70hp yamaha on 22' suncatcher, prop issue?

Bigshannon56

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Apr 26, 2016
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Ok this is a lengthy story, but im going to give as much detail as i possibly can so maybe some of you more knowedgleable fellas can give me some insight as to what my problem is. Ok in 2008 i bought a brand new yamaha 70hp 2 stroke and had it mounted on an 88 model nitro fish and ski boat. Ive ran it every summer and never had a minutes trouble out of it. She starts every single time like clock work, and runs wide open jist like it should. Gps with by myself 41 mph going up stream on the river! Has a 13.5 x 15 prop on it and with no tach guage i never actually saw the exact rpms it ran. Having owned a few outboards, i could always tell it was prelly well wound up, but not to high. Compresseion is perfect. New plugs every 2 years and lower unit oil replaced every spring. But never any carb cleaning or anything major. We took it off and mounted it on a 2006 22' suncatcher pontoon boat that the previously had a 90hp yamaha on it so the controls were all there and our motor hooked right up. I drained the gas tank. Instaled a water/ fuel seperator, and filled it with fresh gas. In my drive way with the muffs on it starts and runs just as it always has. According to tach guage its idling at 700. And revs like it should. Goes into gear and throttles up like it should. We toook it to the river and all works well. Except for when opened up all the way im only getting 2200 rpms and 12 mph according to the guage. Im not sure if the tach is exactly right but to me it sounds like its half theottle and struggling when its wide open. Im convinced its a prop issue. Any feed back is much appreciated.
 

HotTommy

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The first thing I would check is that the throttles are really fully open when the handle on the remote if fully forward. Pull the case off the motor and remove anything that keeps you from seeing into the carbs. With the engine off, push the throttle handle to full open and see if the butterfly valves in the carbs are fully open.

I just ran your description through a propeller selector website and the result wasn't much different from what you have. I don't think the propeller is the main problem.

P.S.
You didn't say how long the engine sat before you noticed this problem. If it sat for more than six months AND the throttles are fully opening, my next step would be to clean the carbs.
 
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Bigshannon56

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I did have tto do some adjusting on the throttle and shifting cable at the turnbuckle. Also pulled the cover off the front to where you can see the butterfy i each carb and they are fully open with the throttle all the way down foreward. It has sat since october and has never done it before when it was on my nitro. I know things happen i just dontt knwnif i should try a new prop first, or have it serviced first. Its never had any carb service so i knownits due for one. Its also never given me any trouble so ive not had a reason to have the carbs cleaned. Maybe i should do more preventative maintence. Also a man at the local prop shop told me that until i put the proper propellar on it wont run right at wot. He reccomended a 13 7/8 x 11 cupped 3 blade pontoon specific propellar. He also says not to rule out any neede carb maintence.
 
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HotTommy

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I'm no expert, but I'd guess the difference between your prop and the other one might be 1,000 RPM, not 3,000. ... A typical carburetor has three fuel circuits - one for idle, another that adds more fuel at mid-range, and another that adds even more fuel at high power. It sounds like you idle circuit is ok, but your mid and/or full power circuits are clogged up. ... If you want more expert advice on the motor, I suggest you visit the Yamaha forum her on iboats and post your questions there. ... BTW, if the carbs do need cleaning, you will be running very lean at high power settings and that is very bad for your engine. .... Lastly, the man at the prop shop needs to sell you a prop. Fix the motor first and then see if you need a prop.
 
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Bigshannon56

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I was told 15 is way to much for the pontoon too. Its jist what the dealershipnput on it when i bought it and had it installed on the nitro. And simce its lighter and what not, i never had an issue with it. Another thing as i said before, ive never had a tach guahe on it. This pontoon has an OMC tach guage and it does work. But i didnt realoze till after some research that the poles have to be set right. So im going to check that out too to maybe get a more accurate rpm reading. It should be set on 6 poles right?
 

HotTommy

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OK, if the tach is wrong and you're actually turning (for example) something like 4400 RPM, then I'll go with the prop change too.
 

