Needing higher rpm

[Jonboy123]

I recently purchased a 18' pontoon and put my 1985 mercury 115 on it. I'm running 25mph but I'm only getting 2200 rpms mayb 2500. Looking at going from a 13.5 diameter 3 blade prop with 17pitch to a 13 with a 14pitch to get the rpms up. Any suggestions? Will this be big enough of a change and will this drop my top speed?

 

[racerone]

Something is not right.----First check settings on tachometer.

 

[jimmbo]

At 25 mph and a 17" Prop, your engine is turning about 3600 rpm. So the tach likely not accurate, but I think a 17 is too much prop for an 18ft Pontoon. That said, I think you need the give the engine a through check over.
First a compression test, post the Numbers
Can the Spark, on all Cylinder jump a 3/8" Gap?
Condition of the Spark Plugs?
Are all 3 carbs supplying Fuel?

 

[Chris1956]

Most 'toons with std power do not go much faster that 25MPH. You need to check the accuracy of the tach and speedo.

 

[Scott Danforth]

An 18' pontoon with a 40hp will do about 22mph. Many times adding more HP just makes more noise and burns more fuel

Is the toon transom rated for 115hp?

Is the motor high enough? Where is the AV plate with respect to the water while you are underway? Most pontoon installations have the motor way too deep

Is the compression on all cylinders over 120 and equal on all cylinders? Or is cylinder #3 really low?

 

[Jonboy123]

I recently purchased a 18' pontoon and put my 1985 mercury 115 on it. I'm running 25mph but I'm only getting 2200 rpms mayb 2500. Looking at going from a 13.5 diameter 3 blade prop with 17pitch to a 13 with a 14pitch to get the rpms up. Any suggestions? Will this be big enough of a change and will this drop my top speed?

The motor seems to be sitting dece

At 25 mph and a 17" Prop, your engine is turning about 3600 rpm. So the tach likely not accurate, but I think a 17 is too much prop for an 18ft Pontoon. That said, I think you need the give the engine a through check over.
First a compression test, post the Numbers
Can the Spark, on all Cylinder jump a 3/8" Gap?
Condition of the Spark Plugs?
Are all 3 carbs supplying Fuel?

At 25 mph and a 17" Prop, your engine is turning about 3600 rpm. So the tach likely not accurate, but I think a 17 is too much prop for an 18ft Pontoon. That said, I think you need the give the engine a through check over.
First a compression test, post the Numbers
Can the Spark, on all Cylinder jump a 3/8" Gap?
Condition of the Spark Plugs?
Are all 3 carbs supplying Fuel?

I just pulled it off my astro glass bass boat and it had been having a carb adjustment issue. I think it might be running a little rich. I definitely think the tach could be off also. Cheap Chinese stuff. I just did all those run through on it this last spring.

 

[Scott Danforth]

just pulled it off my astro glass bass boat and it had been having a carb adjustment issue. I think it might be running a little rich.

Can't just pull a motor off a planing hull and slap on a displacement hull and hope for the best

Go thru the fuel system

Check compression
Check ignition
Check mounting height

Verify HP rating of boat

 

[Jonboy123]

An 18' pontoon with a 40hp will do about 22mph. Many times adding more HP just makes more noise and burns more fuel

Is the toon transom rated for 115hp?

Is the motor high enough? Where is the AV plate with respect to the water while you are underway? Most pontoon installations have the motor way too deep

Is the compression on all cylinders over 120 and equal on all cylinders? Or is cylinder #3 really low?

I used the motor I had. I know I wasn't gonna gain any speed. The motor is sitting pretty good in the water. Probably about the same as my last boat. I ran through the motor last spring and it ran pretty decent all summer. I was having an issue with a carb adjustment and need to try to synchronize them again.

 

[racerone]

You cannot safely run one of those motors with carburetor issues.----A plugged carburetor means you might be starving 2 cylinders of oil.----And it needs a new water pump impeller every 3 to 5 years.

 

[QBhoy]

25 mph is good going, considering you’re way over propped.
A rough calculation might suggest that you’re more likely to be doing late 3000’s rpm or close to 4000 rpm even, in real life.
Is the engine the same age as the boat and a 1985 too ? As in a straight 6 cylinder 115 ?
Is the gearbox original and the usual ratio ?

