinders, 3 carbs, so maybe it’s runnin’ on 2 of the 3 cylinders. I notice each carb has a different part number – interesting!.

    Here’s a link to exploded view of carb parts: https://shop.yamahaonlineparts.com/OEMpartfinder.htm#/Yamaha_Outboard/50TLRC_-_2004/CARBURETOR/ca86f2bb-ecd3-4be4-8622-337eac184d69/8f531dd5-ab24-423b-acdf-10d1574f6de4/y

    I been thinking about it more and If a cylinder is dead, I can find the dead one by starting it with different spark plug wires disconnected and see which one makes no difference in how it runs at idle. I’ll be doing this out of water with garden hose clamped to cooling intake. I may also notice that one spark plug stays cold. When removed, one plug may be wet. I also have a seldom used in-line neon spark flasher/indicator I can put between the spark plug and its wire. If the sun isn’t too bright, I will see the neon bulb flash if there is spark voltage.

    I also plan on a look at the throttle linkage to see if some adjustable part (if any) has come loose, preventing carbs and/or spark advance from going to max., or maybe one carb has throttle disconnected, if that’s possible.

    It’s a tiller motor, so easy to advance throttle and see what moves….to a point – the intake cowl on the carbs blocks view of some stuff. I seem to remember it has cables going in from the tiller – maybe something slipped inthe throttle cable causing it to not go to WOT.

    If I start it with a spark plug wire off, do I have to ground it to keep ignition parts from self-destructing?

    Dave

    • This reply was modified 1 month ago by Avataroutbdnut2.
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    billw

    US Member - 2 Years
    Replies: 1219
    Topics: 39
    #205120

    Grounding the spark plug leads or at least through some kind of spark checker is highly recommended. As they always say, electricity is lazy. If there is too big a gap for the spark to jump, it will eventually find an easier path through the insulation in the coil winding, someplace. Japanese ignition parts are absolutely as good as it gets, but nothing is THAT good, for long.

    Does the owner report that it stalls at low speeds? If not, I am thinking your garden hose test may not show anything but three running cylinders…..

    Very wise move to observe the throttle action before doing anything as rash as a carb job on that thing.

    Don’t forget that the “fuel system” includes the tank and line! One time, I saw a Yamaha that idled well and would run mid-range just fine; but it wouldn’t go fast. The owner had attached the fuel line to the tank backwards. At low speed, there was enough fuel getting by the backwards primer ball valves to keep running! I have also seen many plastic tanks, that have a plastic withdrawal tube. The ethanol damages the tube, which causes air to leak into the system at the top of the tube. That could definitely cause high speed problems. remember that trouble shooting is a matter of “divide and conquer.” It would be best to take a ride with an entirely different, known-good tank and line and see if he gets a different result in speed.

    Long live American manufacturing!

    • This reply was modified 1 month ago by Avatarbillw.
    • This reply was modified 1 month ago by Avatarbillw.
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    outbdnut2

    US Member - 2 Years
    Replies: 1254
    Topics: 65
    #205125

    Thanks for the tank reminder. His fuel tank is factory built-in under the floor of his 16′ Lund fishing boat so no chance of the fuel line being reversed, but, good idea – I could take the tank from my 25 HP Yamaha 4-stroke on my pontoon and try it. He hasn’t mentioned stalling at low speeds. He does have a lake one mile from his house, so we could drop it in so I can see how it runs at high throttle if I can’t figure it out in his driveway. He uses 10% ethanol gas due to limited availability of ethanol-free where he lives, so that could have created a problem over its 16 years..

    I once cobbled in my Yamaha tank to the MerCruiser inboad/outbaord engine in my ski boat when it would bog at sustained high speeds, and it ran fine on the Yamaha tank. I found the anti-syphon check valve was bad with it’s spring broken and jammed up. A couple weeks later, same symptom, and ran on the temporary tank OK. This time it was the pickup screen in the built-in tank clogged with crud that somehow got in there.
    Dave

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    outbdnut2

    US Member - 2 Years
    Replies: 1254
    Topics: 65
    #205698

    I took a look at this motor and found #2 spark plug was somewhat wet with gas after having not been run for about 10 days. The other three plugs looked good. All throttle/carb linkage checked out OK. I replaced the wet plug with an old plug that looked good. Before putting it in, I grounded it and cranked and it had healthy spark in its gap (I didn’t have a way with me of looking for a longer spark in free air). It seemed to idle OK , but this was all I could do in the driveway. The next day, my friend took it to a lake and it still would only go about 3/4 speed. Based on all this, I suspect #2 cylinder’s carb has a problem. He dropped it off at a dealer Saturday. I’ll let you know when I hear what they find. I’ve never been afraid of carbs on cars or motors, but my friend is a particular guy and I didn’t want to be learning these carbs on his motor.
    Dave

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    billw

    US Member - 2 Years
    Replies: 1219
    Topics: 39
    #205709

    If he is a particular guy, I would have him get a newer engine with EFI. MUCH more reliable than a carb 4-stroke. Carbs are going to be nothing but problems for him and costly to have repaired professionally.

    Long live American manufacturing!

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