Bigshannon56

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Ok ive been tinkering with it this afternoon. The tach guage says its for 5 and 6 pole outboards. I took a pic of it. Today i just got to looking at everything. I may also have a fuel issue that could be part of the problem. I installed a fuel/wter seperator on this pontoon. Turns out that i can fill the filtersqueeze the bulb and she starts and runs for a bit then continues to run but a bit rough. If i hook my bulb and hose directly the the tanks built in nipple, it primes, tightens and runs more smoothly. So is it possible for it to get just enough gas to run but not enough to get to wot by the fuel water seperator loosing its prime when the throttle is all the way open?
 

ahicks

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Any kind of restriction will do that. Water separator, plugged fuel filter, or collapsed fuel line. Suggest you remove anything suspicious and see if you can blow through them easily. If so, fine. If not, they need to be replaced.
 

seldomseen

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Removing one spark plug wire at a time while at wide open throttle will show if all three cylinders are fireing.
 

Bigshannon56

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Apr 26, 2016
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Ok so heres an update. First of all let me say that when the motor was installed on this pontoon, the throttle cable was way out of adjustment. When i got it in the right place it didnt match up quite right with the timing lines on the timing arm. Since im not a yamaha mechanic and have not really ever researched my outboard, i really didnt know how they operated. The motor was set by the yamaha dealer when i bought it and i never have done much to it bedises plugs, lower unit oil and fresh gas and stabilzer since ive owned it. Ive also never had it do what its doing now. So after i realize things are out of sync, i got ahold of an online copy of the service manual for the 70hp 2 stroke yamaha and went thruough the link adjustment procedure for the link arms starting with checking to see if the timing plate matched the mark on the flywheeel with #1 cylinder out all the way. Before undoing anything i noticed that with the throttle all the way foreward, the timing indiciator was not quite all the way to the full advance timing mark. So i adjust everytjing to spec and we take it out this after noon. According to the tach, we got 2800 rmps wot and the engine seems to either ventilate or cavitate, im not sure. But its not wot. Its kinda like it would be when i had it on my nitro and it trimmed up to far. This rpm guage could be faulty. But even if it is, its not runing the rpms it used to. Another thing is this boat had a 90hp 4 stroke yamaha on it. When i put my 70 on it we just mounted it where the 90 was. I have two more holes i could drop it down. Or do i still need to try changing my prop? Or both? I honestly believe uts getting plenty of fuel although initially after installing the water seperator i had trouble geting the bulb to prime and keep it. Ine loose fitting later and i had that problem solved. Now it primes, and and pressurizes and fuel doesnt seem to be the priblem. I know its due for a service, carb cleaning, and what not. And if i cant figure this out, ill have no choice but to take it to a yamaha dealer and hope they can get it going right. Its gotta be something simple.
 

HotTommy

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Shannon,
Back when I flew for the Air Force, we pilots were expected to describe any problems with the airplane at the end of each flight. The mechanics taught me to not try and diagnose the problem in my description. All they wanted was an accurate description of the situation. For example, "Each time I put the gear down, the tachometer indicated zero RPM until the gear was fully down," not "Something is wrong with the tachometer." You've been presenting diagnosed problems that we later find were not diagnosed correctly. Do us and yourself a favor by offering more description and less diagnosis unless you are absolutely certain it is correct.

Here's what I take from your descriptions so far:
  • You took a good running 70 HP Yamaha that had sat for several months from another boat and put it on your 22' pontoon boat.
  • You hung it at the same height as the engine that was previously on the pontoon boat.
  • You changed some fuel fittings for the motor.
  • You had some difficulty fitting the control cables to the motor.
  • You suspect the tachometer is not reading correctly.
  • Your speedometer indicates 12 MPH at WOT.
  • A guy at the prop shop suggested a different prop would help.
  • You suspect the propeller is cavitating.
  • You suspect the engine is not running exactly right.
Assuming my summary is correct, here is my advice:
  • If you still have the old motor, measure the leg length (from transom mount height to cavitation plate) and see if it is the same as the new motor. If the new motor is shorter, that may be causing the cavitation problem. In any case, the cavitation plate on your engine should be several inches below the water.
  • If you don't want to take the boat to an experienced mechanic to fix the tach and tune the engine (i.e., adjust the timing linkage and clean the carbs), go to the Yamaha forum of iboats.com and ask for help. Be very open about what you DON'T know so they will provide the kind of detailed guidance you need.
  • After the engine is properly mounted and tuned, record your WOT speed and RPM and visit the Propeller forum here on iboats.com for help in selecting a new propeller.
Good luck.
 

