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  1. #1
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    1998 Yamaha V4 2 stroke problems. I'm stumped and need help.

    Long story but I'll try to hit the highlights with all the pertinent info.

    Bought a boat last fall. It's got a 1998 Yamaha 115 V4 2 stroke on it. The boat is an Xpress H56 with Hyper lift hull. I took a boat mechanic with me to check out the motor before buying. He noticed that the prop shaft was slightly bent. Ended up buying the boat, the mechanic had a machine shop cold straighten the shaft. I put a 20P Laser II prop on this spring to help with bow lift as I added on to the front deck this winter before recarpetting. Boat ran great, then about 2-3 weeks ago the lower started ticking. Got worse and got loud. I removed the bottom plug and there was metal all over the plug. The oil looked like metallic paint. As luck would have it, the mechanic who I paid nicely, in cash last fall to check and go through my lower, dismissed having anything to do with the lower unit failure, without so much as the courtesy of looking at it. He diagnosed his lack of responsibilty for the problem over a text message.

    So I order an SEI lower and install. I also put on a 6" jack plate. I did the 5 hour break in exactly as described. During this period of running, I worked up to 2500 RPM's and while running at that speed the motor started spitting and sputtering. I thought I had sucked something into a carb, or thought that maybe water got through my water seperating filter. After a few minutes of spitting and sputtering, everything levels back out. I do the 6-10 hour break in exactly as described, the motor runs fine. When the time comes to finally give her full throttle I did and the motor was VERY sluggish. As in it took upwards of 45 seconds - 1 minute to get on plane and the top end was turning around 4200 RPM's.

    Since the motor spit and sputtered, I assume the loss of power is a carb issue. I removed both carbs, tore them down and cleaned them. I removed every jet that had a screw slot across it, cleaned them and reinstalled. The floats looked good as did the needles. I decided since the motor was 20 years old I would replace all the fuel lines from the motor to the carbs, including a new OEM primer bulb and also installed a new OEM fuel pump.

    I took her back out and it did the same exact thing. I ran the jack plate up to the max and it did the exact same thing. I removed the jack plate, put the motor back on in exactly the same hole position as it was in prior to the lower replacement. The motor did the exact same thing. A buddy of mine suggested that I try a 16 pitch prop to get my RPM's back up so I did.

    The motor jumped up on plane and topped out at 5400 RPM's, but I lost 8 mph. Dropped from 45-37. The motor sounds great.

    I decided that the new SEI lower must be the wrong gear ratio. I checked my old lower and it's a 2:1. SEI only lists a 2:1 for my motor so I dropped it this afternoon to check it. I checked and double checked and it's indeed a 2:1 as well.

    So, that means that I've lost power, somewhere, somehow, right?

    Compression on all 4 cylinders is 120-122. I tested spark with a gap tester this afternoon set on 7/16" and all 4 are jumping that gap well. The fuel system is spotless with all new lines. The idle timing is spot on. I have not checked the WOT timing yet but will. There is no fuel being spit out the front of the carbs so I think the reed valves are okay.

    I am at a loss here. Granted I'm not a boat mechanic but I'm a fairly mechanical guy. I don't have all the testers and such, and I've scheduled an appointment at a reputable Yamaha mechanic shop, but that's over 2 weeks from now.

    Does anyone have an idea what the cause of this could be or what else I can check? This sounds like one of those $1000 "we can't find an issue" problems on the horizon to me.

    But, a new but same gear ratio lower unit on the same motor, with the same prop, should turn the same RPM's and run the same speed, correct? What am I overlooking?

  2. Member
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    #2
    I have a 94 model and went through same thing carbs and fuel system cleaned. Turned out to be a power pack going out. When it acts up tap on it and see if anything changes. That's what my mechanic did to identify prob. Seemed like a fuel prob but wasn't. You are describing exactly what mine was doing. Make sure to buy OEM. I learned the hard way on several cheaper parts on my enfine

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    #3
    Oh and it's the little black box with all the small wires coming out of it. I would bet it is the prob. Mine would go a week or two between issues but got worse quickly. Hope this helps because it drove me crazy

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    #4
    Thanks for the reply. It doesn't really act up, it just has no power with the 20 pitch prop. Not out of the hole, mid range or top end. With the 17 pitch prop it runs great. Good hole shot, good top end RPM's, just slow due to the smaller prop. It's a little tough to crank first time of the day but it idles fine, no missing, and sounds smooth on top end, but like I said no power.

