24' Pontoon 85 HP Yamaha C85TLRP Prop Question

chadwt75

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Apr 20, 2015
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I recently bought a 1991 sweetwater 24' pontoon boat with a yamaha 1991 85HP motor. I took it to the lake and after half-trottle it would not gain rpm, then after 5 min the heat alarm would go off.. I went through everything on the motor and it "seemed" ok. Then I looked at the prop and noticed it was a 13.5 x 16. I dont have a tach to read the rpm's. Does that seem like a prop problem? Seems logical that the motor cant pull the prop therefore it gets some rpms then cant pull the high pitch prop. Any advice would be nice! Or do I have a motor problem? Btw its a 3 blade prop.
 
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HotTommy

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chadwt75,
First, know that there are forums here on iboats for both prop questions and motor specific questions. You might get better answers on one of them. .... As for your question, my GUESS is that the heat alarm is not associated with the prop. As long as the cooling system is working adequately, it shouldn't matter what speed you are going. When you say you went through everything and it seemed ok, does that mean you examined the water pump? Tested the thermostat? Etc.? ..... If this becomes a motor discussion, you'd really be better off over on the Yamaha forum.
 
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chadwt75

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HotTommy

First of all thanks for replying. All I'm saying is my boat doesn't rev over half thottle. New impeller and thermostat and carb kit.
 

ahicks

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Sep 16, 2013
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Well, there ARE a ton of variables, but that does sound like a lot of pitch, maybe too much prop for a pontoon boat. They're usually down closer to 11 -13 pitch or smaller.
 

1983 ercoa 21'

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Jan 12, 2014
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Find a local prop shop they will know what gear ratio your lower unit has and will be able to recommend a prop . I agree with above it should be in the 11 - 13 range. Lugging that motor will cause heat problems.
 

lmuss53

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My 20 foot boat, when I was running a 90 hp Johnson worked best with a 13 pitch prop, When the pitch was too steep at 17 or even 19 it would lug for a minute but always eventually blow out the prop and overrev.

I think you do have an engine issue, possibly a fuel delivery issue, or a bad coil.It sounds to me like you are running on 2 cylinders. You rebuilt the carbs, did you check the fuel pump?

I also think you might get better results if you ask the mods to move your post to the Yamaha page.

Do not continue to run the motor without resolving this issue, that is a recipe for disaster. Before I knew any better I blew up an 85 Evinrude on a 24 foot pontoon by running it with too much prop for just a few minutes, It leaned out the motor, dried out a cylinder wall and locked up. I had pulled the motor off a 17 foot runabout and put it on the pontoon. We took it to the lake and put it in to see how fast it would go, 15 minutes later we were getting towed in and the motor needed a powerhead.

Another question, do you know the history of this boat? Was it running correctly with this motor prop combination for the previous owner?
 

chadwt75

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Ok I bought a carb kit then when i pulled them apart they looked like new, so I checked the float height on them and all was good. I checked the pick up tube in the tank, it was clean as can be. I bought a new attwood fuel line and bulb and conectors. Fuel pump looks band new. I put new spark plugs in too. As for the previous owner I think they were fighting this same problem because they obviously spent some time and money doing fuel system work. As for the coils I have not been able to get the correct readings the manual says (ohms) but they are all equal. I thought maybe my timing advance wasnt working properly but havent checked that either my timing light is not here at the moment. Compression is good too.So either ignition or prop? Any idea the best place to get a prop, I found one on ebay for 69.00 but if its not the right pitch I'm not sure if I can return it. Also I need a tach but the one from Sim Yamaha was like 225.00


How do I get the discussion moved? This is my first time to ever post to a forum.

P.S. thank y'all for taking the time to answer my question.
 

lmuss53

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I'm broke and cheap so I buy lots of stuff on ebay.

I would go here on iboats and find a part number for a 13 pitch prop to fit your motor and then search ebay and craigslist to find a good used one. I wouldn't count on returning one because it isn't the right pitch, so shop carefully.

The tach can wait, but they are sort of universal and ebay is a good place for them too. If you get the prop right you can just about hear that the engine is running in the right rpm range.

Because a fuel pump looks ok doesn't mean it is. Try pumping the ball while the motor is acting up, if it surges or runs strong for a minute you need a fuel pump kit. If you were comfortable rebuilding carbs the pump rebuild will be simple for you. If I bought a motor with an unknown history on the fuel pump I would rebuild it just so I'd know it was new, it's a good $20 investment.

Again don't run this a lot like this, a 2 stroke motor that is starving for fuel is also starving for lubrication. If you leave it at half throttle or a little less does it still alarm?

Try a different tank just for the heck of it, borrow one from a friend who knows it works well.

Is the tank vent open?

ahicks is right there are a ton of things that this could be.

Private message a moderator and ask him to move your thread if you want it moved.
 

MaPaHa

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Depending on what you're looking to do with the performance, you will need a good tach to get it right if your looking to spend some money high end prop to get the max performance. If you can find a shop that can point you in a direction for another pitch and let you try one or two used ones to see whats going on then that's another good option without a tach. Sometimes it's hard to tell if you're over propped or if you have a horsepower problem if you don't have a history with the boat.

I found some info on similar Yamaha motors and they were 2.33:1 ratio but I don't know if that's the same for yours. You can go on some of the prop calculator sites and run some numbers. With a 13.5 prop, 2.33:1 gear box and 5,500 rpm (just a guess at rated rpm) and a 20% slippage it shows right at 22 mph which sounds to me like it's in the range for an 85 HP on a 24 foot pontoon but you'd want to know the ratio of that motor for sure. I'm calculating manually with no slippage that it would run 27.43 mph and with 20% slippage it runs 21.9 mph. Is there a chance you have water in the pontoons? It makes one act like a dog for sure. Do you know how fast it was running? If 20-22 mph is reasonable for that boat and it's a 2.33:1 ratio for sure, then you're prop may not be too far off.

If you have a temperature meter and a long lead then you can measure the leaving water temperature and see what's going on there. My old '77 Evinrude 140 would set off the temp alarm but it was running the right temp. Wound up being the temp sensor on one head.
 

steelespike

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Apr 26, 2002
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My info indicates 91-97 85 hp is 2.00 gear ratio.
If you check around you will find pontoon speeds vary widely.
Aparently un related to prop size ,boat weight and length.
I find many times that a longer say 24' may be about the same speed as a smaller and much lighter
pontoon say 20' both with the same pitch.
My research indicates prop size varies about 2" 11"-13" On toons 20 to 24 ft un related to size.
Speeds ranging from about 23 to 27 mph
I think its safe to say the 16" prop is too much and prop size likely should be 11" to 13".
This is assuming a healthy,well setup motor.
 

chadwt75

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Apr 20, 2015
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I have a 12 pitch prop on order. I will post the results as soon as I get it in the water. Also I am going to check the temp sensor with a pot of water and a ohmmeter tomorrow I will report back on that as well. As for a tach any recommendations? The yamaha OEM is 226.00. Motor model number is C85TLRP. Thanks again folks!
 

steelespike

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Look for a brand name prop listed to work on your model motor.
I have found ebay to have a vast selection of most anything I'm looking for.
Unrelated to boats I needed a 3.00 X 4 tube for a scooter tire.No shop in town had one
Hopped on ebay Friday afternoon found many listed picked one in Denver (well established vendor) $ 9.95, free shipping
Arrived in Syracuse Monday morning.
 

lmuss53

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Search ebay for this $40

TACHOMETER / UNIVERSAL FIT 6444
 
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