ahicks

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2 thoughts. First, most tachs require they be set to receive the correct number of "pulses" from an engine. This number may change 90hp vs. 70, and require the tach setting to be changed. Second, a prop that is spinning on a loose hub can easily be mistaken for "cavitation". This can be easily check by marking the hub and prop with alignment marks. The boat is then run, allowed to "cavitate", and the marks checked. If they are no longer in alignment, you know the prop is junk or in need of service.
 

Bigshannon56

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Apr 26, 2016
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Hot tommy, you are exactly spot on with your description of my situation. When we bought the pontoon 3 weeks ago, the engine that was previously on it had already been taken off and was sold. So i cant compare it to mine. The man i bought the pontoon from is a used boat dealer, but is also very comparable to a used car salesman. Maybe a tad worse if you know what im sayin. Anyway, he suggested mounting it in the same spot the previois motor was hanging. So we did. At his shop before we left with the pontoon. We literally unbolted it from my nitro and bolted it up to the pontoon. With that being said i simply took his word on the heighth. Ok. So before we left there. All the connections were on the pontoon for the engine. Again it had a yamaha on the pontoon and the yamaha 704 controler. My nitro had a slightly different controller, but the connections were all the same except for the pontoon did have one square plug that would not plug into my 70 hp. Other than that plug everything else hooked right up and the motor started on the muffs and we left, so theres everything that initially happened on the install end of the deal.. Yesterday heres exactly what it done on the water..

Started and idled jist fine as usual and shos to be idling around 700

Goes into foreward and reverse no problem.

In foreward accelleration is good and the pontoon takes off and throttle continues to increase and about stops increasing after about 3/4 theottle and today maxed out according to the tach at 2800 rpms. A few more than the last time we had it out. From that to full is no difference. Also you can feel the boat sort of lunging foreward. Rpm guage doesnt correspond with the lunge feeling and sound, but its very much noticible. Also the controling of the boat is very unstable at this speed. If you turn sloghtly it really tries to get away fro you. Trimming the motor has no effect.
 
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ron nh

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Sep 19, 2014
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You just have to bite the bullet and get a mechanic to drive the boat, It will take him all of 2 minutes to tell you whats wrong. Instead of all the guessing.
 

Silver Eagle

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Mar 16, 2010
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I suppose the ski boat was a bow rider or at least a v bottom boat. and I right? now you put that motor on a pontoon boat. They don't sit in the water the same .What's the weight difference between the two boats?
 

Bigshannon56

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Apr 26, 2016
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The boat i had it this motor on was fiberglass and a bit v shaped. Yes. And im not sure if its weight, id say less than 1000 pounds. So it was extremely light! According to the title to the pontoon, it weighs 3500#s. I took a look at my tachometer. Its an older omc tach. Says its for 5 or 6 pulse alternators. And its set on 6. Is this right or should it be set to another selection? It has a round black dial to turn to whatever number a person wants it on. Whats the difference between pulse and pole?
 
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May 6, 2016
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I just experienced the same thing.. Has a 40hp Mercury and replaced it with a 90hp Mercury.. Was told that I needed to adjust the tach as it was set for another motor.. AND when I went to get a prop, I was told to make sure that I explained that it was for a PONTOON boat.. Regular boats get on a plain when running.. Where a pontoons PLOW. I needed a different prop that the one that was on the 90HP. It was off a V bottom hull.. Just a possibility.. My tach was way off til I adjusted it and when I got a different prop, It worked perfect!! Just throwing a possibility out there..
 

Tjpontoon

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May 14, 2016
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Did you get your problem fixed? I have a 09 tritoon 24ft with 90hp 4stroke. Having close to same issue boat runs 4000 rpm at half throttle and wants to start over rev if giving more throttle. It has a 13.25 13 pitch prop on it came with boat when I got it. I was going to try a 11 pitch prop. Was wondering if you had tried one yet.
 
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