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    #5
    I had similar symptoms with mercury black max 150. It took for ever to get on plane . I changed the prop it ran great . Put the old one back on had the same symptoms. Long story short it was the hub bushing in my prop . Soon as I changed it problem solved Just an idea to check before taking it it

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    #6
    It's running like it's down one cylinder.
    You cleaned the carbs but didn't install new plugs after that long period of low RPM running? It takes very little to foul or carbon track a plug, and it's often not visible.
    I'd start there.
    Changing the prop pitch on a previously good running motor to "fix it"? No, unless the LU was a different ratio. The ultimate "masking" of a problem.

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    #7
    I'm not a prop expert but a spun prop should give me more RPM's and less speed. I'm getting lower RPM's and less speed.

    That's why I took the new lower off to verify the gear ratio by turning the prop and counting the revs on the driveshaft as the prop revs were counted.

    I'll put new plugs in today and do more checking. I do know for sure that all 4 cylinders are firing. With good compression on a cylinder and fire to the plug, how could it be down a cylinder, unless it's not getting gas to that cylinder? That's not meant to sound sarcastic. Sincere question that I don't know the answer to.

    Thanks for the ideas. I don't mean to sound like I'm dismissing any of them. I'm not, so if anyone has any ideas of what to check, please let me know.

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    #8
    Have you checked that you are obtaining full throttle at the carbs?

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    #9
    Quote Originally Posted by PSM207 View Post
    Have you checked that you are obtaining full throttle at the carbs?
    Yes, externally and internally. Everything is opening wide open.

    I removed the flywheel this morning thinking it could be a sheared woodruff key but that was not an issue. That all looked great.

    I've got new plugs to install after lunch, then I'm about out of ideas.

    If I can cypher through the Clymer manual and try to test the CDI box but I don't know that I'm sufficiently technical enough with wiring to do that.

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    #10
    Quote Originally Posted by FurFlyin View Post
    I'm not a prop expert but a spun prop should give me more RPM's and less speed. I'm getting lower RPM's and less speed.

    That's why I took the new lower off to verify the gear ratio by turning the prop and counting the revs on the driveshaft as the prop revs were counted.

    I'll put new plugs in today and do more checking. I do know for sure that all 4 cylinders are firing. With good compression on a cylinder and fire to the plug, how could it be down a cylinder, unless it's not getting gas to that cylinder? That's not meant to sound sarcastic. Sincere question that I don't know the answer to.

    Thanks for the ideas. I don't mean to sound like I'm dismissing any of them. I'm not, so if anyone has any ideas of what to check, please let me know.
    Fire to the plug is not fire at the plug.
    I would be putting a set of plugs in it, if it were my motor.

  11. Member
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    #11
    Very true.

    I pulled the plugs to do a leak down test on each cylinder and they were all wet and black. I know that's causing some of the issues. When I positioned the flywheel at TDC1, I could see that the top of #1 piston was wet. None of the others are wet. So that's pointing to an electrical issue, unless the plug was so gunked up it couldn't fire. I've got some tune up in a can soaking in the warm engine now and in a few minutes I'm going to go run it out and change the plugs again.

    I'll run it a bit and pull the plugs and see if I can figure out anything. I'll post my findings.

    BTW, the leak down test was very good on all cylinders.

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    #12
    Quote Originally Posted by FurFlyin View Post
    Very true.

    I pulled the plugs to do a leak down test on each cylinder and they were all wet and black. I know that's causing some of the issues. When I positioned the flywheel at TDC1, I could see that the top of #1 piston was wet. None of the others are wet. So that's pointing to an electrical issue, unless the plug was so gunked up it couldn't fire. I've got some tune up in a can soaking in the warm engine now and in a few minutes I'm going to go run it out and change the plugs again.

    I'll run it a bit and pull the plugs and see if I can figure out anything. I'll post my findings.

    BTW, the leak down test was very good on all cylinders.
    You had fire at every plug wire as proven by the spark tester.
    It runs like it's down/not running on one cylinder, as it's low on power, but is not misfiring.
    The down cylinder is wet.
    ARE YOU SURE you don't want to at least put a new plug in the cylinder that's not running?

    What does everyone else think?

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    #13
    If you notice in what I wrote earlier, I said after doing the decarb I was going to change the plugs again.

    After doing the decarb the plugs were shiny and looked new again, but I still changed them. I really think that the oil buildup in the cylinders was the problem the whole time.

