Need help with a 40hp Mariner

[c-dubb]

I have an early 80's model 40hp Mariner that will crank and idle fine, but when I give it full throttle it takes a few minutes to open up and then it is only turning 4000 rpms. I have tried a different fuel tank, replaced fuel lines, new bubble, cleaned carbs, checked ohms on stator, trigger, and coils. All readings were good but the blue-blue/white wires on the CDI stator were around 2100 ohms, just a little under the 2200-2400 range. Hoping someone can point me in the right direction. Thanks in advance for any help.

 

[Texasmark]

Is this a new condition or what?

 

[c-dubb]

We recently purchased this boat, however it was worse before i tried the things listed above. It wouldn't go over idle speed in the water.

 

[GA_Boater]

Make sure the carbs are completely open when you move the throttle to wide open. The motor doesn't need to be running for this.

 

[c-dubb]

Thanks, I have verified that the carbs are wide open when the throttle is wide open.

 

[Texasmark]

Is this your situation: You crank up at the dock and motor out to the "no wake" markers. You give it full throttle to get the boat on plane and: "but when I give it full throttle it takes a few minutes to open up and then it is only turning 4000 rpms.". Yes or no? If yes, it could be several fuel related problems, but it also could be too much pitch in your prop for the load in the boat/engine tilt position. If you are running the classic 4 cylinder, cross flow descendent from the original Mercury Mark 55, 4 cyl 40 hp, that engine is spec'd at 5500 rpms at the top end. You are way off, but 2 strokers will load up on the low end and crater which may be happening. When you are on the low side of the peak of the torque curve, any load just reduces the torque that much more and it's the domino effect.

Other thing could be low compression. Check and repost.

 

[c-dubb]

Yes Texasmark, that is my situation. I haven't thought about the prop, just looked and the prop has 992406 13p stamped on it. I forgot to mention this motor is on a 17' Basstracker.

 

[Texasmark]

I need an accurate speed at your 4000 rpm, light load in the boat, best trim/engine tilt pin position used when normally running, and then I can run a prop calculation. I ran a 90 on my 17' alum BB. A 40 is not all that much and you well may be over propped.....pitch too high. Get me the number and I'll advise.

 

[c-dubb]

Thanks Texasmark, I will get that info on Saturday and post the results.

 

[Texasmark]

Thanks Texasmark, I will get that info on Saturday and post the results.

Will be waiting. If you can, get me a picture of your wake. Just turn around at the helm, camera about 5' above the deck (floor) and point it at the top rear of the engine. I want to see how far back your wake converges behind the boat when you take your mph reading.

 

[c-dubb]

Ok, here's the results. Max out around 21.5 mph around 4000 rpm's. When I give it full throttle it will pick up to about 3000 rpm's right away and will slowly climb to around 4000 rpm's. We are currently in the third hole from the top with the pin and using a 13p prop. For some reason, the forum won't let me upload even a single picture.

 

[Texasmark]

You are describing an underpowered boat. First off your engine is designed to be running between 5000 and 5500 rpm at Wide Open Throttle. You are at 4000 and having to struggle to get there. I just ran a BAM prop slip calculation for your numbers and came up with a 25 mph speed if your prop were turning in a solid (no slip) and for your 21 mph the slip (prop turning in water) came in at 16% which is not a bad for a general purpose boat.

I removed your 4000 rpm and inserted the middle of the range (5250). Then I removed the 13P prop and solved the equation for what prop pitch it would take to get you up where you belong. The number is 10 and I believe it.

With a 10P prop the load on your engine will be much less. Your hole shot....getting onto plane from dead still in the water will be highly improved. Your boat won't lumber along straining to get going. It will be lively. You should still get your 21 mph but your engine will/should respond immediately and not have to work it's way up to what rpm it can get and the engine will perform much better.

Then get a couple of cans of Sea Foam (fuel additive, red label on a white can $6.25 a pint here at WW) and add to your regular 50:1 pre-mix fuel per the directions on the can. It will clean up the carbon in your combustion chamber and exhaust and all which was caused by you lugging your engine and help your engine to run better, probably faster, and surely smoother.

Iboats sells props. Go to the top of the page and get into the Boat Parts and Accessories section, scroll down to props. Get in that section and fill in the blanks and you should be met with a very reasonably priced, quality prop, and right now a lot of props are free shipping.

Then after you have done all the above, come back in a couple of weeks and tell us how things are going.

Good Boating,
Mark

 

[c-dubb]

Thanks Texasmark for all the effort you are putting in to help me out. Just for giggles I checked my pontoon today and it only has an 11p on a 70hp Yamaha. Do I need to be concerned with the diameter, not sure I wanna trust what is on there currently.

 

[Texasmark]

Thanks Texasmark for all the effort you are putting in to help me out. Just for giggles I checked my pontoon today and it only has an 11p on a 70hp Yamaha. Do I need to be concerned with the diameter, not sure I wanna trust what is on there currently.

Diameters come in a range to suit the lower unit dimensions which are the result of the HP range. Once you get into an engine and HP range, you are pretty much set in a diameter range which varies as the type prop and pitch. Most folks don't pay it that much attention as it was factored into the design of a particular functional prop.

If your Yammie is running in the recommended WOT operating range you are good to go. If above you need some more pitch, if below, less. Rule of thumb which just passed inspection on the selection of your 10P prop is 150 -200 rpm per inch of pitch change, up or down. I like the 200 number as it is what has worked best for me and worked for you on the 10P.

 

[c-dubb]

Ok Texasmark, here are the results with the 10p prop. There was no change coming out of the hole and still took a few minutes to climb in rpms like before. The only difference that I saw was about a 400-500 rpm increase on the top end that put me at 4400-4500 rpms total. I then brought the motor up a notch and got the rpms to increase to 4800-4900 on the top end, still no change out of the hole. I'm starting to wonder if there may be carb issues, had my son pump bubble while going and started running worse. I could also tell it wanted to do more on top end then would back off a little, nothing really noticeable but you could barely hear and feel it. You got any other ideas?

 

[c-dubb]

OK, got tired of fooling with it and took to a boat mechanic. They came back and said that it is getting too much fuel and the top carb was leaking. They recommended rebuilding both carbs and replacing a few other minor things. I'll update when we get it back.