 

[Jonboy123]

Something is not right.----First check settings

You cannot safely run one of those motors with carburetor issues.----A plugged carburetor means you might be starving 2 cylinders of oil.----And it needs

25 mph is good going, considering you’re way over propped.
A rough calculation might suggest that you’re more likely to be doing late 3000’s rpm or close to 4000 rpm even, in real life.
Is the engine the same age as the boat and a 1985 too ? As in a straight 6 cylinder 115 ?
Is the gearbox original and the usual ratio ?

I figured out the pitch on the prop was 23 not 19 like I thought. So I'm way over propped. Both the pontoon and the engine are 1985 and yup its the inline 6 tower of power. Pretty sure the gearing is original. And I was stoked to get 25mph but I wanna get those rpms where they should be. I have a Stainless Prop that's 19 pitch so I'm gonna try putting it on and see if that doesn't help smooth things out a bit. The tach is a digital one I got from Amazon. It does GPS mph, fuel, temp, oil pressure. It was kinda a cheap one so I wondered if it was wrong

 

[jimmbo]

I figured out the pitch on the prop was 23 not 19 like I thought.

You said it was a 17, now it is a 19, so that just threw out my and QBhpy's caluclations out the Window.

And I was stoked to get 25mph but I wanna get those rpms where they should be. I have a Stainless Prop that's 19 pitch so I'm gonna try putting it on and see if that doesn't help smooth things out a bit.

Why torture the Poor Motor even More? You are overpropped, if you haven't damaged the Engine yet, continued operating it like that will do so.

The tach is a digital one I got from Amazon. It does GPS mph, fuel, temp, oil pressure. It was kinda a cheap one so I wondered if it was wrong

How does the Tach get its Info? If it is an Inductive Tach does it have a Setting for 2 Strokes? If not, it is giving a Reading for a 4 Stroke. if so that 2200rpm is really 4400, still 600 rpm below the Minimum WOT Rpm, and about 1000 rpm below where you want that engine running. WOT range for that Motor is 5000-5500

 

[Chris1956]

Yes, your tach cannot display oil pressure as there isn't any. Water temp will go from dead cold at idle to warmer at cruise, but without a thermostat in the motor to hold a steady temp., it is pretty useless.

Now if you could program the oil pressure gauge to read 0-15PSI water pressure, and tap it into the water jacket that would be useful.

BTW - you should have the newer wire colors. As such, the tach should be connected to the grey wire for the tach signal. The tach setting is 6C or 12P, whichever you have.

 

[QBhoy]

Agree with above. Some confusion over prop actual pitch perhaps.
But also agree with the last post. Make sure you have the tachometer set on the right setting. For 2 stroke cycle and 6 cylinders.

 

[Jonboy123]

Agree with above. Some confusion over prop actual pitch perhaps.
But also agree with the last post. Make sure you have the tachometer set on the right setting. For 2 stroke cycle and 6 cylinders.

Ok sorry I was running on a low amount of sleep. The prop that was on it was a 23. I thought it was 17 by one of the set of numbers but put all of them into Google and found out it was a 23. I had a spare that's a 19. As far as the tach goes. It only has a black button on the back so if that's how you adjust it I'll have to try but the instructions were in Chinese. I know by running the motor all summer it might be close to half the rpms it needs. That's why I only ran it for an hr and that was just to test if the pontoon was safe and not leaking and see if it was sitting decently in the water. Which it was. I know the carbs are running rich and plan on adjusting once I get back out which might be a while. I'm going to try to run the 19 I have and see if that gets me up to the proper rpms if not I will go lower. Which I don't plan on running it too long if it doesn't reach the right rpms with the carbs adjusted and the new prop.

 

[Scott Danforth]

Get a quality tach

 

[jimmbo]

17, then 19, now it was 23....

 

[Scott Danforth]

Doesn't really matter what the current prop is until the tach is sorted and the fuel system is sorted

Additionally, the hp rating of the transom pod needs to be confirmed

 

[Jonboy123]

Jimmbo I never said it was 19

 

[Jonboy123]

Jimmbo I never said it was 19

I literally just said I had a spare that's a 19