    I took it out late this evening, but I didn't put the larger prop on. When I had it on the water last, with that 16 pitch prop all it would do was 5400 rpms. Coming out of the hole it would hit around 3000 then climb to 5400 as it planed out. I don't bury the throttle when I start out, I go to about 2/3 throttle. When I did that today I could immediately tell the motor was turning more RPMs just by the sound. By the time I looked down I was turning 5000 RPM's and hadn't broke over yet. As soon as the nose set down I was running 6000 RPM's and I backed off the throttle. I never did open it up all the way because I would have overreved with the small prop. I could run 5800 RPM's on plane with what I'd estimate to be 2/3 throttle.

    If it's fixed, it ended up being something very, very simple that a real marine mechanic would have caught in 5 minutes. If it's not fixed, then it's got to be a firing problem. Or at least I think that's what it would mean.

    If I have time after church tomorrow, I'll put the larger prop back on and try it out again.

    If nothing else, I've learned a whole lot about 2 stroke outboards in the process I've gone through.

    I would never have guessed that idling around for several hours would have loaded that motor up with so much oily residue. When I did the decarbonizing this afternoon, the water in the drum that I was running the motor in, turned jet black. There's a solid coating of oil on top of the water now.

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    #14
    Quote Originally Posted by FurFlyin View Post
    If you notice in what I wrote earlier, I said after doing the decarb I was going to change the plugs again.

    After doing the decarb the plugs were shiny and looked new again, but I still changed them. I really think that the oil buildup in the cylinders was the problem the whole time.

    I took it out late this evening, but I didn't put the larger prop on. When I had it on the water last, with that 16 pitch prop all it would do was 5400 rpms. Coming out of the hole it would hit around 3000 then climb to 5400 as it planed out. I don't bury the throttle when I start out, I go to about 2/3 throttle. When I did that today I could immediately tell the motor was turning more RPMs just by the sound. By the time I looked down I was turning 5000 RPM's and hadn't broke over yet. As soon as the nose set down I was running 6000 RPM's and I backed off the throttle. I never did open it up all the way because I would have overreved with the small prop. I could run 5800 RPM's on plane with what I'd estimate to be 2/3 throttle.

    If it's fixed, it ended up being something very, very simple that a real marine mechanic would have caught in 5 minutes. If it's not fixed, then it's got to be a firing problem. Or at least I think that's what it would mean.

    If I have time after church tomorrow, I'll put the larger prop back on and try it out again.

    If nothing else, I've learned a whole lot about 2 stroke outboards in the process I've gone through.

    I would never have guessed that idling around for several hours would have loaded that motor up with so much oily residue. When I did the decarbonizing this afternoon, the water in the drum that I was running the motor in, turned jet black. There's a solid coating of oil on top of the water now.
    It loaded up and fouled a plug.
    New plugs fixed it.
    The crankcase is still loaded up with fuel and oil, so it now needs to run at least a mile or two at WOT to clean it out.
    That was a long, drawn out process for a dead plug. At least now you can go fishing.
    Last edited by Nova Kaw 650; 05-20-2018 at 06:38 AM.

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    #15
    Great news....I look forward to update with the larger prop.
    I just raised my motor on transom to bottom hole (4.5" PTP) and saw improvement.
    Keep us posted on your results!
    1998 Xpress H51, 90hp Yamaha, 6" Vance JP, MotorGuide Xi3, Lowrance Electronics, Micro Power Pole...

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    #16
    We didn't have church yesterday evening so I put the 20 pitch prop back on and ran the boat. It runs great, again. I do think I need to drop down to a 19 pitch prop because it turns 5400 RPM's at WOT. Now I'm going to put the jack plate back on and go through the process of setting that again.

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    #17

    You'll gain RPM's by raising the jackplate.

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    #18
    Quote Originally Posted by Nova Kaw 650 View Post

    You'll gain RPM's by raising the jackplate.
    Maybe so. I've currently got the motor in it's highest hole and it's high enough that it's trying to blow out while turning, so I may not, but Good Lord willing, I'll find out this afternoon. Today is an abnormal Monday and things are pretty quiet here at the shop. I've got my guys started on their daily projects and I have the boat backed up under the overhead crane. I'm about to pull the motor and put the jack plate back on.

    Despite the ignorance of my actions, I'm a very mechanical guy. I'm a metal fabricator and semi trailer reman guy by trade, but as previously noted, I don't know jack crap about 2 strokes. I'm learning though!

    In my defense, diesel engines don't have spark plugs! LOL