***MERCURY 140HP "TOWER OF POWER" PROP???

Lukelona

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*Question about my Mercury 140hp outboard ("tower of power")***

Just recently bought a 99 Larson 206 but it's got a old a** 1980-82 mercury 140hp outboard
The prop it came with was damaged and in bad shape so I wanted to change it. Long story short it was stuck on shaft so bad I had to cut it off.

I can't find any markings on the old prop to figure out size, pitch etc. I had a old mercruiser SS prop I tried on it it's a 19p but larger diameter. Believe it's 14"+

I did first splash today and left it on the trailer while I ran it and made sure everything was ok before unloading completely... When I put it in gear it stalls out immediately as if you were talking off in 3rd gear in a car. It does not seem engine performance issue, runs perfect, goes in gear perfect in the garden hose, but in the river it's like it's over propped so bad it's stalling the engine.

My question is,
A. Does this sound logical?
B. Does anyone know what size prop I should try? What pitch and diameter?

Figured I'd try on here first before calling around tomorrow.
Thanks in advance for any input
 

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jimmbo

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The 140 was made in 72, in 78 another version was made until 1981, a serial number would be helpful. Your engine has a WOT throttle range of 5300 - 5800 rpm. A 20 ft boat with a 140 is going to need a 15 or 16 inch pitch to get to the rpm range you want. Look carefully on the hub between the blades, Mercury used to cast the part # there, and the last 2 digits were the pitch. If it is not a Mercury prop then the info could be on the blade, on the end of the hub.
As for your engine dying in the water. This week there have numerous posts of 115s doing the same thing. Soooo... First thing to do is a compression test. Then making sure all cylinders are getting spark. Carb cleanliness is important, as is idle mixture adjustment. Those engines are 6 cylinders, they can start and run very well on the muffs out of the water, even with a couple of weak or dead cylinders. All that shows up in the water with back pressure on the exhaust and load on the engine.

You learned why it is important to keep the propshaft greased
 
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Lukelona

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So your saying the prop can't kill the motor when put in gear if it's over propped?
 

jimmbo

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The higher the pitch, the greater the load when in gear. So anytime there is a pitch increase or decrease an adjustment to the idle speed may be necessary.
If the engine isn't firing on all cylinders, it is going to have trouble under load
 
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Lukelona

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I'm gonna do another compression test and spark check all 6 cyl after work today. My buddy is getting me a prop to test that's smaller like it should have on it. If everything checks out I'll try the smaller prop hopefully today
 

jimmbo

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Does it have a Tach? As I said earlier, the WOT rpm range for that engine is 5300 - 5800. That engine will perform best propped at the high end
 

Lukelona

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Does it have a Tach? As I said earlier, the WOT rpm range for that engine is 5300 - 5800. That engine will perform best propped at the high end

It does, but whoever put the motor on sucked and didn't hook everything up.
I pulled the console and wired the tach wire to the tach but it's not working. Totally seperate issue...
I don't think it's charging batteries, I read online if it's not charging the tach won't work as well?
I may start looking for a replacement used engine soon. She's got a few issues and I'm wondering if it's even worth my time and money to fix it...
 

jimmbo

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Probably a bad rectifier

While the 140 is a fine performer when running properly, it will always be a little weak at the bottom end, especially on larger boats, when compared to the Merc 135, 150 V6s.
 
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